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Temperature Measurements
PrinciplesMeasuring Devices
Applications
حيـــــم الر حمن الر الله بســـــم
Definition of Temperature• An expression for the kinetic energy of vibrating
atoms and molecules of matter.• Can be measured by various secondary
phenomena, e.g.,– change of volume or pressure, – electrical resistance, – electromagnetic force, – electron surface charge, or – emission of electromagnetic radiation.
BME 353 - Biomedical Measurements and Instrumentation
212 Oct. 2015
Direct and Indirect• Many engineering applications require direct
measurement of temperature. – Synthetic fuel research, – solar energy conversion and – new engine development are a few of these disciplines.– All industries place new emphasis on energy efficiency.
Hence, the fundamental measurement of temperature assumes new importance.
• Temperature also effects measurement of most physical variables and it must be measured for compensation purposes as well.
BME 353 - Biomedical Measurements and Instrumentation
312 Oct. 2015
Temperature Scale• Celsius, divide the difference between the freezing
and boiling points of water into 100° • Fahrenheit which divide the difference between
the freezing and boiling points of water into 180°• °C = (5 /9) (°F - 32), and °F = (9 /5) °C + 32. • The thermodynamic scale begins at absolute zero,
or 0 Kelvin, the point at which all atoms cease vibrating and no kinetic energy is dissipated.
• 0 K = –273.15° C = –459.67° F. – The official Kelvin scale does not carry a degree sign.
The units are expressed in “kelvins,” not degrees Kelvin.
BME 353 - Biomedical Measurements and Instrumentation
412 Oct. 2015
Reference Temperatures• No temperature divider or adder• We must rely upon temperatures established by
physical phenomena which are easily observed and consistent in nature.
• The International Temperature Scale (ITS) establishes seventeen fixed points and corresponding temperatures. Examples:– the triple-point (the temperature and pressure at which
solid, liquid, and gas phases of a given substance are all present simultaneously in varying amounts) of water = 0.01C,
– triple-point of hydrogen = -259.3467C, and – freezing point of silver = 961.78C.
BME 353 - Biomedical Measurements and Instrumentation
512 Oct. 2015
Heat Gain and Heat Loss
• Heat gain:– Environment– Metabolism– Hot food– Shivering
• Heat loss:– Convection– Conduction– Evaporation– IR radiation
BME 353 - Biomedical Measurements and Instrumentation
612 Oct. 2015
Temperature measuring devices• Temperature can be
measured via a diverse array of sensors. All of them infer temperature by sensing some change in a physical characteristic.
• In the chemical process industries, the most commonly used temperature sensors are thermocouples, resistive devices and infrared devices.
• thermocouples, • resistance temperature
devices– RTD’s and – Thermistors
• infrared radiators, • I.C. sensors, • bimetallic devices, • liquid expansion devices,• change-of-state devices.
BME 353 - Biomedical Measurements and Instrumentation
712 Oct. 2015
Thermocouples • Two strips or wires made
of different metals and joined at one end.
• Changes in temperature at that junction induce changes in the emf between the other ends.
• As temperature goes up, this output emf of the thermocouple rises, though not necessarily linearly.
Metal A Metal A
Metal B
Metal A
Metal B
+ VAB -
VAB = Seebeck voltage
VAB = T, where , the Seebeck coefficient, is the constant of proportionality. For real world thermocouples, is not constant but varies with temperature.
BME 353 - Biomedical Measurements and Instrumentation
812 Oct. 2015
Peltier effect
• If a voltage is applied, then there will be temperature change at the junction. This is called the Peltier effect and can be used for heating and cooling (refrigeration).
BME 353 - Biomedical Measurements and Instrumentation
912 Oct. 2015
Equation of a thermocouple• The output voltage “V” of a simple thermocouple (with a
reference temperature T0 = 0C = 32F) is:
32
31
21
CTBTATV
where T is the temperature of the measuring junction in C, A, B,and C are constants that depend upon the thermocouple material. The sensitivity
volts,
2CTBTATVS
volt/C
BME 353 - Biomedical Measurements and Instrumentation
1012 Oct. 2015
Characteristics of thermocouples
0
20
40
60
80
500 1000 1500 2000
E
J K
R S T
Temperature, C
Milli
volts
Type of Metals + - E Chromel vs Constantan J Iron vs Constantan K Chromel vs Alumel R Platinum vs Platinum
13% Rhodium S Platinum vs Platinum
10% Rhodium T Copper vs Constantan
Constantan is a metal alloy with %60 copper and %40 nickel
BME 353 - Biomedical Measurements and Instrumentation
1112 Oct. 2015
BME 353 - Biomedical Measurements and Instrumentation
1212 Oct. 2015
Resistance Temperature Devices• RTD’s R = R0[1 + (T – T0)]• platinum, nickel, or ni alloys
– fine platinum wire wrapped around a mandrel and covered with a protective coating (also abbreviated PRTD).
– most stable temp trans.• Film RTD
– a platinum or metal-glass slurry film is deposited or screened onto a small flat ceramic substrate, etched with a laser-trimming system, and sealed
– device size itself is small, which means it can respond quickly to step changes in temperature.
