Transducer From Wikipedia, the free encyclopedia Jump to: navigation , search A transducer is a device that converts one type of energy to another. The conversion can be to/from electrical , electro- mechanical , electromagnetic , photonic , photovoltaic , or any other form of energy. While the term transducer commonly implies use as a sensor/detector, any device which converts energy can be considered a transducer. [edit ] Types Transducers may be categorized by application: Sensor , actuator , or combination. A sensor is used to detect a parameter in one form and report it in another form of energy (usually an electrical and/or digital signal). For example, a pressure sensor might detect pressure (a mechanical form of energy) and convert it to electricity for display at a remote gauge . An actuator accepts energy and produces movement (action). The energy supplied to an actuator might be electrical or mechanical (pneumatic , hydraulic , etc.). An electric motor and a loudspeaker are both transducers, converting electrical energy into motion for different purposes. Combination transducers have both functions -- they both detect and create action. For example, a typical ultrasonic transducer switches back and forth many times a second between acting as an actuator to produce ultrasonic waves, and acting as a sensor to detect ultrasonic waves. [edit ] Applications
Transcript
TransducerFrom Wikipedia, the free encyclopedia
Jump to: navigation, search
A transducer is a device that converts one type of energy to another. The conversion can be to/from electrical, electro-mechanical, electromagnetic, photonic, photovoltaic, or any other form of energy. While the term transducer commonly implies use as a sensor/detector, any device which converts energy can be considered a transducer.
[edit] Types
Transducers may be categorized by application: Sensor, actuator, or combination.
A sensor is used to detect a parameter in one form and report it in another form of energy (usually an electrical and/or digital signal). For example, a pressure sensor might detect pressure (a mechanical form of energy) and convert it to electricity for display at a remote gauge.
An actuator accepts energy and produces movement (action). The energy supplied to an actuator might be electrical or mechanical (pneumatic, hydraulic, etc.). An electric motor and a loudspeaker are both transducers, converting electrical energy into motion for different purposes.
Combination transducers have both functions -- they both detect and create action. For example, a typical ultrasonic transducer switches back and forth many times a second between acting as an actuator to produce ultrasonic waves, and acting as a sensor to detect ultrasonic waves.
[edit] Applications
Electromagnetic: o Antenna - converts electromagnetic waves into electric current and vice
versa.o Cathode ray tube (CRT) - converts electrical signals into visual formo Fluorescent lamp , light bulb - converts electrical power into visible lighto Magnetic cartridge - converts motion into electrical formo Photodetector or Photoresistor (LDR) - converts changes in light levels
into resistance changeso Tape head - converts changing magnetic fields into electrical formo Hall effect sensor - converts a magnetic field level into electrical form
Electrochemical: o pH probes o Electro-galvanic fuel cell o Hydrogen sensor
Electromechanical (electromechanical output devices are generically called actuators):
o Electroactive polymers o Galvanometer o Microelectromechanical systems o Rotary motor , linear motoro Vibration powered generator o Potentiometer when used for measuring positiono Load cell converts force to mV/V electrical signal using strain gaugeo Accelerometer o Strain gauge o String Potentiometer o Air flow sensor o Tactile sensor
Electroacoustic: o Loudspeaker , earphone - converts electrical signals into sound (amplified
signal → magnetic field → motion → air pressure)o Microphone - converts sound into an electrical signal (air pressure →
motion of conductor/coil → magnetic field → signal)o Pick up (music technology) - converts motion of metal strings into an
electrical signal (magnetism → electricity (signal))o Tactile transducer - converts solid-state vibrations into electrical signal
(vibration → ? → signal)o Piezoelectric crystal - converts solid-state electrical moduluations into an
electrical signal (vibration → ? → signal)o Geophone - convert a ground movement (displacement) into voltage -
(vibrations → motion of conductor/coil → magnetic field → signal)o Gramophone pick-up - (air pressure → motion → magnetic field →
signal)o Hydrophone - converts changes in water pressure into an electrical formo Sonar transponder (water pressure → motion of conductor/coil →
magnetic field → signal)
Photoelectric : o Laser diode , light-emitting diode - convert electrical power into forms of
Thermoelectric: o RTD Resistance Temperature Detectoro Thermocouple o Peltier cooler o Thermistor (includes PTC resistor and NTC resistor)
Radioacoustic: o Geiger-Müller tube used for measuring radioactivity.o Receiver (radio)
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. A thermocouple converts temperature to an output voltage which can be read by a voltmeter. For accuracy, all sensors need to be calibrated against known standards.
An actuator is a mechanical device for moving or controlling a mechanism or system. It takes energy, usually transported by air, electric current, or liquid, and converts that into some kind of motion.
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism, but other operating principles are also used. Relays find applications where it is necessary to control a circuit by a low-power signal, or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re-transmitting it to another. Relays found extensive use in telephone exchanges and early computers to perform logical operations. A type of relay that can handle the high power required to directly drive an electric motor is called a contactor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protection relays".
nverter (electrical)From Wikipedia, the free encyclopedia
Jump to: navigation, search
An inverter is an electrical device that converts direct current (DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits.
Static inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries.
The electrical inverter is a high-power electronic oscillator. It is so named because early mechanical AC to DC converters were made to work in reverse, and thus were "inverted", to convert DC to AC.
The inverter performs the opposite function of a rectifier.
A solenoid (1827, fr. solénoïde, gr. solen "pipe, channel" + comb. form of gr. eidos "form, shape"[1]) is a three-dimensional coil. In physics, the term solenoid refers to a loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create controlled magnetic fields and can be used as electromagnets. The term solenoid refers specifically to a magnet designed to produce a uniform magnetic field in a volume of space (where some experiment might be carried out).
In engineering, the term solenoid may also refer to a variety of transducer devices that convert energy into linear motion. The term is also often used to refer to a solenoid valve, which is an integrated device containing an electromechanical solenoid which actuates either a pneumatic or hydraulic valve, or a solenoid switch, which is a specific type of relay that internally uses an electromechanical solenoid to operate an electrical switch; for example, an automobile starter solenoid, or a linear solenoid, which is an electromechanical solenoid.
A transistor is a semiconductor device used to amplify and switch electronic signals. It is made of a solid piece of semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, the transistor provides amplification of a signal. Some transistors are packaged individually but many more are found embedded in integrated circuits.
The transistor is the fundamental building block of modern electronic devices, and its presence is ubiquitous in modern electronic systems.
A contactor is an electrically controlled switch (a relay) used for switching a power or control circuit.[1] A contactor is controlled by a circuit which has a much lower power level than the switched circuit. Contactors come in many forms with varying capacities
and features. Unlike a circuit breaker a contactor is not intended to interrupt a short circuit current.
Contactors range from those having a breaking current of several amps and 24 V DC to thousands of amps and many kilovolts. The physical size of contactors ranges from a device small enough to pick up with one hand, to large devices approximately a meter (yard) on a side.
Contactors are used to control electric motors, lighting, heating, capacitor banks, and other electrical loads.
