Energy Auditing & Demand Side Management Lighting and Energy Instruments UNIT – V 5. LIGHTING AND ENERGY INSTRUMENTS 5.1. Good Lighting System Design and Practice Lighting is an essential service in all the industries. The power by the industrial lighting varies between 2 to 10% of the total power depending on the type of industry. In hotels, lighting consumes up to 30% of total electrical energy. Innovation and continuous improvement in the field of lighting has given rise to tremendous energy saving opportunities in this area. Lighting is an area, which provides a major scope to achieve energy efficiency at the design stage, by incorporating modern energy efficient lamps, luminaires and gears, apart from good operational practices. 5.1.1. Basic Terms in Lighting System and Features (A) Lamps Lamp is equipment, which produces light. The most commonly used lamps are described briefly as follows: Incandescent lamps Incandescent lamps produce light by means of a filament heated to incandescence by the flow of electric current through it. The principal parts of an incandescent lamp, also known as GLS (General Lighting Service) lamp include the filament, the bulb, the fill gas and the cap. Reflector lamps Reflector lamps are basically incandescent, provided with a high quality internal mirror, which follows exactly the parabolic shape of the lamp. The reflector is resistant to corrosion, thus making the lamp maintenance free and output efficient. Gas discharge lamps P.SURESH BABU, AITS, RAJAMPET 99
Transcript
Energy Auditing & Demand Side Management Lighting and Energy Instruments
UNIT – V5. LIGHTING AND ENERGY INSTRUMENTS
5.1. Good Lighting System Design and Practice
Lighting is an essential service in all the industries. The power by the industrial lighting varies
between 2 to 10% of the total power depending on the type of industry. In hotels, lighting consumes up
to 30% of total electrical energy. Innovation and continuous improvement in the field of lighting has
given rise to tremendous energy saving opportunities in this area.
Lighting is an area, which provides a major scope to achieve energy efficiency at the design stage, by
incorporating modern energy efficient lamps, luminaires and gears, apart from good operational
practices.
5.1.1. Basic Terms in Lighting System and Features
(A) Lamps
Lamp is equipment, which produces light. The most commonly used lamps are described briefly as
follows:
Incandescent lamps
Incandescent lamps produce light by means of a filament heated to incandescence by the flow of
electric current through it. The principal parts of an incandescent lamp, also known as GLS (General
Lighting Service) lamp include the filament, the bulb, the fill gas and the cap.
Reflector lamps
Reflector lamps are basically incandescent, provided with a high quality internal mirror, which follows
exactly the parabolic shape of the lamp. The reflector is resistant to corrosion, thus making the lamp
maintenance free and output efficient.
Gas discharge lamps
The light from a gas discharge lamp is produced by the excitation of gas contained in either a tubular or
elliptical outer bulb.
The most commonly used discharge lamps are as follows:
Fluorescent Tube Lamps (FTL)
Compact Fluorescent Lamps (CFL)
Mercury Vapor Lamps (HPMV)
Sodium Vapor Lamps (HPSV)
Metal Halide Lamps
(B) Luminaire
Luminaire is a device that distributes, filters or transforms the light emitted from one or more
lamps. The luminaire includes all the parts necessary for fixing and protecting the lamps, except the
lamps themselves. In some cases, luminaires also include the necessary circuit auxiliaries, together with
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the means for connecting them to the electric supply. The basic physical principles used in optical
luminaire are reflection, absorption, transmission and refraction.
(C) Control Gear
The gears used in the lighting equipment are as follows:
Ballast or Choke
A current limiting device, to counter negative resistance characteristics of any discharge lamps.
In case of fluorescent lamps, it aids the initial voltage build-up required for starting.
Igniters
These are used for starting high intensity Metal Halide and Sodium vapor lamps.
