Transcript Temperature Measurements

‫الرحيـــــِ ِم‬
َّ ‫الر ْح َم ِن‬
َّ ِ‫ـــــم هللا‬
ِ ‫ِب ْس‬
Temperature Measurements
Principles
Measuring Devices
Applications
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.
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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.
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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.
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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 =
0.01C,
– triple-point of hydrogen = -259.3467C, and
– freezing point of silver = 961.78C.
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Heat Gain and Heat Loss
• Heat gain:
–
–
–
–
Environment
Metabolism
Hot food
Shivering
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• Heat loss:
–
–
–
–
Convection
Conduction
Evaporation
IR radiation
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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.
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• thermocouples,
• resistance temperature
devices
– RTD’s and
– Thermistors
•
•
•
•
•
infrared radiators,
I.C. sensors,
bimetallic devices,
liquid expansion devices,
change-of-state devices.
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Thermocouples
Metal A
Metal A
Metal B
+
VAB
-
Metal A
Metal B
VAB = Seebeck voltage
• 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.
VAB = T, where , the Seebeck coefficient, is the constant of proportionality. For real
world thermocouples,  is not constant but varies with temperature.
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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).
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Equation of a thermocouple
• The output voltage “V” of a simple thermocouple (with a
reference temperature T0 = 0C = 32F) is:
1
1
2
V  AT  BT  CT 3
2
3
volts,
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
V
2
S
 A  BT  CT
T
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volt/C
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Characteristics of thermocouples
80
E
60
Millivolts
K
J
40
20
T
R
S
0
500 1000 1500 2000
Temperature, C
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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
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Resistance Temperature Devices
– 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 lasertrimming 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
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• Thermistors
– NTC
– PTC
• most sensitive temperature
transducer
V or T
• RTD’s R = R0[1 + (T – T0)]
• platinum, nickel, or ni alloys
Thermistor
RTD
Thermocouple
Temperature, C
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Equation of a thermistor
Steinhart-Hart equation:
a simpler equation:
1
 A  B(ln R)  C (ln R) 3
T
T
1
C
(ln R)  A
R  R0e
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(
T0 T
TT0
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The Self-Heating Problem
100
0 slope
Voltage, V
10
- slope
1.0
+ slope
0.1
0.10
1.0
10.0
100.0
Current, mA
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Integrated Circuit (I.C.) Sensors
+
+
1 A/K
To DVM
Current sensor
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10 mV/K
1 M
To DVM
Voltage sensor
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RTD
T
 Self powered
 Simple
 Rugged
 Inexpensive
 Wide variety of
physical forms
 Wide temperature
range





Non-linear
Low voltage
Reference required
Least stable
Least sensitive
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Resistance
Voltage
Disadvantages
Advantages
Temperature
I.C. Sensor
R
R
Resistance
V
Thermistor
Temperature
 Most stable
 Most accurate
 More linear than
thermocouple
T
Temperature
 High output
 Fast
 Two-wire ohmic
measurement
 Expensive
 Non-linear
 Slow
 Limited
temperature range
 Current source
 Fragile
required
 Small resistance
 Current source
change
required
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 Self-heating
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measurement
T
Voltage or current
Thermocouple
V or I
Temperature
 Most linear
 Highest output
 Inexpensive
 T < 250C
 Power supply
required
 Self-heating
 Limited
configurations
17
T
Bimetallic Devices
Metal A
Metal B
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Fluid-Expansion Devices
• Types:
Safety bulb
50
– 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
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Capillary tube
Stem
0
Temperature
sensing bulb
19
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
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Radiation Detectors (IR Sensors)
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100%
0.00312
0.003
80
60
0.002
40
0.001
20
T = 300 K
5
10
15
20
25
% Total power
Spectral radient emittance, W-cm-2·mm-1
m= 9.66 m
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.
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Fused silica
100
Sapphire
Arsenic trisulfide
Thallium
bromide
iodine
50
10
0
1
10
100
Wavelength, m
Spectral transmission for a number of optical materials.
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All thermal detectors
100
Indium antimonide (InSb)
(photovoltaic)
60
Lead sulfide (PbS)
20
0
1
2
3
4
5
6
7
8
Wavelength, m
Spectral sensitivity of photon and thermal detectors.
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Shutter
Ear
IR
Ambient sensor
Micro
processor
Ta
Tb
MUX
A/D
Amp.
Sensor
Waveguide
Window
Shutter
switch
Digital
display
The infrared thermometer opens a shutter to expose the sensor
to radiation from the tympanic membrane.
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Details of the fiber/sensor arrangement for the GaAs
semiconductor temperature probe.
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