Motor is a device that creates motion, not an engine; it usually refers to either an electrical motor or an internal combustion engine. It may also refer to:
Electric motor , a machine that converts electricity into a mechanical motion o AC motor , an electric motor that is driven by alternating current
Synchronous motor , an alternating current motor distinguished by a rotor spinning with coils passing magnets at the same rate as the alternating current and resulting magnetic field which drives it
Induction motor , also called a squirrel-cage motor, a type of asynchronous alternating current motor where power is supplied to the rotating device by means of electromagnetic induction
o DC motor , an electric motor that runs on direct current electricity Brushed DC electric motor , an internally commutated electric
motor designed to be run from a direct current power source Brushless DC motor , a synchronous electric motor which is
powered by direct current electricity and has an electronically controlled commutation system, instead of a mechanical commutation system based on brushes
o Electrostatic motor , a type of electric motor based on the attraction and repulsion of electric charge
o Servo motor , an electric motor that operates a servo, commonly used in robotics
o Internal fan-cooled electric motor , an electric motor that is self-cooled by a fan, typically used for motors with a high energy density
A thermostat is a device for regulating the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The name is derived from the Greek words thermos "hot" and statos "a standing". The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer fluid as needed, to maintain the correct temperature.
A thermostat may be a control unit for a heating or cooling system or a component part of a heater or air conditioner. Thermostats can be constructed in many ways and may use a variety of sensors to measure the temperature. The output of the sensor then controls the heating or cooling apparatus.
The first electric room thermostat was invented in 1883 by Warren S. Johnson.[1][2]
Common sensor technologies include:
Bimetallic mechanical or electrical sensors Expanding wax pellets Electronic thermistors and semiconductor devices Electrical thermocouples Thermostatic Radiator Valve (TRV)
A Honeywell electronic thermostat in a retail store
Earlier technologies included mercury thermometers with electrodes inserted directly through the glass, so that when a certain (fixed) temperature was reached the contacts would be closed by the mercury. These were accurate within a degree.
These may then control the heating or cooling apparatus using:
Direct mechanical control Electrical signals Pneumatic signals
o 1.1 Bimetal o 1.2 Wax pellet o 1.3 Gas expansion
2 Electrical o 2.1 Bimetallic thermostatic components o 2.2 Simple two wire thermostats
2.2.1 Millivolt thermostats 2.2.2 24 volt thermostats 2.2.3 Ignition sequences in modern systems 2.2.4 Line voltage thermostats
o 2.3 Combination heating/cooling regulation 3 Heat pump regulation
o 3.1 Digital o 3.2 Household thermostat location
4 Dummy thermostats 5 See also 6 References
7 External links
[edit] Mechanical
[edit] Bimetal
The examples and perspective in this article may not represent a worldwide view of the subject. Please improve this article and discuss the issue on the talk page.
On a steam or hot-water radiator system, the thermostat may be an entirely mechanical device incorporating a bimetal strip. Generally, this is an automatic valve which regulates the flow based on the temperature. For the most part, their use in North America is now rare, as modern under-floor radiator systems use electric valves, as do some older retrofitted systems. They are still widely employed on central heating radiators throughout Europe, however.
Mechanical thermostats are used to regulate dampers in rooftop turbine vents, reducing building heat loss in cool or cold periods.
An automobile passenger compartment's heating system has a thermostatically controlled valve to regulate the water flow and temperature to an adjustable level. In older vehicles the thermostat controls the application of engine vacuum to actuators that control water valves and flappers to direct the flow of air. In modern vehicles, the vacuum actuators may be operated by small solenoids under the control of a central computer.
A thermostat is used in internal combustion engines to maintain the engine at its optimum operating temperature by regulating the flow of coolant to an external heat sink, usually an air cooled radiator.
This type of thermostat operates mechanically. It makes use of a wax pellet inside a sealed chamber. The wax is solid at low temperatures but as the engine heats up the wax melts and expands. The sealed chamber has an expansion provision that operates a rod which opens a valve when the operating temperature is exceeded. The operating temperature is fixed, but is determined by the specific composition of the wax, so thermostats of this type are available to maintain different temperatures, typically in the range of 70 to 90°C (160 to 200°F). Modern engines run hot, that is, over 80°C (180°F), in order to run more efficiently and to reduce the emission of pollutants. Most thermostats have a small bypass hole to vent any gas that might get into the system, e.g., air introduced during coolant replacement, which also allows a small flow of coolant past the thermostat when it is closed. This bypass flow ensures that the thermostat experiences the temperature change in the coolant as the engine heats up; without it a stagnant region of coolant around the thermostat could shield it from temperature changes in the coolant adjacent to the combustion chambers and cylinder bores.
While the thermostat is closed, the flow of coolant in the loop is greatly slowed, allowing coolant surrounding the combustion chambers to warm up rapidly. The thermostat stays closed until the coolant temperature reaches the nominal thermostat opening temperature. The thermostat then progressively opens as the coolant temperature increases to the optimum operating temperature, increasing the coolant flow to the radiator. Once the optimum operating temperature is reached, the thermostat progressively increases or decreases its opening in response to temperature changes, dynamically balancing the coolant recirculation flow and coolant flow to the radiator to maintain the engine temperature in the optimum range as engine heat output, vehicle speed, and outside ambient temperature change. If the load on the engine increases, increasing the heat input to the cooling system, or the vehicle speed decreases or air temperature increases, decreasing the radiator heat output, the thermostat will open further to increase the flow of coolant to the radiator, preventing the engine from overheating. If the conditions reverse, the thermostat will reduce its opening to maintain the coolant temperature.
Under normal operating conditions the thermostat is open to about half of its stroke travel, so that it can open further or reduce its opening to react to changes in operating conditions. A correctly designed thermostat will never be fully open or fully closed while the engine is operating normally, or overheating or overcooling would occur. For instance,
If more cooling is required, e.g., in response to an increase in engine heat output which causes the coolant temperature to rise, the thermostat will increase its opening to allow more coolant to flow through the radiator and increase engine cooling. If the thermostat were already fully open, then it would not be able to increase the flow of coolant to the radiator, hence there would be no more cooling capacity available, and the increase in heat output by the engine would result in overheating.
If less cooling is required, e.g., in response to decrease in ambient temperature which causes the coolant temperature to fall, the thermostat will decrease its opening to restrict the coolant flow through the radiator and reduce engine cooling. If the thermostat were already fully closed, then it would not be able to reduce cooling in response to the fall in coolant temperature, and the engine temperature would fall below the optimum operating range.
Modern cooling systems contain a relief valve in the form of a spring-loaded radiator pressure cap, with a tube leading to a partially filled expansion reservoir. Owing to the high temperature, the cooling system will become pressurized to a maximum set by the relief valve. The additional pressure increases the boiling point of the coolant above that which it would be at atmospheric pressure.
The wax product used within the thermostat requires a specific process to produce. Unlike a standard paraffin wax, which has a relatively wide range of carbon chain lengths, a wax used in the thermostat application has a very narrow range of carbon molecule chains. The extent of the chains is usually determined by the melting
characteristics demanded by the specific end application. To manufacture a product in this manner requires very precise levels of distillation, which is difficult or impossible for most wax refineries.
[edit] Gas expansion
Thermostats are sometimes used to regulate gas ovens. It consists of a gas-filled bulb connected to the control unit by a slender copper tube. The bulb is normally located at the top of the oven. The tube ends in a chamber sealed by a diaphragm. As the thermostat heats up the gas expands applying pressure to the diaphragm which reduces the flow of gas to the burner.