(D) Illuminance
This is the quotient of the luminous flux incident on an element of the surface at a point of
surface containing the point, by the area of that element. The lighting level produced by alighting
installation is usually qualified by the illuminance produced on a specified plane. In most cases, this
plane is the major plane of the tasks in the interior and is commonly called the working plane. The
illuminance provided by an installation affects both the performance of the tasks and the appearance of
the space.
(E) Lux (lx)
This is the illuminance produced by a luminous flux of one lux, uniformly distributed over a
surface area of one square meter. One lux is equal to one lumen per square meter.
(F) Luminous Efficacy (lm/W)
This is the ratio of luminous flux emitted by a lamp to the power consumed by the lamp. It is a
reflection of efficiency of energy conversion from electricity to light form.
(G) Color Rendering Index (RI)
Is a measure of the degree to which the colors of surfaces illuminated by a given light source
confirm to those of the same surfaces under a reference illuminant; suitable allowance having been
made for the state of Chromatic adaptation.
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5.1.2. Lamp Types and their features
Table shows the various types of lamp available along with their features.
Table: Luminous Performance Characteristics of Commonly Used Luminaries
Type of LampLumens/Watt
CRI Typical Application Typical Life (h)Range Averag
e
Incandescent 8--18 14 ExcellentHomes, restaurants, general lighting, emergency lighting
1000
Fluorescent Lamps 46-60 50Good w.r.t.
coatingOffices, shops, hospitals, homes 5000
Compact Fluorescent Lamps (CFL)
40-70 60 Very good Hotels, shops, homes, offices
8000-10000
High Pressure Mercury (HPMV) 44-57 50 Fair
General lighting in factories, garages, car parking, flood lighting
5000
Halogen lamps 18-24 20 ExcellentDisplay, flood lighting, stadium exhibition grounds, construction areas
2000-4000
High Pressure Sodium (HPSV) SON
67-121 90 FairGeneral lighting in factories, ware houses, street lighting
6000-12000
Low Pressure Sodium (LPSV) SOX
101-175 150 Poor Roadways,, tunnels,
canals, street lighting6000-12000
5.1.3. Methodology of Lighting System Energy Efficiency Study
A step-by-step approach for assessing energy efficiency of lighting system is given below:
Step-1: Inventorise the Lighting System elements & transformers in the facility as per following typical
format (Table given below).
Table: Device Rating, Population and Use Profile
S. No. Plant Location
Lighting Device & Ballast Type
Rating in Watts Lamp & Ballast
Population Numbers
No. of Hours/Day
Table: Lighting Transformer/Rating and Population Profile
S. No. Plant Location
Lighting Transformer Rating
(kVA)
Numbers Installed
Meter Provisions Available Volts/Amps/kW/Energy
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In case of distribution boards (instead of transformers) being available, fuse ratings may be inventoried
along the above pattern in place of transformer kVA.
Step-2: With the aid of a lux meter, measure and document the lux levels at various plant locations at
working level, as daytime lux and night time lux values alongside the number of lamps “ON” during
measurement.
Step-3: With the aid of portable load analyzer, measure and document the voltage, current, power factor
and power consumption at various input points, namely the distribution boards or the lighting voltage
transformers at the same as that of the lighting level audit.
Step-4: Compare the measured lux values with standard values as reference and identify locations as
under-llt and over-llt areas.
Step-5: Collect and analyze the failure rates of lamps, ballasts and the actual life expectancy levels from
the past data.
Step-6: Based on careful assessment and evaluation, bring out improvement options, which could
include:
1) Maximize sunlight use through use of transparent roof sheets, north light roof, etc.
2) Examine scope for replacements of lamps by more energy efficient lamps, with due
consideration to luminaire, color rendering index, lux level as well as expected life comparison.
3) Replace conventional magnetic ballasts by more energy efficient ballasts, with due consideration
to life and power factor apart from watt loss.
4) Select interior colors for light reflection.
5) Modify layout for optimum lighting.
