Precision Temperature and Humidity Instrumentation
How Thermocouples Work
Thermocouples consist of two wires of dissimilar metals joined at one end, which we will refer to as the measuring junction. This end that is joined together is inserted into the fluid or solid that we wish to measure the temperature of.
If there is a temperature difference between the measuring junction and the other end of the wires, a small voltage will be produced which is proportional to the temperature difference between the ends of the wires. It is important to note that voltage is not produced at the junction of the thermocouple wires, rather it is produced along the portions of the thermocouple wires that experience temperature differences.
All thermocouple circuits must have two junctions, a measuring junction and a reference junction. And the voltage produced is related to the difference between the temperature of the measuring junction and the temperature of the reference junction. Therefore, we must know the temperature of the reference junction in order to calculate the temperature of the measuring junction from the measurement of the small voltage produced by the thermocouple circuit.
In early work with thermocouples, the reference junction was placed in an ice water bath because this arrangement could assure that the reference junction was at or very near a known temperature, the melting point of water, 0 °C. Ice point reference junctions are still used today by some systems that require very high accuracy in temperature measurement.
Tables of millivolts produced by a thermocouple circuit at different temperatures are commonly based on a reference junction temperature of 0 °C. And tables for other reference junction temperatures can be created through computational methods.
Because of this usage, the reference junction is often referred to as the cold junction, and the measuring junction is often called the hot junction. In reality, the reference junction does not have to be at 0 °C, the only requirement is that its temperature must be known and accounted for. And the reference junction does not have to be colder than the measuring junction, it can be at a higher temperature than the measuring junction. In any instance where the reference junction is warmer or hotter than the measuring junction it would be a misnomer to refer to it as the cold junction. Therefore at GEC Instruments we never refer to a thermocouple reference junction as the cold junction. It could be colder, hotter or the same temperature as the measuring junction. And for the same reasons, we never refer to the measuring junction as the hot junction. We prefer to call it the measuring junction, because this is a more correct and more descriptive terminology.
In most modern electronic thermocouple measuring systems, the reference junction occurs at the point where the end of the thermocouple opposite the measuring junction end is connected to the input of the voltage measuring circuitry. This reference junction temperature must be determined and accounted for in the final determination of the temperature of the measuring junction.
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