[edit] Electrical
[edit] Bimetallic thermostatic components
These are small circular self-contained units with a mounting flange for attachment to a plate or metal part of a heater or air-conditioner, exposed to the ambient temperature. The internal sensor generally consists of a bimetallic disk with an electrical contact in its centre. At the switching temperature, the disk flips from concave to convex (or vice versa) causing the contact to open or close depending on the required mode of switching (normally closed or normally open). This device can also be applied as an overheating prevention switch.
The illustration is the interior of a common two wire heat-only household thermostat, used to regulate a gas-fired heater via an electric gas valve. Similar mechanisms may also be used to control oil furnaces, boilers, boiler zone valves, electric attic fans, electric furnaces, electric baseboard heaters, and household appliances such as refrigerators, coffee pots, and hair dryers. The power through the thermostat is provided by the heating device and may range from millivolts to 240 volts in common North American construction, and is used to control the heating system either directly (electric baseboard heaters and some electric furnaces) or indirectly (all gas, oil and forced hot water systems). Due to the variety of possible voltages and currents available at the thermostat, caution must be taken when selecting a replacement device.
1. Set point control lever. This is moved to the right for a higher temperature. The round indicator pin in the center of the second slot shows through a numbered slot in the outer case.
2. Bimetallic strip wound into a coil. The center of the coil is attached to a rotating post attached to lever (1). As the coil gets colder the moving end — carrying (4) — moves clockwise.
3. Flexible wire. The left side is connected via one wire of a pair to the heater control valve.
4. Moving contact attached to the bimetal coil.thence to the heater's controller.5. Magnet . This ensures a good contact when the contact closes. It also provides
hysteresis to prevent short heating cycles, as the temperature must be raised several degrees before the contacts will open. As an alternative, some thermostats instead use a mercury switch on the end of the bimetal coil. The weight of the mercury on the end of the coil tends to keep it there, also preventing short heating cycles. However, this type of thermostat is banned in many countries due to its highly and permanently toxic nature if broken. When replacing these thermostats they must be regarded as chemical waste.
6. Fixed contact screw. This is adjusted by the manufacturer. It is connected electrically by a second wire of the pair to the thermocouple and the heater's electrically operated gas valve.
Not shown in the illustration is a separate bimetal thermometer on the outer case to show the actual temperature at the thermostat.
[edit] Millivolt thermostats
As illustrated in the use of the thermostat above, the power is provided by a thermocouple, heated by the pilot light. This produces little power and so the system must use a low power valve to control the gas. This type of device is generally considered obsolete as pilot lights waste a surprising amount of gas (in the same way a dripping faucet can waste a huge amount of water over an extended period), and are also no longer used on stoves, but are still to be found in many gas water heaters. (Their poor efficiency is acceptable in water heaters, since most of the energy "wasted" on the pilot light is still being coupled to the water and therefore helping to keep the tank warm. It also makes it unnecessary for an electrical circuit to be run to the water heater. For tankless (on
demand) water heaters, pilot ignition is preferable because it is faster than hot-surface ignition and more reliable than spark ignition.)
Some programmable thermostats will control these systems.
[edit] 24 volt thermostats
The majority of modern heating/cooling/heat pump thermostats operate on low voltage (typically 24 volts AC) control circuits. The source of the 24 volt AC power is a control transformer installed as part of the heating/cooling equipment. The advantage of the low voltage control system is the ability to operate multiple electromechanical switching devices such as relays, contactors, and sequencers using inherently safe voltage and current levels.[3] Built into the thermostat is a provision for enhanced temperature control using anticipation. A heat anticipator generates a small amount of additional heat to the sensing element while the heating appliance is operating. This opens the heating contacts slightly early to prevent the space temperature from greatly overshooting the thermostat setting. A mechanical heat anticipator is generally adjustable and should be set to the current flowing in the heating control circuit when the system is operating. A cooling anticipator generates a small amount of additional heat to the sensing element while the cooling appliance is not operating. This causes the contacts to energize the cooling equipment slightly early, preventing the space temperature from climbing excessively. Cooling anticipators are generally non-adjustable. Electromechanical thermostats use resistance elements as anticipators. Most electronic thermostats use either thermistor devices or integrated logic elements for the anticipation function. In some electronic thermostats, the thermistor anticipator may be located outdoors, providing a variable anticipation depending on the outdoor temperature. Thermostat enhancements include outdoor temperature display, programmability, and system fault indication. While such 24 volt thermostats are incapable of operating a furnace when the mains power fails, most such furnaces require mains power for heated air fans (and often also hot-surface or electronic spark ignition) so no functionality is lost. In other circumstances such as piloted wall and "gravity" (fanless) floor and central heaters the low voltage system described previously may be capable of remaining functional when electrical power is unavailable.
[edit] Ignition sequences in modern systems Gas
1. Start drafting fan (if the furnace is relatively recent) to create a column of air flowing up the chimney
2. Heat ignitor or start spark-ignition system3. Open gas valve to ignite main burners4. Wait (if furnace is relatively recent) until the heat exchanger is at proper
operating temperature before starting main blower fan or circulator pump
1. Similar to gas, except rather than opening a valve, the furnace will start an oil pump to inject oil into the burner
Electric
1. The blower fan or circulator pump will be started, and a large electromechanical relay or TRIAC will turn on the heating elements
Coal (including grains such as corn, wheat, and barley, or pellets made of wood, bark, or cardboard)
1. Generally rare today (though grains and pellets are increasing in popularity); similar to gas, except rather than opening a valve, the furnace will start a screw to drive coal/grain/pellets into the firebox
With non-zoned (typical residential, one thermostat for the whole house) systems, when the thermostat's R (or Rh) and W terminals are connected, the furnace will go through its startup rituals and produce heat.
With zoned systems (some residential, many commercial systems — several thermostats controlling different "zones" in the building), the thermostat will cause small electric motors to open valves or dampers and start the furnace or boiler if it's not already running.
Most programmable thermostats will control these systems.
[edit] Line voltage thermostats
Line voltage thermostats are most commonly used for electric space heaters such as a baseboard heater or a direct-wired electric furnace. If a line voltage thermostat is used, system power (in the United States, 120 or 240 volts) is directly switched by the thermostat. With switching current often exceeding 40 amperes, using a low voltage thermostat on a line voltage circuit will result at least in the failure of the thermostat and possibly a fire. Line voltage thermostats are sometimes used in other applications, such as the control of fan-coil (fan powered from line voltage blowing through a coil of tubing which is either heated or cooled by a larger system) units in large systems using centralized boilers and chillers, or to control circulation pumps in hydronic heating applications.
Some programmable thermostats are available to control line-voltage systems. Baseboard heaters will especially benefit from a programmable thermostat which is capable of continuous control (as are at least some Honeywell models), effectively controlling the heater like a lamp dimmer, and gradually increasing and decreasing heating to ensure an extremely constant room temperature (continuous control rather than relying on the averaging effects of hysteresis). Systems which include a fan (electric furnaces, wall heaters, etc.) must typically use simple on/off controls.
Depending on what is being controlled, a forced-air air conditioning thermostat generally has an external switch for heat/off/cool, and another on/auto to turn the blower fan on constantly or only when heating and cooling are running. Four wires come to the centrally-located thermostat from the main heating/cooling unit (usually located in a closet, basement, or occasionally in the attic): One wire supplies a 24 volts AC power connection to the thermostat, while the other three supply control signals from the thermostat, one for heat, one for cooling, and one to turn on the blower fan. The power is supplied by a transformer, and when the thermostat makes contact between power and another wire, a relay back at the heating/cooling unit activates the corresponding function of the unit.