6) Providing individual / group controls for lighting for energy efficiency such as:
a. On / off type voltage regulation type (for illuminance control)
b. Group control switches / units.
c. Occupancy sensors
d. Photocell controls
e. Timer operated controls
f. Pager operated controls
g. Computerized lighting control programs
7) Install input voltage regulators / controllers for energy efficiency as well as longer life
expectancy for lamps where higher voltages, fluctuations are expected.
8) Replace energy efficient displays like LED’s in place of lamp type displays in control panels /
instrumentation areas, etc.
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5.1.3. Some Good Practices in Lighting
Installation of energy efficient fluorescent lamps in place of “Conventional” fluorescent lamps
Energy efficient lamps are based on the highly sophisticated tri-phosphor fluorescent powder
technology. They offer excellent color rendering properties in addition to the very high luminous
efficacy.
Installation of Compact Fluorescent Lamps (CFLs) in place of Incandescent lamps
Compact fluorescent lamps are generally considered best for replacement of lower wattage
incandescent lamps. These lamps have efficacy ranging from 55 to 65 lumens/watt. The average rated
lamp life is 10,000 hours, which is 10 times longer than that of a normal incandescent lamps. CFLs are
highly suitable for places such as Living rooms, Hotel lounges, Bars, Restaurants, Pathways, Building
entrances, Corridors, etc.
Installation of metal halide lamps in place of mercury / sodium vapor lamps
Metal halide lamps provide high color rendering index when compared with mercury & sodium
vapor lamps. These lamps offer efficient white light. Hence, metal halide is the choice for color critical
applications where, higher illumination levels are required. These lamps are highly suitable for
applications such as assembly line, inspection areas, painting shops, etc. It is recommended to install
metal halide lamps where color rendering is critical.
Installation of High Pressure Sodium Vapor (HPSV) lamps for applications where color rendering is
not critical
High pressure sodium vapor (HPSV) lamps offer more efficacy. But the color rendering
property of HPSV is very low. Hence, it is recommended to install HPSV lamps for applications such
street lighting, yard lighting, etc.
Installation of LED panel indicator lamps in place of filament lamps
Panel indicator lamps are used widely in industries for monitoring, fault indication, signaling,
etc. Conventionally filament lamps are used for the purpose, which has got the following disadvantages:
High energy consumption (15 W/lamp)
Failure of lamps is high (Operation life less than 10,000 hours)
Very sensitive to the voltage fluctuations Recently, the conventional filament lamps
are being replaced with Light Emitting Diodes (LEXs).
The LEDs have the following merits over the filament lamps.
Lesser power consumption (Less than 1 W/lamp)
Withstand high voltage fluctuation in the power supply.
Longer operating life (more than 1,00,000 hours)
It is recommended to install LEDs for panel indicator lamps at the design stage.
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5.2. Light Distribution
Energy efficiency cannot be obtained by mere selection of more efficient lamps alone. Efficient
luminaires along with the lamp of high efficacy achieve the optimum efficiency. Mirror-optic
luminaires with a high output ratio and bat-wing light distribution can save energy.
For achieving better efficiency, luminaires that are having light distribution characteristics appropriate
for the task interior should be selected. The luminaires fitted with a lamp should ensure that discomfort
glare and veiling reflections are minimized. Installation of suitable luminaires depends upon the height
– Low, Medium & High Bay. Luminaires for high intensity discharge lamp are classified as follows:
Low bay, for heights less than 5 meters.
Medium bay, for heights between 5-7 meters.
High bay, for heights greater than 7 meters.
System layout and fixing of the luminaires play a major role in achieving energy efficiency. This
also varies from application to application. Hence, fixing the luminaires at optimum height and usage of
mirror optic luminaries leads to energy efficiency.
5.3. Light Control
The simplest and the most widely used form of controlling a lighting installation is “On-Off”
switch. The initial investment for this set up is extremely low, but the resulting operations costs may be
high. This does not provide the flexibility to control the lighting, where it is not required.
