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8.17 A J-type thermocouple referenced to 0
C indicates 4.115 mV. What is the temperature of the
measuring junction?
8.18 A temperature measurement requires an uncertainty of Æ2
C (5%) at a temperature of 200
C. A
standard T-type thermocouple is to be used with a readout device that provides electronic ice-point
reference junction compensation, and has a stated instrument uncertainty of 0.5
C (95%) with 0.1
C
resolution. Determine whether the uncertainty constraint is met at the design stage.
8.19 A temperature difference of 3.0
C is measured using a thermopile having three pairs of measuring
junctions, arranged as shown in Figure 8.23.
a. Determine the output of the thermopile for J-type thermocouple wire, if all pairs of junctions sense
the 3.0
C temperature difference. The average temperature of the junctions is 80
C.
b. If the thermopile is constructed of wire that has a maximum emf variation from the NIST standard
values of Æ0.8% (95%), and the voltage measuring capabilities in the system are such that the
uncertainty is Æ0.0005 V (95%), perform an uncertainty analysis to estimate the uncertainty in the
measured temperature difference at the design stage.
8.20 Complete the following table for a J-type thermocouple:
Temperature (
C)
Measured
Reference
emf (mV)
100
0
0
À0.5
100
50
50
2.5
8.21 A J-type thermopile is constructed as shown in Figure 8.22, to measure a single temperature. For a
four-junction thermopile referenced to 0
C, what would be the emf produced at a temperature of
125
C? If a voltage measuring device was available that had a total uncertainty of Æ0.0001 V (95%),
how many junctions would be required in the thermopile to reduce the uncertainty in the measured
temperature to Æ0.1
C (95%)?
8.22 You are employed as a heating, ventilating, and air conditioning engineer. Your task is to decide
where in a residence to place a thermostat, and how it is to be mounted on the wall. A thermostat
contains a bimetallic temperature measuring device that serves as the sensor for the control logic of
the heating and air conditioning system for the house. Consider the heating season. When the
temperature of the sensor falls 1
C below the set-point temperature of the thermostat, the furnace is
activated; when the temperature rises 1
C above the set point, the furnace is turned off. Discuss
where the thermostat should be placed in the house, what factors could cause the temperature of the
sensor in the thermostat to be different from the air temperature, and possible causes of discomfort
for the occupants of the house. How does the thermal capacitance of the temperature sensor affect the
operation of the thermostat? Why are thermostats typically set 5
C higher in the air conditioning
season?
8.23 A J-type thermocouple for use at temperatures between 0
and 100
C was calibrated at the steam
point in a device called a hypsometer. A hypsometer creates a constant temperature environment at
the saturation temperature of water, at the local barometric pressure. The steam-point temperature is
strongly affected by barometric pressure variations. Atmospheric pressure on the day of this
370 Chapter 8 Temperature Measurements
14:54:4 Page 370
8.17 A J-type thermocouple referenced to 0
C indicates 4.115 mV. What is the temperature of the
measuring junction?
8.18 A temperature measurement requires an uncertainty of Æ2
C (5%) at a temperature of 200
C. A
standard T-type thermocouple is to be used with a readout device that provides electronic ice-point
reference junction compensation, and has a stated instrument uncertainty of 0.5
C (95%) with 0.1
C
resolution. Determine whether the uncertainty constraint is met at the design stage.
8.19 A temperature difference of 3.0
C is measured using a thermopile having three pairs of measuring
junctions, arranged as shown in Figure 8.23.
a. Determine the output of the thermopile for J-type thermocouple wire, if all pairs of junctions sense
the 3.0
C temperature difference. The average temperature of the junctions is 80
C.
b. If the thermopile is constructed of wire that has a maximum emf variation from the NIST standard
values of Æ0.8% (95%), and the voltage measuring capabilities in the system are such that the
uncertainty is Æ0.0005 V (95%), perform an uncertainty analysis to estimate the uncertainty in the
measured temperature difference at the design stage.
8.20 Complete the following table for a J-type thermocouple:
Temperature (
C)
Measured
Reference
emf (mV)
100
0
0
À0.5
100
50
50
2.5
8.21 A J-type thermopile is constructed as shown in Figure 8.22, to measure a single temperature. For a
four-junction thermopile referenced to 0
C, what would be the emf produced at a temperature of
125
C? If a voltage measuring device was available that had a total uncertainty of Æ0.0001 V (95%),
how many junctions would be required in the thermopile to reduce the uncertainty in the measured
temperature to Æ0.1
C (95%)?
8.22 You are employed as a heating, ventilating, and air conditioning engineer. Your task is to decide
where in a residence to place a thermostat, and how it is to be mounted on the wall. A thermostat
contains a bimetallic temperature measuring device that serves as the sensor for the control logic of
the heating and air conditioning system for the house. Consider the heating season. When the
temperature of the sensor falls 1
C below the set-point temperature of the thermostat, the furnace is
activated; when the temperature rises 1
C above the set point, the furnace is turned off. Discuss
where the thermostat should be placed in the house, what factors could cause the temperature of the
sensor in the thermostat to be different from the air temperature, and possible causes of discomfort
for the occupants of the house. How does the thermal capacitance of the temperature sensor affect the
operation of the thermostat? Why are thermostats typically set 5
C higher in the air conditioning
season?
8.23 A J-type thermocouple for use at temperatures between 0
and 100
C was calibrated at the steam
point in a device called a hypsometer. A hypsometer creates a constant temperature environment at
the saturation temperature of water, at the local barometric pressure. The steam-point temperature is
strongly affected by barometric pressure variations. Atmospheric pressure on the day of this
370 Chapter 8 Temperature Measurements