A thermostat, when set to "cool", will only turn on when the ambient temperature of the surrounding room is above the set temperature. Thus, if the controlled space has a temperature normally above the desired setting when the heating/cooling system is off, it would be wise to keep the thermostat set to "cool", despite what the temperature is outside. On the other hand, if the temperature of the controlled area falls below the desired degree, then it is advisable to turn the thermostat to "heat".
[edit] Heat pump regulation
The heat pump is a refrigeration based appliance which reverses refrigerant flow between the indoor and outdoor coils. This is done by energizing a reversing valve (also known as a "4-way" or "change-over" valve). During cooling, the indoor coil is an evaporator removing heat from the indoor air and transferring it to the outdoor coil where it is rejected to the outdoor air. During heating, the outdoor coil becomes the evaporator and heat is removed from the outdoor air and transferred to the indoor air through the indoor coil. The reversing valve, controlled by the thermostat, causes the change-over from heat to cool. Residential heat pump thermostats generally have an "O" terminal to energize the reversing valve in cooling. Some residential and many commercial heat pump thermostats use a "B" terminal to energize the reversing valve in heating. The heating capacity of a heat pump decreases as outdoor temperatures fall. At some outdoor temperature (called the balance point) the ability of the refrigeration system to transfer heat into the building falls below the heating needs of the building. A typical heat pump is fitted with electric heating elements to supplement the refrigeration heat when the outdoor temperature is below this balance point. Operation of the supplemental heat is controlled by a second stage heating contact in the heat pump thermostat. During heating, the outdoor coil is operating at a temperature below the outdoor temperature and condensation on the coil may take place. This condensation may then freeze onto the coil, reducing its heat transfer capacity. Heat pumps therefore have a provision for occasional defrost of the outdoor coil. This is done by reversing the cycle to the cooling mode, shutting off the outdoor fan, and energizing the electric heating elements. The electric heat in defrost mode is needed to keep the system from blowing cold air inside the building. The elements are then used in the "reheat" function. Although the thermostat may indicate the system is in defrost and electric heat is activated, the defrost function is
not controlled by the thermostat. Since the heat pump has electric heat elements for supplemental and reheats, the heat pump thermostat provides for use of the electric heat elements should the refrigeration system fail. This function is normally activated by an "E" terminal on the thermostat. When in emergency heat, the thermostat makes no attempt to operate the compressor or outdoor fan.
[edit] Digital
See also: Programmable thermostat
Residential digital thermostat
Newer digital thermostats have no moving parts to measure temperature and instead rely on thermistors or other semiconductor devices such as a resistance thermometer (resistance temperature detector). Typically one or more regular batteries must be installed to operate it, although some so-called "power stealing" digital thermostats use the common 24 volt AC circuits as a power source, but will not operate on thermopile powered "millivolt" circuits used in some furnaces. Each has an LCD screen showing the current temperature, and the current setting. Most also have a clock, and time-of-day and even day-of-week settings for the temperature, used for comfort and energy conservation. Some advanced models have touch screens, or the ability to work with home automation or building automation systems.
Digital thermostats use either a relay or a semiconductor device such as triac to act as switch to control the HVAC unit. Units with relays will operate millivolt systems, but often make an audible "click" noise when switching on or off.
More expensive models have a built-in PID controller, so that the thermostat knows ahead how the system will react to its commands. For instance, setting it up that temperature in the morning at 7 a.m. should be 21°C, makes sure that at that time the temperature will be 21°C, where a conventional thermostat would just start working at that time. The PID controller decides at what time the system should be activated in order
to reach the desired temperature at the desired time. It also makes sure that the temperature is very stable (for instance, by reducing overshoots[citation needed]).
Most digital thermostats in common residential use in North America and Europe are programmable thermostats, which will typically provide a 30% energy savings if left with their default programs; adjustments to these defaults may increase or reduce energy savings.[citation needed] The programmable thermostat article provides basic information on the operation, selection and installation of such a thermostat.
[edit] Household thermostat location
The thermostat should be located away from the room's cooling or heating vents or device, yet exposed to general airflow from the room(s) to be regulated. An open hallway may be most appropriate for a single zone system, where living rooms and bedrooms are operated as a single zone. If the hallway may be closed by doors from the regulated spaces then these should be left open when the system is in use. If the thermostat is too close to the source controlled then the system will tend to "short cycle", and numerous starts and stops can be annoying and in some cases shorten equipment life. A multiple zoned system can save considerable energy by regulating individual spaces, allowing unused rooms to vary in temperature by turning off the heating and cooling.
[edit] Dummy thermostats
It has been reported that many thermostats in office buildings are non-functional dummy devices, installed to give tenants' employees an illusion of control.[4][5] These dummy thermostats are in effect a type of placebo button.
[edit] See also
On-off control Automatic control OpenTherm Programmable thermostat Sensor
A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor for measurement and control[1] and can also be used to convert heat into electric power. They are inexpensive[2] and interchangeable, are supplied fitted with standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy: system errors of less than one kelvin (K) can be difficult to achieve.[citation needed]
Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.
Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.
Contents
[hide] 1 Principle of operation 2 Practical use
o 2.1 Voltage–temperature relationship o 2.2 Cold junction compensation
3 Power production 4 Grades
o 4.1 Extension wire 5 Types
o 5.1 K o 5.2 E o 5.3 J o 5.4 N o 5.5 Platinum types B, R, and S o 5.6 T o 5.7 C o 5.8 M o 5.9 Chromel-gold/iron
6 Laws for thermocouples o 6.1 Law of homogeneous material o 6.2 Law of intermediate materials o 6.3 Law of successive or intermediate temperatures
7 Aging of thermocouples 8 Thermocouple comparison 9 Applications
o 9.1 Steel industry o 9.2 Heating appliance safety
o 9.3 Thermopile radiation sensors o 9.4 Manufacturing o 9.5 Radioisotope thermoelectric generators o 9.6 Process plants
10 See also 11 References
12 External links
[edit] Principle of operation
Main article: Seebeck effect
In 1821, the German–Estonian physicist Thomas Johann Seebeck discovered that when any conductor is subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect or Seebeck effect. Any attempt to measure this voltage necessarily involves connecting another conductor to the "hot" end. This additional conductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose the original. Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate different voltages, leaving a small difference in voltage available for measurement. That difference increases with temperature, and is between 1 and 70 microvolts per degree Celsius (µV/°C) for standard metal combinations.
[edit] Practical use
[edit] Voltage–temperature relationship
The nonlinear relationship between the temperature difference (ΔT) and the output voltage (mV) of a thermocouple can be approximated by a polynomial:
The coefficients an are given for n from 0 to between 5 and 13 depending upon the metals. In some cases better accuracy is obtained with additional non-polynomial terms[3]. A database of voltage as a function of temperature, and coefficients for computation of temperature from voltage and vice-versa for many types of thermocouple is available online[3].
In modern equipment the equation is usually implemented in a digital controller or stored in a look-up table;[4] older devices use analog circuits.
[edit] Cold junction compensation
Thermocouples measure the temperature difference between two points, not absolute temperature. To measure a single temperature one of the junctions—normally the cold junction—is maintained at a known reference temperature, and the other junction is at the temperature to be sensed.