Hence, a flexible lighting system has to be provided, which will offer switch-off or reduction in
lighting level, when not needed. The following light control systems can be adopted at design stage:
Grouping of lighting system, to provide greater flexibility in lighting control
Grouping of lighting system, which can be controlled manually or by timer control.
Installation of microprocessor based controllers
Another modern method is usage of microprocessor / infrared controlled dimming or switching
circuits. The lighting control can be obtained by using logic units located in the ceiling, which can take
pre-programmed commands and activate specified lighting circuits. Advanced lighting control system
uses movement detectors or lighting sensors, to feed signals to the controllers.
Optimum usage of day lighting
Whenever the orientation of a building permits, day lighting can be used in combination with
electric lighting. This should not introduce glare or a severe imbalance of brightness in visual
environment. Usage of day lighting (in offices/air conditioned halls ) will have to be very limited,
because the air conditioning load will increase on account of the increased solar heat dissipation into the
area. In many cases, a switching method, to enable reduction of electric light in the window zones
during certain hours, has to be designed.
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Installation of “exclusive” transformer for lighting
In most of the industries, lighting load varies between 2 to 10%. Most of the problems faced by
the lighting equipment and the “gears” are due to the “voltage” fluctuations. Hence, the lighting
equipment has to be isolated from the power feeders. This provides a better voltage regulation for the
lighting. This will reduce the voltage related problems, which in turn increases the efficiency of the
lighting system.
Installation of servo stabilizer for lighting feeder
Wherever, installation of exclusive transformer for lighting is not economically attractive, servo
stabilizer can be installed for the lighting feeders. This will provide stabilized voltage for the lighting
equipment.
The performance of “gears” such as chokes, ballasts, will also improve due to the stabilized
voltage. This set up also provide, the option to optimize the voltage level fed to the lighting feeder. In
many plants, during the non-peaking hours, the voltage levels are on the higher side. During this period,
voltage can be optimized, without any significant drop in the illumination level.
Installation of high frequency (HF) electronic ballasts in place of conventional ballasts
New high frequency (28-32 kHz) electronic ballasts have the following advantages over the
traditional magnetic ballasts:
Energy savings up to 35%; less heat dissipation, which reduces the air conditioning load
Lights instantly
Improved Power Factor
Operates in low voltage load
Less in weight
Increases the life of lamp
The advantage of HF electronic ballasts, out weight the initial investment (higher costs when
compared with conventional ballast). In the past the failure rate of electronic ballast in Indian Industries
was high. Recently, many manufacturers have improved the design of the ballast leading to drastic
improvement in their reliability. The life of the electronic ballast is high especially when, used in a
lighting circuit fitted with a automatic voltage stabilizer. The Table below gives the type of luminaire,
gear and controls used in different areas of industry.
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Table: types of Luminaire with their Gear and Controls Used in Different Industrial Locations.
Location Source Luminaire Gear Controls
Plant HID/FTL
Industrial rall reflector: High bay Medium bay Low bay
Conventional/Low loss electronic ballast
Manual/electronic
Office FTL/CFL FTL/CFL Electronic/low loss Manual/auto
Yard HID/FTL Flood light Suitable Manual
Road Peripheral HID/PL Street light luminaire Suitable Manual
5.4. Electrical Energy Audit Instruments
The basis of any energy accounting system is the measurement of the usage of electricity in the
various items of plant. The system must include the basic measuring device plus the instrument for
direct or remote indication of the actual measurement. The sensor or measuring device converts some
physical property into a voltage or some other output which can be applied to a secondary device; for
example, a thermocouple converts a temperature to a voltage which can be measured by a voltmeter.
It is important to seek advice form instrument manufacturers or consultants on the selection of
measuring device, since purchase cost, reliability and accuracy are important features. Furthermore, it is
necessary to determine the measurements which can be most appropriately made for calculating each
energy flow required. Certain characteristics of the measuring device may be significant; for example,
physical size, resistance to corrosion, temperature changes, vibration.
