I'm a mechanical engineer working on acquiring some temperature data using thermocouples.
Q. I wanted to know the effect of wire length on a thermocouple's accuracy.
I don't have much of electrical engineering knowledge. I did some amount of search and people have mentioned that thermocouples don't experience much of voltage drop even when they are really long (100-150m, infact long thermocouples are used in powerplants for temperature measurements. I wasn't able to figure out the physical reason behind it. I want to particularly understand effect of voltage drop in particular, I'm not interested noise due to interference. Any leads would be really helpful!
Best Answer
Wire cable resistance is insignificant when compared to the input impedance of the amplifier which is so high that the wire doesn't affect the measurement.
For example, the LT1052 (which would be great for a thermocouple) has a 30pA input bias current, which corresponds to over 100MΩ of resistance. Wire cable even in the 100Ω range with a 30pA current would be about 3nV of error.
What does cause greater problems are temperature effects from other thermocouples, such as the connectors to the board and the thermocouple junctions formed between the pins of the opamp IC and the PCB. It is necessary to make sure that the connector is thermocouple effects are matched to reduce error.
Another problem is the voltage offset of the op amp, because themocouple voltages are in the mV range, voltage offsets contribute to error, so look for a low voltage offset (and one that doesn't change with the temperature of the op amp much)