Having a junction of known temperature, while useful for laboratory calibration, is not convenient for most measurement and control applications. Instead, they incorporate an artificial cold junction using a thermally sensitive device such as a thermistor or diode to measure the temperature of the input connections at the instrument, with special care being taken to minimize any temperature gradient between terminals. Hence, the voltage from a known cold junction can be simulated, and the appropriate correction applied. This is known as cold junction compensation.
Alternatively cold junction compensation can be performed by computation using look-up tables [4] and polynomial interpolation.
[edit] Power production
A thermocouple can produce current, which means it can be used to drive some processes directly, without the need for extra circuitry and power sources. For example, the power from a thermocouple can activate a valve when a temperature difference arises. The electrical energy generated by a thermocouple is converted from the heat energy which must be supplied to the hot side to maintain the electric potential. A continuous flow of heat is necessary because the current flowing through the thermocouple tends to cause the hot side to cool down and the cold side to heat up (the Peltier effect).
Thermocouples can be connected in series to form a thermopile, where all the hot junctions are exposed to a higher and all the cold junctions to a lower temperature. The output is the sum of the voltages across the individual junctions, giving larger voltage and power output. Using the radioactive decay of transuranic elements as a heat source, this arrangement has been used to power spacecraft on missions too far from the Sun to utilize solar power.
[edit] Grades
Thermocouple wire is available in several different metallurgical formulations per type, typically, in decreasing levels of accuracy and cost: special limits of error, standard, and extension grades.
Extension grade wires made of the same metals as a higher-grade thermocouple are used to connect it to a measuring instrument some distance away without introducing additional junctions between dissimilar materials which would generate unwanted voltages; the connections to the extension wires, being of like metals, do not generate a voltage. In the case of platinum thermocouples, extension wire is a copper alloy, since it would be prohibitively expensive to use platinum for extension wires. The extension wire is specified to have a very similar thermal coefficient of EMF to the thermocouple, but only over a narrow range of temperatures; this reduces the cost significantly.
The temperature-measuring instrument must have high input impedance to prevent any significant current draw from the thermocouple, to prevent a resistive voltage drop across the wire. Changes in metallurgy along the length of the thermocouple (such as termination strips or changes in thermocouple type wire) will introduce another thermocouple junction which affects measurement accuracy.
[edit] Types
Certain combinations of alloys have become popular as industry standards. Selection of the combination is driven by cost, availability, convenience, melting point, chemical properties, stability, and output. Different types are best suited for different applications. They are usually selected based on the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria include the inertness of the thermocouple material, and whether it is magnetic or not. Standard thermocouple types are listed below with the positive electrode first, followed by the negative electrode.
[edit] K
Type K (chromel–alumel) is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/°C, chromel positive relative to alumel.[5] It is inexpensive, and a wide variety of probes are available in its −200 °C to +1350 °C range. Type K was specified at a time when metallurgy was less advanced than it is today, and consequently characteristics vary considerably between samples. One of the constituent metals, nickel, is magnetic; a characteristic of thermocouples made with magnetic material is that they undergo a step change in output when the magnetic material reaches its Curie point (around 354 °C for type K thermocouples).
[edit] E
Type E (chromel–constantan)[4] has a high output (68 µV/°C) which makes it well suited to cryogenic use. Additionally, it is non-magnetic.
Type J (iron–constantan) has a more restricted range than type K (−40 to +750 °C), but higher sensitivity of about 55 µV/°C.[2] The Curie point of the iron (770 °C) causes an abrupt change in the characteristic, which determines the upper temperature limit.
[edit] N
Type N (Nicrosil–Nisil) (Nickel-Chromium-Silicon/Nickel-Silicon) thermocouples are suitable for use at high temperatures, exceeding 1200 °C, due to their stability and ability to resist high temperature oxidation. Sensitivity is about 39 µV/°C at 900 °C, slightly lower than type K. Designed to be an improved type K, it is becoming more popular.
[edit] Platinum types B, R, and S
Types B, R, and S thermocouples use platinum or a platinum–rhodium alloy for each conductor. These are among the most stable thermocouples, but have lower sensitivity than other types, approximately 10 µV/°C. Type B, R, and S thermocouples are usually used only for high temperature measurements due to their high cost and low sensitivity.
B
Type B thermocouples use a platinum–rhodium alloy for each conductor. One conductor contains 30% rhodium while the other conductor contains 6% rhodium. These thermocouples are suited for use at up to 1800 °C. Type B thermocouples produce the same output at 0 °C and 42 °C, limiting their use below about 50 °C.
R
Type R thermocouples use a platinum–rhodium alloy containing 13% rhodium for one conductor and pure platinum for the other conductor. Type R thermocouples are used up to 1600 °C.
S
Type S thermocouples are constructed using one wire of 90% Platinum and 10% Rhodium (the positive or "+" wire) and a second wire of 100% platinum (the negative or "-" wire). Like type R, type S thermocouples are used up to 1600 °C. In particular, type S is used as the standard of calibration for the melting point of gold (1064.43 °C).
[edit] T
Type T (copper–constantan) thermocouples are suited for measurements in the −200 to 350 °C range. Often used as a differential measurement since only copper wire touches the probes. Since both conductors are non-magnetic, there is no Curie point and thus no abrupt change in characteristics. Type T thermocouples have a sensitivity of about 43 µV/°C.
Type C (tungsten 5% rhenium – tungsten 26% rhenium) thermocouples are suited for measurements in the 0 °C to 2320 °C range. This thermocouple is well-suited for vacuum furnaces at extremely high temperatures. It must never be used in the presence of oxygen at temperatures above 260 °C.
[edit] M
Type M thermocouples use a nickel alloy for each wire. The positive wire contains 18% molybdenum while the negative wire contains 0.8% cobalt. These thermocouples are used in vacuum furnaces for the same reasons as with type C. Upper temperature is limited to 1400 °C. It is less commonly used than other types.
[edit] Chromel-gold/iron
In chromel-gold/iron thermocouples, the positive wire is chromel and the negative wire is gold with a small fraction (0.03–0.15 atom percent) of iron. It can be used for cryogenic applications (1.2–300 K and even up to 600 K). Both the sensitivity and the temperature range depends on the iron concentration. The sensitivity is typically around 15 µV/K at low temperatures and the lowest usable temperature varies between 1.2 and 4.2 K.
[edit] Laws for thermocouples
[edit] Law of homogeneous material
A thermoelectric current cannot be sustained in a circuit of a single homogeneous material by the application of heat alone, regardless of how it might vary in cross section. In other words, temperature changes in the wiring between the input and output do not affect the output voltage, provided all wires are made of the same materials as the thermocouple.
[edit] Law of intermediate materials
The algebraic sum of the thermoelectric forces in a circuit composed of any number of dissimilar materials is zero if all of the junctions are at a uniform temperature. So If a third metal is inserted in either wire and if the two new junctions are at the same temperature, there will be no net voltage generated by the new metal.
[edit] Law of successive or intermediate temperatures
If two dissimilar homogeneous materials produce thermal emf1 when the junctions are at T1 and T2 and produce thermal emf2 when the junctions are at T2 and T3 , the emf generated when the junctions are at T1 and T3 will be emf1 + emf2 .