Such a great variety of electrical instruments are available, only the most important meters form an
energy viewpoint will be considered.
Ammeter
Voltmeter
Wattmeter
Watt-hour meter
Maximum demand meter
Ammeter and Voltmeter
An ammeter measures current and a voltmeter measure voltage. However, the nature of the
property to be measured should be considered; is it steady with only slowly changing magnitude,
pulsating or intermittent. The properties which could be indicated by electrical meters may be defined:
a. Peak Value: The positive (negative) peak value of a signal is the maximum positive (negative)
value of that signal.
b. Average Value: The average of a periodic signal may be defined in several ways. The average
of the signal over a full cycle is the full cycle average. The average of the positive (negative)
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parts of the signal is the positive (negative) pulse average. The average of the instaneous
absolute values of the function is the absolute average.
c. Effective Value: The effective value of a periodic voltage (current) is defined as the magnitude
of a constant d.c. voltage (current) which dissipates the same average power in a given resistor
as is dissipated by the periodic voltage (current).
All meters respond to one of these three values and many meters have scales indicating correctly
the r.m.s. value of a sinusoidal even though they do not respond to true r.m.s. The sinusoidal wave is,
however, the shape most frequently encountered in practice. Consequently, it is necessary to use caution
in interpreting the indications of meters when measuring signals and the advice of instrument suppliers
and manufacturers is indispensable.
Wattmeter
This instrument measures the basic unit of electrical power, the watt. It is a function of current,
voltage and power factor and measures only the component of the current that is in phase with the
voltage, that is, the component flowing through the circuit resistance. Thus W represents the real work
done by the machine.
The current component that is out of phase with the voltage, VAR, magnetizes the circuit and
flows even when a motor is not driving any load. This is the idle or wattles current. The voltage
multiplied by the line current VA gives the apparent power. Thus,
VA = Vector sum of VAR and W
Power factor = W/VA
The wattmeter measures W.
Watt-hour meter
This instrument is time dependent and measures power multiplied by time. It measures total
electrical energy supplied and not just instantaneous power as depicted by the wattmeter.
Maximum demand meter
This is basically a wattmeter whose motor drives an indicating maximum demand mechanism in
addition to the normal Wh recorder. The maximum demand measuring periods are kept constant in
time, for example, 15 minute periods, and the energy consumed is integrated over this interval. The
system is used to indicate the highest average power consumed during a predetermined period.
Energy Monitors (Data Loggers)
Energy monitors are used to measure the single and three-phase power parameters such as current,
voltage, power factor, frequency, active power, apparent power, reactive power, and energy over a
period of time.
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5.5. Wattmeter
An instrument that measures electric power. See electric power measurement
A variety of wattmeters are available to measure the power in ac circuits. They are generally
classified by names descriptive of their operating principles. Determination of power in dc circuits is
almost always done by separate measurements of voltage and current. However, some of the
instruments described will also function in dc circuits, if desired.
Probably the most useful instrument in the measurement of ac power at commercial frequencies
is the indicating (deflecting) electro dynamic wattmeter. It is similar in principle to the double-coil dc
ammeter or voltmeter in that it depends on the interaction of the fields of two sets of coils, one fixed and
the other movable. The moving coil is suspended, or pivoted, so that it is free to rotate through a limited
angle about an axis perpendicular to that of the fixed coils. As a single-phase wattmeter, the moving
(potential) coil, usually constructed of fine wire, carries a current proportional to the voltage applied to
the measured circuit, and the fixed (current) coils carry the load current. This arrangement of coils is
due to the practical necessity of designing current coils of relatively heavy conductors to carry large
values of current. The potential coil can be lighter because the operating current is limited to low values.
See Ammeter, Voltmeter.
