Temperature Measurement Notes | EduRev

Created by: Vinod Sharma

: Temperature Measurement Notes | EduRev

 Page 1


Objectives_template
file:///G|/optical_measurement/lecture9/9_1.htm[5/7/2012 11:55:38 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
The Lecture Contains:
Resistance Thermometers
Thermocouples
Wind Tunnel
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 2


Objectives_template
file:///G|/optical_measurement/lecture9/9_1.htm[5/7/2012 11:55:38 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
The Lecture Contains:
Resistance Thermometers
Thermocouples
Wind Tunnel
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture9/9_2.htm[5/7/2012 11:55:38 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
Resistance Thermometers
The hot wire probe referred above can be used in the constant current mode as a resistance
thermometer to determine the local fluid temperature. Here we exploit the fact that the wire resistance
changes with temperature. This variation is close to linear and is of the form
A typical value of  for commonly used metals such as aluminum, gold, platinum and tungsten is 
C. To measure resistance a small current of around 1 mA is passed through the wire
and the instantaneous voltage drop across the wire is measured. This method permits the
tracking of thermal transients and fluctuations in the fluid. However it suffers from an error that occurs
due to Joule heating of the wire itself. For this reason resistance thermometers are more
commonly used with circuits that employ a null method to measure resistance. The current flow in
such circuits can be made extremely small.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 3


Objectives_template
file:///G|/optical_measurement/lecture9/9_1.htm[5/7/2012 11:55:38 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
The Lecture Contains:
Resistance Thermometers
Thermocouples
Wind Tunnel
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture9/9_2.htm[5/7/2012 11:55:38 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
Resistance Thermometers
The hot wire probe referred above can be used in the constant current mode as a resistance
thermometer to determine the local fluid temperature. Here we exploit the fact that the wire resistance
changes with temperature. This variation is close to linear and is of the form
A typical value of  for commonly used metals such as aluminum, gold, platinum and tungsten is 
C. To measure resistance a small current of around 1 mA is passed through the wire
and the instantaneous voltage drop across the wire is measured. This method permits the
tracking of thermal transients and fluctuations in the fluid. However it suffers from an error that occurs
due to Joule heating of the wire itself. For this reason resistance thermometers are more
commonly used with circuits that employ a null method to measure resistance. The current flow in
such circuits can be made extremely small.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture9/9_3.htm[5/7/2012 11:55:38 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
Figure 2.14: Placement of Resistance Thermometer.
Figure 2.15: Wheatstone Bridge Setup.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 4


Objectives_template
file:///G|/optical_measurement/lecture9/9_1.htm[5/7/2012 11:55:38 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
The Lecture Contains:
Resistance Thermometers
Thermocouples
Wind Tunnel
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture9/9_2.htm[5/7/2012 11:55:38 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
Resistance Thermometers
The hot wire probe referred above can be used in the constant current mode as a resistance
thermometer to determine the local fluid temperature. Here we exploit the fact that the wire resistance
changes with temperature. This variation is close to linear and is of the form
A typical value of  for commonly used metals such as aluminum, gold, platinum and tungsten is 
C. To measure resistance a small current of around 1 mA is passed through the wire
and the instantaneous voltage drop across the wire is measured. This method permits the
tracking of thermal transients and fluctuations in the fluid. However it suffers from an error that occurs
due to Joule heating of the wire itself. For this reason resistance thermometers are more
commonly used with circuits that employ a null method to measure resistance. The current flow in
such circuits can be made extremely small.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture9/9_3.htm[5/7/2012 11:55:38 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
Figure 2.14: Placement of Resistance Thermometer.
Figure 2.15: Wheatstone Bridge Setup.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture9/9_4.htm[5/7/2012 11:55:39 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
Consider the measurement of temperature of a heated surface as shown in Figure 2.14. Wheatstone
bridge is commonly used for direct measurement of wire resistance, as shown in Figure 2.15 where 
is the DC power source with a stability of better than 1 mV,  is a galvanometer used to check for
null current, ,  are control resistors that reduce the current level in the circuit and , are
the lead and wire resistances, respectively. The presence of  in the parallel arm of the bridge
compensates for temperature gradients that will exist in the lead wires. When the bridge is balanced
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Page 5


Objectives_template
file:///G|/optical_measurement/lecture9/9_1.htm[5/7/2012 11:55:38 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
The Lecture Contains:
Resistance Thermometers
Thermocouples
Wind Tunnel
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture9/9_2.htm[5/7/2012 11:55:38 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
Resistance Thermometers
The hot wire probe referred above can be used in the constant current mode as a resistance
thermometer to determine the local fluid temperature. Here we exploit the fact that the wire resistance
changes with temperature. This variation is close to linear and is of the form
A typical value of  for commonly used metals such as aluminum, gold, platinum and tungsten is 
C. To measure resistance a small current of around 1 mA is passed through the wire
and the instantaneous voltage drop across the wire is measured. This method permits the
tracking of thermal transients and fluctuations in the fluid. However it suffers from an error that occurs
due to Joule heating of the wire itself. For this reason resistance thermometers are more
commonly used with circuits that employ a null method to measure resistance. The current flow in
such circuits can be made extremely small.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture9/9_3.htm[5/7/2012 11:55:38 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
Figure 2.14: Placement of Resistance Thermometer.
Figure 2.15: Wheatstone Bridge Setup.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture9/9_4.htm[5/7/2012 11:55:39 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
Consider the measurement of temperature of a heated surface as shown in Figure 2.14. Wheatstone
bridge is commonly used for direct measurement of wire resistance, as shown in Figure 2.15 where 
is the DC power source with a stability of better than 1 mV,  is a galvanometer used to check for
null current, ,  are control resistors that reduce the current level in the circuit and , are
the lead and wire resistances, respectively. The presence of  in the parallel arm of the bridge
compensates for temperature gradients that will exist in the lead wires. When the bridge is balanced
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Objectives_template
file:///G|/optical_measurement/lecture9/9_5.htm[5/7/2012 11:55:39 AM]
 Module 2: Review of Probes and Transducers
 Lecture 9: Temperature measurement
 
Thermocouples
We describe below the use of thermocouples in temperature measurement. Their ease of operation,
reliability, applicability over a wide range of temperatures ( 100 to  C) and the ability to follow
transients have made thermocouples very popular in research as well as industrial applications.
Thermocouples employ a phenomenon called Seebeck effect. When two junctions formed between
two dissimilar metals are maintained at different temperatures a break anywhere in the circuit will
exhibit an emf called thermo-emf (Figure 2.16).
Figure 2.16: Thermocouple Arrangement.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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