Temperature gradients and different types of wires in thermocouples

Question

I use long (10-m) copper-constantan thermocouples for work to measure temperature. The temperature gradient is only present in the 15 cm nearest the hot junction; the remaining 9.85 m of these thermocouples contain no temperature gradient. Currently, we use 10-m copper wires and 10-m constantan wires that go all the way from the hot junction back to the datalogger.

Since the temperature gradient exists only in the 15 cm nearest the hot junction, is it necessary to use constantan and copper wires to go all the way from the hot junction to the datalogger? It'd be a lot cheaper for us to use constantan only for the 15 cm nearest the hot junction.

Based on my knowledge of the thermoelectric effect, the Seebeck effect, and Kirchhoff's law, I believe that we only need to use the different types of metals across the temperature gradient nearest the hot junction. For the remainder of the thermocouple where there is no temperature gradient, I believe we can use copper wires for both the constantan and the copper side of the circuit, and a quick online search confirms this suspicion. I am not an electrical engineer, though, so I'd love to hear from someone who knows more than me.

Here's a sketch:

Since temperature gradients only exist across the two wires connected to the hot junction, it seems like currents will only be induced in these two wires. The two segments of copper wire farther away from the hot junction will not have any current induced in them; the only current that will flow through them is that which gets induced in the two wires that are part of the hot junction.

Answer 1

If you change wire material, then you establish a junction that you have to know the temperature of. That's why thermocouple extension wires and sockets are made of the thermocouple materials, so you don't need to know or control their temperature.

By all means, put a junction from thermocouple wires to copper at the sense end, and copper back to thermocouple for connection to your datalogger (I assume your datalogger has a thermocouple metal input rather than a copper input), but then you will have two junctions whose temperatures you need to know in order to offset the reading on the data logger. If they're equal, then no correction needed.

If your logger has a copper input (yikes, unknown connection block temperature!), then you could go thermocouple to copper once near your sense point, and copper all the way to your logger. You would still need to know the temperature of that connection, but you wouldn't lose any accuracy at the logger connection block.

Answer 2

As proposed your cold-junction temperature sensor should be part of an isothermal (ie. all the same temperature, isolated from disturbances from air currents or heat flowing down the wires) volume A. Ideally the isothermal block has a chunk of metal or something of that ilk to slow temperature changes down so that the dynamics of the CJC sensor do not play much of a part.

In many, if not most, cases of thermocouple instrumentation it is inconvenient to locate and thermally isolate the junctions and an accurate CJC sensor in the field so wires of thermocouple material (or a suitable proxy) are brought back to the instrument and the CJC sensor is located in an isothermal block as in volume B. Heat flows down the wires as shown by the small red arrows.

In your special case of a type T thermocouple, the top junction of Cu-Cu wires are similar alloy so there is little EMF at the top junction and it could be left out of the isothermal area but they won't be identical so it had better be treated like any other thermocouple (with the understanding that the Copper-Constantan junction temperature is actually more important to monitor). The error due to the temperature differences between A and B will be the same.

The consequence of having the CJC sensor in area B while having the wires transition in area A is that any total differences in temperature between A and B will result in almost exactly the same error in the reading (with the given temperatures).

In other wires if the hot junction is at 50°, Area A is at 25.5°C and area B is at 24.5°C your reading will be 49.0°C, a -1°C error. That's a relatively large error in terms of the reading. If the measurement of the temperature is part of a control loop, the controller will cause the hot junction to run at 51°C.

Note that if the hot junction temperature was 500°C, and the other temperatures were the same, then the -1°C error might not be deemed as important.

The above statements are true for most thermocouple types (J, K, T, E, N), and not necessarily for other types.