A thermal converter consists of a resistive heater in close thermal contact with one or more
thermocouples. When current flows through the heater, the temperature rises. Thermocouples give an
output voltage proportional to the square of the current, and so make suitable transducers for the
construction of thermal wattmeters. See thermal converters, thermocouple, Thermoelectricity.
The electrostatic force between two conductors is proportional to the product of the square of the
potential difference between them and the rate of change of capacitance with displacement. A
differential electrostatic instrument may therefore be used to construct a quarter-squares wattmeter. In
spite of the problems of matching the capacitance changes of the two elements and the small forces
available, electrostatic wattmeters were used as standards for many years.
Digital wattmeters combine the advantages of electronic signal processing and a high-resolution,
easily read display. Electrical readout of the measurement is also possible. A variety of electronic
techniques for carrying out the necessary multiplication of the signals representing the current and
voltage have been used. Usually the electronic multiplier is an analog system which gives as its output a
voltage proportional to the power indication required. This voltage is then converted into digital form in
one of the standard ways. Many of the multipliers were originally developed for use in analog
computers. See analog computer.
The instruments described are designed for single-phase power measurement. In polyphase
circuits, the total power is the algebraic sum of the power in each phase. This summation is assisted by
simple modifications of single-phase instruments. See alternating current
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5.6. Data logger
A data logger (also data logger or data recorder) is an electronic device that records data over
time or in relation to location either with a built in instrument or sensor or via external instruments and
sensors. Increasingly, but not entirely, they are based on a digital processor (or computer). They
generally are small, battery powered, portable, and equipped with a microprocessor, internal memory
for data storage and sensors. Some data loggers interface with a personal computer and utilize software
to activate the data logger and view and analyze the collected data, while other have a local interface
device (Keypad, LCD) and can be used as a stand-alone device.
Data loggers vary between general purpose types for a range of measurement applications to
very specific devices for measuring in one environment or application type only. It is common for
general purpose types to be programmable however many remain as static machines with only a limited
number or no changeable parameters. Electronic data loggers have replaced chart recorders in many
applications.
One of the primary benefits of using data logger s is the ability to automatically collect data on a
24-hour basis. Upon activation, data loggers are typically deployed and left unattended to measure and
record information for the duration of the monitoring period. This allows for a comprehensive, accurate
picture of the environmental conditions being monitored, such as air temperature and relative humidity.
The cost of data loggers has been declining over the years as technology improves and costs are
reduced. Simple single channel data loggers cost as little as $25. More complicated loggers may costs
hundreds or thousands of dollars.
Data logging versus Data Acquisition
The terms data logging and data acquisition are often used interchangeably. However, in a
historical convert they are quite different. A data logger is a data acquisition system, but a data
acquisition system is not necessarily a data logger.
Data loggers typically have slower sample rates. A maximum sample rate of 1 Hz may be
considered to be very fast for a data logger, yet very slow for a typical data acquisition system.
Data loggers are implicitly stand-alone devices, while typical data acquisition system must
remain tethered to a computer to acquire data. This stand-alone aspect of data loggers implies on-board
memory that is used to store acquired data. Sometimes this memory is very large to accommodate many
days, or even months, of unattended recording. This memory may be battery-backed static random
access memory, flash memory or EEPROM. Earlier data loggers used magnetic tape, punched paper
tape, or directly viewable records such as “strip chart recorders”.
Given the extended recording times of data loggers, they typically feature a time and date
stamping mechanism to ensure that each recorded data value is employ built-in real-time clocks whose
published drift can be an important consideration when choosing between data loggers.
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Data loggers range from simple single-channel input to complex multi-channel instruments.
Typically, the simpler the device the less programming flexibility. Some more sophisticated instruments
allow for cross-channel computations and alarms based on predetermined conditions. The newest of
data loggers can serve web pages, allowing numerous people to monitor a system remotely.
