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the measured emf will be unchanged from the open-circuit emf, which corresponds to the
temperature difference between T 1 and T 2 , if T 3 ¼ T 4 . Another practical consequence of this
law is that copper extension wires may be used to transmit thermocouple emfs to a measuring
device.
3. Law of successive or intermediate temperatures: If two dissimilar homogeneous materials
that form a thermocouple circuit produce emf 1 when the junctions are at T 1 and T 2 and
produce emf 2 when the junctions are at T 2 and T 3 , the emf generated when the junctions are
at T 1 and T 3 will be emf 1 þ emf 2 . This law allows a thermocouple calibrated for one
reference temperature, say T 2 , to be used at another reference temperature, such as T 3 , to
determine temperature T 1 .
Basic Temperature Measurement with Thermocouples
Let’s first examine a historically significant method of using a thermocouple circuit to measure
temperature. Figure 8.17 shows two basic thermocouple circuits, using a chromel–constantan
thermocouple and an ice bath to create a reference temperature. In Figure 8.17a, the
thermocouple wires are connected directly to a potentiometer to measure the emf. In Figure
8.17b, copper extension wires are employed, creating two reference junctions. The law of
intermediate materials ensures that neither the potentiometer nor the extension wires will
change the emf of the circuit, as long as the two connecting junctions at the potentiometer and
the two in the ice bath experience no temperature difference. All that is required to be able to
measure temperature with this circuit is to know the relationship between the output emf and
the temperature of the measuring junction, for the particular reference temperature. One
method of determining this relationship is to calibrate the thermocouple. However, we shall
see that for reasonable levels of uncertainty for temperature measurement, standard materials
and procedures allow thermocouples to be accurate temperature measuring devices without the
necessity of calibration.
Reference Junction
The provisions for a reference junction should provide a temperature that is accurately known,
stable, and reproducible. A very common reference junction temperature is provided by the ice
point, 0
C, because of the ease with which it can be obtained.
The creation of a reference junction temperature of 0
C is accomplished in either of two
basic ways. Prior to the development of an electronic means of creating a reference point in the
electric circuit, an ice bath served to provide the reference junction temperature. An ice bath is
typically made by filling a vacuum flask, or Dewar, with finely crushed ice, and adding just
enough water to create a transparent slush. When done correctly, the method is surprising
accurate and reproducible. A few ice cubes floating in water do not create a 0
C environment!
Ice baths can be constructed to provide a reference junction temperature to an uncertainty
within Æ0:01
C.
Electronic reference junctions provide a convenient means of the measurement of temperature
without the necessity to construct an ice bath. Numerous manufacturers produce commercial
334 Chapter 8 Temperature Measurements
14:53:58 Page 334
the measured emf will be unchanged from the open-circuit emf, which corresponds to the
temperature difference between T 1 and T 2 , if T 3 ¼ T 4 . Another practical consequence of this
law is that copper extension wires may be used to transmit thermocouple emfs to a measuring
device.
3. Law of successive or intermediate temperatures: If two dissimilar homogeneous materials
that form a thermocouple circuit produce emf 1 when the junctions are at T 1 and T 2 and
produce emf 2 when the junctions are at T 2 and T 3 , the emf generated when the junctions are
at T 1 and T 3 will be emf 1 þ emf 2 . This law allows a thermocouple calibrated for one
reference temperature, say T 2 , to be used at another reference temperature, such as T 3 , to
determine temperature T 1 .
Basic Temperature Measurement with Thermocouples
Let’s first examine a historically significant method of using a thermocouple circuit to measure
temperature. Figure 8.17 shows two basic thermocouple circuits, using a chromel–constantan
thermocouple and an ice bath to create a reference temperature. In Figure 8.17a, the
thermocouple wires are connected directly to a potentiometer to measure the emf. In Figure
8.17b, copper extension wires are employed, creating two reference junctions. The law of
intermediate materials ensures that neither the potentiometer nor the extension wires will
change the emf of the circuit, as long as the two connecting junctions at the potentiometer and
the two in the ice bath experience no temperature difference. All that is required to be able to
measure temperature with this circuit is to know the relationship between the output emf and
the temperature of the measuring junction, for the particular reference temperature. One
method of determining this relationship is to calibrate the thermocouple. However, we shall
see that for reasonable levels of uncertainty for temperature measurement, standard materials
and procedures allow thermocouples to be accurate temperature measuring devices without the
necessity of calibration.
Reference Junction
The provisions for a reference junction should provide a temperature that is accurately known,
stable, and reproducible. A very common reference junction temperature is provided by the ice
point, 0
C, because of the ease with which it can be obtained.
The creation of a reference junction temperature of 0
C is accomplished in either of two
basic ways. Prior to the development of an electronic means of creating a reference point in the
electric circuit, an ice bath served to provide the reference junction temperature. An ice bath is
typically made by filling a vacuum flask, or Dewar, with finely crushed ice, and adding just
enough water to create a transparent slush. When done correctly, the method is surprising
accurate and reproducible. A few ice cubes floating in water do not create a 0
C environment!
Ice baths can be constructed to provide a reference junction temperature to an uncertainty
within Æ0:01
C.
Electronic reference junctions provide a convenient means of the measurement of temperature
without the necessity to construct an ice bath. Numerous manufacturers produce commercial
334 Chapter 8 Temperature Measurements