Thermoelements are often used at high temperatures and in reactive furnace atmospheres. In this case the practical lifetime is determined by aging. The thermoelectric coefficients of the wires in the area of high temperature change with time and the measurement voltage drops. The simple relationship between the temperature difference of the joints and the measurement voltage is only correct if each wire is homogeneous. With an aged thermocouple this is not the case. Relevant for the generation of the measurement voltage are the properties of the metals at a temperature gradient. If an aged thermocouple is pulled partly out of the furnace, the aged parts from the region previously at high temperature enter the area of temperature gradient and the measurement error is significantly increased. However an aged thermocouple that is pushed deeper into the furnace gives a more accurate reading.
[edit] Thermocouple comparison
The table below describes properties of several different thermocouple types. Within the tolerance columns, T represents the temperature of the hot junction, in degrees Celsius. For example, a thermocouple with a tolerance of ±0.0025×T would have a tolerance of ±2.5 °C at 1000 °C.
TypeTemperature range °C
(continuous)
Temperature range °C
(short term)
Tolerance class
one (°C)
Tolerance class two
(°C)
IEC Color code
BS Color code
ANSI Color code
K 0 to +1100−180 to +1300
±1.5 between −40 °C and 375 °C±0.004×T between 375 °C and 1000 °C
±2.5 between −40 °C and 333 °C±0.0075×T between 333 °C and 1200 °C
J 0 to +700 −180 to +800
±1.5 between −40 °C and 375 °C±0.004×T between 375 °C and 750 °C
±2.5 between −40 °C and 333 °C±0.0075×T between 333 °C and 750 °C
±1.5 between −40 °C and 375 °C±0.004×T between 375 °C and 800 °C
±2.5 between −40 °C and 333 °C±0.0075×T between 333 °C and 900 °C
Chromel/AuFe
−272 to +300 n/a
Reproducibility 0.2% of the voltage; each sensor needs individual calibration.
[edit] Applications
Thermocouples are suitable for measuring over a large temperature range, up to 2300 °C. They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such applications thermistors and resistance temperature detectors are more suitable. Applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.
[edit] Steel industry
Type B, S, R and K thermocouples are used extensively in the steel and iron industries to monitor temperatures and chemistry throughout the steel making process. Disposable, immersible, type S thermocouples are regularly used in the electric arc furnace process to accurately measure the temperature of steel before tapping. The cooling curve of a small steel sample can be analyzed and used to estimate the carbon content of molten steel.
[edit] Heating appliance safety
Many gas-fed heating appliances such as ovens and water heaters make use of a pilot flame to ignite the main gas burner when required. If it goes out gas may be released, which is a fire risk and a health hazard. To prevent this some appliances use a thermocouple in a fail-safe circuit to sense when the pilot light is burning. The tip of the thermocouple is placed in the pilot flame, generating a voltage which operates the supply valve which feeds gas to the pilot. So long as the pilot flame remains lit, the thermocouple remains hot, and the pilot gas valve is held open. If the pilot light goes out, the thermocouple temperature falls, causing the voltage across the thermocouple to drop and the valve to close.
Some systems, known as millivolt control systems, extend this concept to the main gas valve as well. Not only does the voltage created by the pilot thermocouple activate the pilot gas valve, it is also routed through a thermostat to power the main gas valve as well. Here, a larger voltage is needed than in a pilot flame safety system described above, and a thermopile is used rather than a single thermocouple. Such a system requires no external source of electricity for its operation and so can operate during a power failure, provided all the related system components allow for this. Note that this excludes common forced air furnaces because external power is required to operate the blower motor, but this feature is especially useful for un-powered convection heaters.
A similar gas shut-off safety mechanism using a thermocouple is sometimes employed to ensure that the main burner ignites within a certain time period, shutting off the main burner gas supply valve should that not happen.
Out of concern for energy wasted by the standing pilot, designers of many newer appliances have switched to an electronically controlled pilot-less ignition, also called intermittent ignition. With no standing pilot flame, there is no risk of gas buildup should the flame go out, so these appliances do not need thermocouple-based safety pilot safety switches. As these designs lose the benefit of operation without a continuous source of electricity, standing pilots are still used in some appliances. The exception is later model instantaneous water heaters that utilise the flow of water to generate the current required to ignite the gas burner, in conjunction with a thermocouple as a safety cut-off device in the event the gas fails to ignite, or the flame is extinguished.
[edit] Thermopile radiation sensors
See also: bolometer
Thermopiles are used for measuring the intensity of incident radiation, typically visible or infrared light, which heats the hot junctions, while the cold junctions are on a heat sink. It is possible to measure radiative intensities of only a few μW/cm2 with commercially available thermopile sensors. For example, some laser power meters are based on such sensors.
[edit] Manufacturing
Thermocouples can generally be used in the testing of prototype electrical and mechanical apparatus. For example, switchgear under test for its current carrying capacity may have thermocouples installed and monitored during a heat run test, to confirm that the temperature rise at rated current does not exceed designed limits.
[edit] Radioisotope thermoelectric generators
Thermopiles can also be applied to generate electricity in radioisotope thermoelectric generators.
Chemical production and petroleum refineries will usually employ computers for logging and limit testing the many temperatures associated with a process, typically numbering in the hundreds. For such cases a number of thermocouple leads will be brought to a common reference block (a large block of copper) containing the second thermocouple of each circuit. The temperature of the block is in turn measured by a thermistor. Simple computations are used to determine the temperature at each measured location.
[edit] See also
Bolometer Giuseppe Domenico Botto Resistance thermometer Thermistor List of sensors International Temperature Scale of 1990
[edit] References
1. ̂ "Thermocouple temperature sensors". Temperatures.com. http://www.temperatures.com/tcs.html. Retrieved 2007-11-04.
2. ^ a b Ramsden, Ed (September 1, 2000). "Temperature measurement". Sensors. http://www.sensorsmag.com/sensors/temperature/temperature-measurement-1030. Retrieved 2010-02-19.
3. ^ a b c "NIST ITS-90 Thermocouple Database". http://srdata.nist.gov/its90/main/.4. ^ a b c Baker, Bonnie C. (September 1, 2000). "Designing the embedded
temperature circuit to meet the system's requirements". Sensors. http://www.sensorsmag.com/sensors/Technologies+In+Depth%2FSensors%2FTemperature/Designing-the-Embedded-Temperature-Circuit-to-Meet/ArticleStandard/Article/detail/361649. Retrieved 2007-11-04.
5. ̂ Manual on the Use of Thermocouples in Temperature Measurements. ASTM, 1974
[edit] External links
Thermocouple Operating Principle - University Of Cambridge Thermocouple Drift - University Of Cambridge Fundamental of thermocouples(pdf) Thermocouple design guide Mineral-Insulated Thermocouple Know-How Thermocouple Color Code Chart and Specifications Thermocouple Attachment - A Primer Thermocouple design
A thermistor is a type of resistor whose resistance varies with temperature. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.
Thermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range [usually −90 °C to 130 °C].
Contents
[hide] 1 Basic operation 2 Steinhart-Hart equation 3 B parameter equation 4 Conduction model 5 Self-heating effects 6 Applications 7 History 8 References 9 See also
10 External links
[edit] Basic operation
Thermistor symbol
Assuming, as a first-order approximation, that the relationship between resistance and temperature is linear, then:
ΔR = change in resistanceΔT = change in temperaturek = first-order temperature coefficient of resistance
Thermistors can be classified into two types, depending on the sign of k. If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, or posistor. If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. Resistors that are not thermistors are designed to have a k as close to zero as possible, so that their resistance remains nearly constant over a wide temperature range.
