Thermocouple vs NTC

Question

I want to test the accuracy of the NTC soldered in a current sensing board with a shunt resistor. For that, I attached a thermocouple type K to the junction of the shunt using epoxy and applied a current of 300A to register the temperatures captured by both the NTC and the thermocouple using CAN. The results are attached below, red is Thermocouple and blue is NTC, the problem is that I'm expecting the thermocouple temperature to be higher or equal to the NTC's and not have much difference between them (2 or 3°C difference), which is not the case. I want to understand:

• the potential causes of this result.
• If it's possible for the NTC's temperature to be higher, why is that?
• Are there better ways to attach the thermocouple?

Answer 1

First, as MCG pointed out in their comment, make sure that the sensors are calibrated and verified.

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I attached a thermocouple type K to the junction of the shunt using epoxy

I do a similar thing when I want to measure, for example, surface temperatures of some critical components on a PCB. I use double-component epoxy (namely DP100) to secure the T/C in place.

the potential causes of this result.

One thing I noticed with this method (using epoxy with T/Cs) is that if somehow the T/C is not in good contact, or in other words if there's any epoxy between the test surface and the T/C the results differ by a substantial amount. That's because the poor thermal conductivity of the epoxy (less than 0.02 W/m-K for DP100 in my case). A recent example: I measured the solder-point temperatures from the drains of two synchronous rectifier MOSFETs in DFN5x6-package placed side-by-side, one showed 98°C whilst the other did 105°C. After re-attaching the sensors "properly" the difference reduced to ~3°C.

One other thing is the convection. If you use a ball-type T/C like this one and attach it to a surface you need to isolate the rest of the sensor because a very small surface of the T/C is in contact with the target surface so the remaining may get "cooled down" by the air convection. The isolation is normally done by the epoxy but it still worth considering.

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If it's possible for the NTC's temperature to be higher, why is that?

As for the NTC, the only thing comes to my mind is the self heating due to the biasing. You use an NTC as part of a voltage divider or you drive it with a constant current. Either way, the current flowing through it will cause a power dissipation which then results in self heating. You need to check the datasheet for the maximum allowed bias currents to bring the minimal self heating.

Answer 2

You may have error due to heat conducted down the thermocouple leads, which will reduce the measured temperature (bring it closer to ambient, to be more accurate). To minimize this use the smallest gauge of thermocouple that is practical and keep as much length of the leads in contact with the surface you are trying to measure as is practical.

You can find stick-on polyimide-insulated ribbon thermocouples that are way better than cheap bead thermocouples (and are available in popular alloy combinations such as ISA/ASTM E, J, K, T). A bit of insulation on the back and Robert's your uncle.

Cold junction stability is important when you are trying to measure close to room temperature, so make sure your measuring instrument is up to the job and that you have properly connected the sensor.

300A is also quite a bit of current in relation to the tens of uV/°C T/C output and you may have some effect due to that.