• Film RTD’s are less stable
• Thermistors – NTC– PTC
• most sensitive temperature transducer
Thermocouple
Thermistor
RTD
Temperature, C
V o
r T
BME 353 - Biomedical Measurements and Instrumentation
1312 Oct. 2015
Equation of a thermistor
Steinhart-Hart equation:
3)(ln)(ln1 RCRBAT
a simpler equation: CAR
T
)(ln1
)(
00
0
TTTT
eRR
BME 353 - Biomedical Measurements and Instrumentation
1412 Oct. 2015
The Self-Heating Problem
0.1
1.0
10
100
0.10 1.0 10.0 100.0
Current, mA
Vol
tage
, V
+ slope
0 slope
- slope
BME 353 - Biomedical Measurements and Instrumentation
1512 Oct. 2015
Integrated Circuit (I.C.) Sensors +
To DVM 1 M
1 A/K
To DVM 1 M
1 A/K To DVM
+
10 m
V/K
Current sensor Voltage sensor
BME 353 - Biomedical Measurements and Instrumentation
1612 Oct. 2015
Temperature
Vol
tage
V
T
Thermocouple
Temperature
Resis
tanc
e
R
T
RTD
Temperature T
Thermistor
Temperature Vol
tage
or c
urre
nt V or I
T
I.C. Sensor
Resis
tanc
e
R A
dvan
tage
s
Disa
dvan
tage
s
Self powered Simple Rugged Inexpensive Wide variety of
physical forms Wide temperature
range
Non-linear Low voltage Reference required Least stable Least sensitive
Most stable Most accurate More linear than thermocouple
Expensive Slow Current source required
Small resistance change
Four-wire measurement
High output Fast Two-wire ohmic measurement
Most linear Highest output Inexpensive
Non-linear Limited temperature range
Fragile Current source required
Self-heating
T < 250 C Power supply
required Self-heating Limited
configurations BME 353 - Biomedical
Measurements and Instrumentation1712 Oct. 2015
Bimetallic Devices
Metal A
Metal B
BME 353 - Biomedical Measurements and Instrumentation
1812 Oct. 2015
Fluid-Expansion Devices• Types:
– the mercury type: an environmental hazard, so there are regulations governing the shipment of devices that contain it.
– the organic-liquid type.– gas instead of liquid type
• No electric power, do not pose explosion hazards, and are stable even after repeated cycling.
• On the other hand, – they do not generate data that are easily
recorded or transmitted, and – they cannot make spot or point
measurements.
50
0
Safety bulb
Capillary tube
Stem
Temperature sensing bulb
BME 353 - Biomedical Measurements and Instrumentation
1912 Oct. 2015
Chemical (Change-of-State) Sensors
• Change-of-state temperature sensors – labels, pellets, crayons,lacquers or liquid crystals whose appearance
changes when a certain temperature is reached.– They are used, for instance, with steam traps – when a trap exceeds a
certain temperature, a white dot on a sensor label attached to the trap will turn black.
– Response time typically takes minutes, so these devices often do not respond to transient temperature changes, and accuracy is lower than other types of sensors.
– the change in state is irreversible, except in the case of liquid-crystal displays.
– Even so, change-of-state sensors can be handy when one needs confirmation that the temperature of a piece of equipment or a material has not exceeded a certain level, for instance for technical or legal reasons, during product shipment
BME 353 - Biomedical Measurements and Instrumentation
2012 Oct. 2015
Radiation Detectors (IR Sensors)
BME 353 - Biomedical Measurements and Instrumentation
2112 Oct. 2015
Spectral radiant emittance versus wavelength for a blackbody at 300 K on the left vertical axis; percentage of total energy on the right vertical axis.
5
0.001
0.002
0.0030.00312
10 15 20
T = 300 K
m= 9.66 m
25
20
40
60
80
100%
% T
otal
pow
er
Spec
tral r
adie
nt e
mitt
ance
, W-c
m-2·m
m-1
BME 353 - Biomedical Measurements and Instrumentation
2212 Oct. 2015
10
10
50
100
10
Fused silica
SapphireArsenic trisulfide
Thalliumbromideiodine
Wavelength, m
100
Spectral transmission for a number of optical materials.
BME 353 - Biomedical Measurements and Instrumentation
2312 Oct. 2015
1 2 3
Wavelength, m
Indium antimonide (InSb)(photovoltaic)
Lead sulfide (PbS)
All thermal detectors
0
20
60
100
4 5 6 7 8
Spectral sensitivity of photon and thermal detectors.
BME 353 - Biomedical Measurements and Instrumentation
2412 Oct. 2015
EarIR
ShutterAmbient sensor
SensorAmp.
MUX A/D
Shutterswitch
WindowWaveguide
Microprocessor
Digitaldisplay
TaTb
The infrared thermometer opens a shutter to expose the sensor to radiation from the tympanic membrane.
BME 353 - Biomedical Measurements and Instrumentation
2512 Oct. 2015
Details of the fiber/sensor arrangement for the GaAs semiconductor temperature probe.
BME 353 - Biomedical Measurements and Instrumentation
2612 Oct. 2015