Instead of the temperature coefficient k, sometimes the temperature coefficient of resistance α (alpha) or αT is used. It is defined as[1]
For example, for the common PT100 sensor, α = 0.00385 or 0.385 %/°C. This αT coefficient should not be confused with the α parameter below.
[edit] Steinhart-Hart equation
In practice, the linear approximation (above) works only over a small temperature range. For accurate temperature measurements, the resistance/temperature curve of the device must be described in more detail. The Steinhart-Hart equation is a widely used third-order approximation:
where a, b and c are called the Steinhart-Hart parameters, and must be specified for each device. T is the temperature in kelvins and R is the resistance in ohms. To give resistance as a function of temperature, the above can be rearranged into:
The error in the Steinhart-Hart equation is generally less than 0.02 °C in the measurement of temperature[citation needed]. As an example, typical values for a thermistor with a resistance of 3000 Ω at room temperature (25 °C = 298.15 K) are:
[edit] B parameter equation
NTC thermistors can also be characterised with the B parameter equation, which is essentially the Steinhart Hart equation with c = 0 and B = 1/b.
where the temperatures are in kelvins and R0 is the resistance at temperature T0 (usually 25 °C = 298.15 K). Solving for R yields:
or, alternatively,
where . This can be solved for the temperature:
The B-parameter equation can also be written as . This can be used to convert the function of resistance vs. temperature of a thermistor into a linear function of lnR vs. 1 / T. The average slope of this function will then yield an estimate of the value of the B parameter.
[edit] Conduction model
Many NTC thermistors are made from a pressed disc or cast chip of a semiconductor such as a sintered metal oxide. They work because raising the temperature of a semiconductor increases the number of electrons able to move about and carry charge - it promotes them into the conduction band. The more charge carriers that are available, the more current a material can conduct. This is described in the formula:
I = electric current (amperes)n = density of charge carriers (count/m³)A = cross-sectional area of the material (m²)v = velocity of charge carriers (m/s)
e = charge of an electron ( coulomb)
The current is measured using an ammeter. Over large changes in temperature, calibration is necessary. Over small changes in temperature, if the right semiconductor is used, the resistance of the material is linearly proportional to the temperature. There are many different semiconducting thermistors with a range from about 0.01 kelvin to 2,000 kelvins (−273.14 °C to 1,700 °C).
Most PTC thermistors are of the "switching" type, which means that their resistance rises suddenly at a certain critical temperature. The devices are made of a doped polycrystalline ceramic containing barium titanate (BaTiO3) and other compounds. The dielectric constant of this ferroelectric material varies with temperature. Below the Curie point temperature, the high dielectric constant prevents the formation of potential barriers between the crystal grains, leading to a low resistance. In this region the device has a small negative temperature coefficient. At the Curie point temperature, the dielectric constant drops sufficiently to allow the formation of potential barriers at the grain boundaries, and the resistance increases sharply. At even higher temperatures, the material reverts to NTC behaviour. The equations used for modeling this behaviour were derived by W. Heywang and G. H. Jonker in the 1960s.
Another type of PTC thermistor is the polymer PTC, which is sold under brand names such as "Polyswitch" "Semifuse", and "Multifuse". This consists of a slice of plastic with carbon grains embedded in it. When the plastic is cool, the carbon grains are all in contact with each other, forming a conductive path through the device. When the plastic heats up, it expands, forcing the carbon grains apart, and causing the resistance of the device to rise rapidly. Like the BaTiO3 thermistor, this device has a highly nonlinear resistance/temperature response and is used for switching, not for proportional temperature measurement.
Yet another type of thermistor is a silistor, a thermally sensitive silicon resistor. Silistors are similarly constructed and operate on the same principles as other thermistors, but employ silicon as the semiconductive component material.
[edit] Self-heating effects
When a current flows through a thermistor, it will generate heat which will raise the temperature of the thermistor above that of its environment. If the thermistor is being used to measure the temperature of the environment, this electrical heating may introduce a significant error if a correction is not made. Alternatively, this effect itself can be
exploited. It can, for example, make a sensitive air-flow device employed in a sailplane rate-of-climb instrument, the electronic variometer, or serve as a timer for a relay as was formerly done in telephone exchanges.
The electrical power input to the thermistor is just:
where I is current and V is the voltage drop across the thermistor. This power is converted to heat, and this heat energy is transferred to the surrounding environment. The rate of transfer is well described by Newton's law of cooling:
where T(R) is the temperature of the thermistor as a function of its resistance R, T0 is the temperature of the surroundings, and K is the dissipation constant, usually expressed in units of milliwatts per degree Celsius. At equilibrium, the two rates must be equal.
The current and voltage across the thermistor will depend on the particular circuit configuration. As a simple example, if the voltage across the thermistor is held fixed, then by Ohm's Law we have I = V / R and the equilibrium equation can be solved for the ambient temperature as a function of the measured resistance of the thermistor:
The dissipation constant is a measure of the thermal connection of the thermistor to its surroundings. It is generally given for the thermistor in still air, and in well-stirred oil. Typical values for a small glass bead thermistor are 1.5 mW/°C in still air and 6.0 mW/°C in stirred oil. If the temperature of the environment is known beforehand, then a thermistor may be used to measure the value of the dissipation constant. For example, the thermistor may be used as a flow rate sensor, since the dissipation constant increases with the rate of flow of a fluid past the thermistor.
[edit] Applications
NTC thermistors are used as resistance thermometers in low-temperature measurements of the order of 10 K.
NTC thermistors can be used as inrush-current limiting devices in power supply circuits. They present a higher resistance initially which prevents large currents from flowing at turn-on, and then heat up and become much lower resistance to allow higher current flow during normal operation. These thermistors are usually
much larger than measuring type thermistors, and are purposely designed for this application.
NTC thermistors are regularly used in automotive applications. For example, they monitor things like coolant temperature and/or oil temperature inside the engine and provide data to the ECU and, indirectly, to the dashboard.
Thermistors are also commonly used in modern digital thermostats and to monitor the temperature of battery packs while charging.
[edit] History
The first NTC thermistor was discovered in 1833 by Michael Faraday, who reported on the semiconducting behavior of silver sulfide. Faraday noticed that the resistance of silver sulphide decreased dramatically as temperature increased. Because early thermistors were difficult to produce and applications for the technology were limited, commercial production of thermistors did not begin until the 1930s.[2]
[edit] References
1. ̂ Thermistor Terminology. U.S. Sensor2. ̂ McGee, Thomas (1988). "9". Principles and Methods of Temperature
Measurement.
[edit] See also
Iron-hydrogen resistor Thermocouple
[edit] External links
Wikimedia Commons has media related to: Thermistors
Thermistor Sensor for Catheter NTC Thermistor Basic Properties Practical Temperature Measurements - Agilent Application Note Temperature Measurement Tools: Thermistors, Thermocouples Thermistor App Note Thermistor ADC using Parallel Port The thermistor at bucknell.edu Measurement Specialties thermistor technical data sheets Thermistor calculation software at sourceforge.net Thermistor Samples and Application help
The temperature coefficient is the relative change of a physical property when the temperature is changed by 1 K.
