Minimizing Measurement Errors When Making Thermocouple to Transmitter Connections Industry Technology Paper
How To Minimize Errors When Making Thermocouple to Transmitter Connections
The most important thing to remember about the thermocouple is it that it’s the temperature difference between ends that produces the charge imbalance that drives the small voltages, not the junction of the two dissimilar metals that form the thermocouple circuit.
While you can form your own TC circuit with different combinations of two conductors, standard TC types using specific metals and alloys are available that produce larger, stable, and predictable output voltages relative to applied thermal gradients.
These standard types have their voltage over temperature tabulated in TC type tables relative to a cold junction at 0°C and 0.000mV. These TC voltages could have been referenced to another temperature than 0°C, but 0°C was chosen because it is easily reproducible within ±0.2°C using a mixture or slurry of ice and water. By holding the cold junction at 0°C, the temperature of the other end directly corresponds to its thermoelectric voltage found by look-up in the standard TC type table.
Three Fundamental Principles of Thermocouples
To correctly apply thermocouples to temperature measurement, it’s important to understand three thermoelectric principles that govern TC behavior and that give important clues for properly conditioning them.
The first basic principle is The Law of Homogeneous Materials: A thermoelectric current cannot be sustained in a circuit composed of a single homogeneous material by the application of heat alone, and regardless of how the material may vary in cross-section.
This tells us:
- No current flows in a conductor circuit by the application of heat alone when made of a single metal.
- Two different metals are required to form a thermocouple.
- The size or gage of wire does not affect the voltage produced.
- The voltage produced is independent of temperature variations along the TC path.
The TC produces a voltage difference between ends independent of the temperature distribution along its length allowing you to pass TC wires through hot and cold areas without affecting its measurement if the wire material is kept consistent along the wire path (i.e. using TC connection blocks & extension wires).
The second principle that governs TC behavior is The Law of Intermediate Materials: The algebraic sum of the thermoelectric emf’s in a circuit composed of any number of dissimilar materials is zero if all the junctions are maintained at a uniform temperature.
This tells us:
- If a different metal than the TC material is used to connect either or both TC wires, the measured voltage will not be affected if the different metal is kept at the same temperature across its transition or isothermal.
Extending this principle to the cold junction at the open end of a TC circuit where the measurement is taken (inclusive of the lead metal, solder, copper board traces, etc.), it may be considered isothermal when its combined temperature remains constant, usually following a period of heat exchange with its surroundings where its temperature may adjust slowly over time before finally reaching thermal equilibrium (its warm-up period).
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