The unattended and remote nature of many data logger applications implies the need in some
applications to operate from a DC power source, such as a battery. Solar power may be used to
supplement these power sources. These constraints have generally led to ensure that the devices they
market are extremely power efficient relative to computers. In many cases they are required to operate
in harsh environmental conditions where computers will not function reliably.
This unattended nature also dictates that data loggers must be extremely reliable. since they may operate
for long periods nonstop with little or no human supervision, and may be installed in harsh or remote
locations, it is imperative that so long as they have power they will not fail to log data for any reason.
Manufacturers go to great length to ensure that the devices can be depended on in these applications. As
such data loggers are almost completely immune to the problems that might affect a general-purpose
computer in the same application, such as program crashes and the instability of some operating
systems.
Applications
Applications of data logging include:
Unattended weather station recording (such as wind speed / direction, temperature, relative
humidity, solar radiation).
Unattended hydrographic recording (such as water level, water depth, water flow, water pH,
water conductivity).
Unattended soil moisture level recording.
Unattended gas pressure recording.
Offshore buoys for recording a variety of environmental conditions.
Road traffic counting.
Measure temperatures (humidity, etc) of perishables during shipments: Cold chain.
Process monitoring for maintenance and troubleshooting applications.
Process monitoring to verify warranty conditions.
Wildlife research.
Measure vibration and handling shock (drop height) environment of distribution packaging.
Tank level monitoring.
Deformation monitoring of any object with geodetic or geotechnical sensors controlled by an
automatic deformation monitoring system.
Environmental monitoring.
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Vehicle Testing.
Monitoring of relay status in railway signaling.
For science education enabling ‘measurement’, ‘scientific investigation’ and an appreciation of
‘change’
Record trend data at regular intervals in veterinary vital signs monitoring.
Load profile recording for energy consumption management.
5.7. Temperature Measurement
To control the temperature in buildings and determine the heat content of process streams it is
necessary to measure temperatures accurately. There are several devices available, most of which can
be set up as indicating and/or recording instruments.
5.8. Thermocouples
The principle is that two dissimilar wires are fused at each end and when one junction is heated,
an e.m.f. is produced causing a current to flow round the loop. the e.m.f. generated, E, is given by the
following equation:
log E = A log t + B
where,
t = temperature and A and B are constants depending on the wires forming the junction.
1. A device for measuring temperature consisting of a pair of wires of different metals or
semiconductors joined at both ends. One junction is at the temperature to be measured, the
second at a fixed temperature. The electromotive force generated depends upon the temperature
difference.
2. A similar device with only one junction between two dissimilar metals or semiconductors
A device in which the temperature difference between the ends of a pair of dissimilar metal
wires is deduced from a measurement of the difference in the thermoelectric potentials developed along
the wires. The presence of a temperature gradient in a metal or alloy leads to an electric potential
gradient being set up along the temperature gradient. This thermoelectric potential gradient is
proportional to the temperature gradient and varies from metal to metal. It is the fact that the
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thermoelectric emf is different in different metals and alloys for the same temperature gradient that
allows the effect to be used for the measurement of temperature.
The basic circuit of a thermocouple is shown in the illustration. The thermocouple wires, made
of different metals or alloys A and B, are joined together at one end H, called the hot (or measuring)
junction, at a temperature T1. The other ends, CA and CB (the cold or reference junctions), are
maintained at a constant reference temperature T0, usually but not necessarily 320F (00C). From the cold
junctions, wires, usually of copper, lead to a voltmeter V at room temperature Tr. Due to the
thermoelectric potential gradients being different along the wires A and B, there exists a potential
difference between CA and CB. This can be measured by the voltmeter, provided that CA and CB are at
the same temperature and that the lead wires between CA and V and CB and V are identical (or that V
is at the temperature T0, which is unusual). Such a thermocouple will produce a thermoelectric emf
between CA and CB which depends only upon the temperature difference T1 - T0. See temperature
measurement, thermoelectricity.
