In its’ simplest form a thermocouple consists of two thermoelectrically dissimilar materials joined at two points. If there is a temperature differential present between these points an electric current will flow due to a voltage differential between the two connection points. The actual current produced in the circuit depends on this voltage and the electrical resistance of the circuit.
In early thermocouple use, an ammeter was introduced somewhere in the circuit to measure the current. By knowing the current, the thermoelectric characteristics of the materials (generally wires) and the temperature at one of the two connection points (the reference junction), it was possible to calculate the temperature at the other junction (the measuring junction). The electrical resistance of the circuit affected the current flow, so the meters were calibrated using a specified loop resistance. Although actually measuring current, these meters were often calibrated in temperature. However, they were only accurate if the total circuit resistance was correctly matched to the meter.
Modern instrumentation is generally based on a circuit that measures voltage directly by balancing a reference voltage against the circuit voltage. When the current flowing in the circuit is adjusted to zero, the balancing voltage is exactly equal to the thermocouple voltage. That voltage is then used to calculate temperature. Such an instrument is not sensitive to loop resistance because there is zero current flowing.
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