Thermocouple Reference Junction
To measure the temperature it is necessary to connect the thermocouple wires to a measuring instrument, or to copper lead wires that in turn connect to the instrument. This connection is known as a “cold junction”. The cold junction temperature needs to be known to measure the hot junction temperature correctly.
The value of the thermocouple EMF is proportional to the temperature difference between the hot and cold thermocouple junctions and is given by:
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E = S (T1 - T2) |
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S is the thermocouple Seebeck coefficient |
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T1 is the temperature at the hot junction |
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T2 is the temperature at the cold junction |
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Ideally, the thermocouple signal should only be proportional to the highest temperature viz. E = ST1 therefore the part ST2 should be zero. This is possible when T2 is equal to zero.
Under laboratory conditions this can be achieved by immersing the cold junction in an ice bath. An accuracy of +/- 0.01°C is achievable.
For industrial and aerospace applications a technique called “Cold Junction Compensation” (CJC) is used. Electronic measuring equipment generates a signal proportional to ST2, which is added to the EMF value given by the equation above, resulting in:
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E = S (T1 - T2) + ST2 = ST1 |
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To generate this signal an additional temperature sensor is placed very close to the cold junction to measure its temperature (refer to the diagram below).
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