Letter designations and compositions for standardized thermocouples
Type designation Materials
B Platinum-30% rhodium/platinum-6% rhodium
E Nickel-chromium alloy/a copper – nickel alloy
J Iron/another slightly different copper-nickel alloy
K Nickel-chromium alloy/nickel-aluminum alloy
R Platinum-13% rhodium/Platinum
S Platinum-10% rhodium/Platinum
T Copper/a copper-nickel alloy
After T.J. Quinn, Temperature, Academic Press, 1983.
A large number of pure metal and alloy combinations have been studied as thermocouples, and
the seven most widely used are listed in the table. The thermocouples in the table together cover the
temperature range from about – 4200F (-2500c or 20 K) to about 33000F (18000C). The most accurate
and reproducible are the platinum/rhodium thermocouples, types R and S, while the most widly used
industrial thermocouples are probably types K, T, and E.
5.9. Optical and radiation pyrometers
These instruments depend on the intensity of radiation emitted from a hot body and while optical
pyrometers depend on visual observation of the indicator radiation pyrometers use a receiver such as a
thermocouple. Pyrometers are often used for measuring very high temperatures above the range covered
by other instruments.
A temperature measuring device, originally an instrument that measures temperatures beyond
the range of thermometers, but now in addition a device that measures thermal radiation in any
P.SURESH BABU, AITS, RAJAMPET 112
Energy Auditing & Demand Side Management Lighting and Energy Instruments
temperature range. This article discusses radiation pyrometers; for other temperature-measuring devices
see bolometer, Thermistor, Thermocouple.
The illustration shows a very simple type of radiation pyrometer. Part of the thermal radiation
emitted by a hot object is intercepted by a lens and focused onto a thermopile. The resultant heating of
the thermopile causes it to generate an electrical signal (proportional to the thermal radiation) which can
be displayed on a recorder.
Unfortunately, the thermal radiation emitted by the object depends not only on its temperature
but also on its surface characteristics. The radiation existing inside hot, opaque objects is so-called
blackbody radiation, which is a unique function of temperature and wavelength and is the same for all
opaque materials. However, such radiation, when it attempts to escape from the object, is partly
reflected at the surface. In order to use the output of the pyrometer as a measure of target temperature,
the effect of the surface characteristics must be eliminated. A cavity can be formed in an opaque
material and the pyrometer sighted on a small opening extending from the cavity to the surface. The
opening has no surface reflection, since the surface has been eliminated. Such a source is called a
blackbody source, and is said to have an emittance of 1.00 By attaching thermocouples to the black-
body source, a curve of pyrometer output voltage versus blackbody temperature can be constructed.
Pyrometers can be classified generally into types requiring that the field of view be filled, such
as narrow-band and total-radiation pyrometers; and types not requiring that the field of view be filled,
such as optical and ratio pyrometers. The latter depend upon making some sort of comparison between
two or more signals.
The optical pyrometer should more strictly be called the disappearing-filament pyrometer. In
operation, an image of the target is focused in the plane of a wire that can be heated electrically. A
rheostat is used to adjust the current through the wire until the wire blends into the image of the target
(equal brightness condition), and the temperature is then read from a calibrated dial on the rheostat.
P.SURESH BABU, AITS, RAJAMPET 113
Energy Auditing & Demand Side Management Lighting and Energy Instruments
The ratio, or “two-color,” pyrometer makes measurement in tow wavelength regions and electronically
takes the ratio of these measurements. If the emittance is the same for both wavelengths, the emittance
cancels out of the result, and the true temperature of the target is obtained. This so-called gray-body
assumption is sufficiently valid in some cases so that the “color temperature” measured by a ratio
pyrometer is close to the true temperature. See Thermometer.
A general guide to the selection of thermometer type is shown in the following table.
Temperature range Measuring device
Range between 1 K and room temperature Iron-gold: Chromel thermocouples
Ambient temperature up to 3500 C Liquid expansion (mercury-in-steel)Electrical resistance thermometer