In the following formula, let R be the physical property to be measured and T be the temperature at which the property is measured. T0 is the reference temperature, and ΔT is the difference between T and T0. Finally, α is the (linear) temperature coefficient. Given these definitions, the physical property is:
Here α has the dimensions of an inverse temperature (1/K or K−1).
This equation is linear with respect to temperature. For quantities that vary polynomially or logarithmically with temperature, it may be possible to calculate a temperature coefficient that is a useful approximation for a certain range of temperatures. For quantities that vary exponentially with temperature, such as the rate of a chemical reaction, any temperature coefficient would be valid only over a very small temperature range.
Different temperature coefficients are specified for various applications, including nuclear, electrical and magnetic.
Contents
[hide] 1 Negative temperature coefficient 2 Reversible temperature coefficient 3 Temperature coefficient of electrical resistance
o 3.1 Positive temperature coefficient of resistance 4 Coefficient of thermal expansion 5 Temperature coefficient of elasticity 6 Temperature coefficient of reactivity 7 References
8 Bibliography
[edit] Negative temperature coefficient
A negative temperature coefficient (NTC) occurs when the thermal conductivity of a material rises with increasing temperature, typically in a defined temperature range. For most materials, the thermal conductivity will decrease with increasing temperature.
Materials with a negative temperature coefficient have been used in floor heating since 1971. The negative temperature coefficient avoids excessive local heating beneath carpets, bean bag chairs, mattresses etc., which can damage wooden floors, and may infrequently cause fires.
Most ceramics exhibit NTC behaviour, which is governed by an Arrhenius equation over a wide range of temperatures:
where R is resistance, A and B are constants, and T is absolute temperature (K). The constant B is related to the energies required to form and move the charge carriers responsible for electrical conduction – hence, as the value of B decreases, the material becomes insulating. Practical and commercial NTC resistors aim to combine modest resistance with a value of B that provides good sensitivity to temperature. Such is the importance of the B constant value, that it is possible to characterize NTC thermistors using the B parameter equation:
where R0 is resistance at temperature T0. Therefore, many materials that produce acceptable values of R0 include materials that have been alloyed or possess variable cation valence states and thus contain a high natural defect center concentration. The value of B strongly depends on the energy required to dissociate the charge carriers that are used for the electrical conduction from these defect centers.
[edit] Reversible temperature coefficient
Residual magnetic flux density or Br changes with temperature and it is one of the important characteristics of magnet performance. Some applications, such as interial gyroscopes and traveling-wave tubes (TWTs), need to have constant field over a wide temperature range. The reversible temperature coefficient (RTC) of Br is defined as:
To address these requirements, temperature compensated magnets were developed in the late 1970s.[1] For conventional SmCo magnets, Br decreases as temperature increases. Conversely, for GdCo magnets, Br increases as temperature increases within certain temperature ranges. By combining samarium and gadolinium in the alloy, the temperature coefficient can be reduced to nearly zero.
[edit] Temperature coefficient of electrical resistance
See also: Table of materials' resistivities
The temperature dependence of electrical resistance and thus of electronic devices (wires, resistors) has to be taken into account when constructing devices and circuits. The
temperature dependence of conductors is to a great degree linear and can be described by the approximation below.
where
ρ0 just corresponds to the specific resistance temperature coefficient at a specified reference value (normally T = 0 °C)[2]
That of a semiconductor is however exponential:
where S is defined as the cross sectional area and α and b are coefficients determining the shape of the function and the value of resistivity at a given temperature.
For both, α is referred to as the resistance temperature coefficient.[3]
This property is used in devices such as thermistors.
[edit] Positive temperature coefficient of resistance
A positive temperature coefficient (PTC) refers to materials that experience an increase in electrical resistance when their temperature is raised. Materials which have useful engineering applications usually show a relatively rapid increase with temperature, i.e. a higher coefficient. The higher the coefficient, the greater an increase in electrical resistance for a given temperature increase.
[edit] Coefficient of thermal expansion
Main article: Coefficient of thermal expansion
The physical dimensions of matter can be affected by temperature. The coefficient of thermal expansion for a given sample of matter can be used to approximate its change in volume given a change in temperature. A similar coefficient, the linear thermal expansion coefficient, is also often used to measure the change of length of an object in one-dimension.
The coefficient of thermal expansion is often used to develop thermometers. Here lengths of materials can express temperature. The coefficient is also used for several types of thermostats.
[edit] Temperature coefficient of elasticity
The elastic modulus of elastic materials varies with temperature, typically decreasing with higher temperature.
[edit] Temperature coefficient of reactivity
In nuclear engineering, the temperature coefficient of reactivity is a measure of the change in reactivity (resulting in a change in power), brought about by a change in temperature of the reactor components or the reactor coolant. This may be defined as
Where ρ is reactivity and T is temperature. The relationship shows that αT is the value of the partial differential of reactivity with respect to temperature and is referred to as the "temperature coefficient of reactivity". As a result, the temperature feedback provided by αT has an intuitive application to passive nuclear safety. A negative αT is broadly cited as important for reactor safety, but wide temperature variations across real reactors (As opposed to a theoretical homogeneous reactor) limit the usability of a single metric as a marker of reactor safety.[4]
In water moderated nuclear reactors, the bulk of reactivity changes with respect to temperature are brought about by changes in the temperature of the water. However each element of the core has a specific temperature coefficient of reactivity (e.g. the fuel or cladding). The mechanisms which drive fuel temperature coefficients of reactivity are different than water temperature coefficients. While water expands as temperature increases, causing longer neutron travel times during moderation, fuel material will not expand appreciably. Changes in reactivity in fuel due to temperature stem from a phenomenon known as doppler broadening, where resonance absorption of fast neutrons in fuel filler material prevents those neutrons from thermalizing (slowing down).[5]
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. The reverse conversion of electrical energy into mechanical energy is done by a motor; motors and generators have many similarities. A generator forces electrons in the windings to flow through the external electrical circuit. It is somewhat analogous to a water pump, which creates a flow of water but does not create the water inside. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, compressed air or any other source of mechanical energy.
A gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. (Gas turbine may also refer to just the turbine element.)
Energy is added to the gas stream in the combustor, where air is mixed with fuel and ignited. Combustion increases the temperature, velocity and volume of the gas flow. This is directed through a nozzle over the turbine's blades, spinning the turbine and powering the compressor.
Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power aircraft, trains, ships, generators, and even tanks.
Resistance thermometers, also called resistance temperature detectors or resistive thermal devices (RTDs), are temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature. As they are almost invariably made of platinum, they are often called platinum resistance thermometers (PRTs). They are slowly replacing the use of thermocouples in many industrial applications below 600 °C, due to higher accuracy and repeatability.[1]
In physics, velocity is the rate of change of position. It is a vector physical quantity; both speed and direction are required to define it. In the SI (metric) system, it is measured in meters per second: (m/s) or ms−1. The scalar absolute value (magnitude) of velocity is speed. For example, "5 meters per second" is a scalar and not a vector, whereas "5 meters
per second east" is a vector. The average velocity v of an object moving through a
displacement during a time interval (Δt) is described by the formula:
The rate of change of velocity is acceleration – how an object's speed or direction changes over time, and how it is changing at a particular point in time.
