My Thermocouple Thermometer with datalogger shows invalid measurement or NULL

[Annamalai]

Hello people, I have a Thermocouple Thermometer with Data logger (THE-373 K/J Datalogger Thermocouple Thermometer) which I use to measure the temperature of RF amplifier by pasting the probes on the amplifier case with a tape which works perfectly but as soon as I connect it to a PC with a microUSB cable for Real time Data acquisition it starts showing very high measurement values instead of usual 45-50 deg C it goes to 300, 400 deg C and then to NULL.

This happens when "both microUSB cable and RF amplifier is powered and working",  but if any of those aren't there for e.g. without micro USB cable it properly shows  temperature of RF amplifier (with power ON) and vice versa case with USB cable but with RF amplifier power off then too it works. I couldn't figure why does it behaves like this.

User Manual of the Thermometer : https://cd50.net/wp-content/uploads/2023/07/37_E.pdf

[darkspr1te]

You might need to 'electrically' isolate the probe from the amp, it sounds like  you are getting leakage down the casing of the probe.
if you have any heat transfer pads that might work


darkspr1te

[BeBuLamar]

Generally if the thermocouple is broken it would read as very high temperature. I don't know if NULL mean broken thermocouple or not.

[HKJ]

I use to measure the temperature of RF amplifier by pasting the probes on the amplifier case with a tape which works perfectly but as soon as I connect it to a PC with a microUSB cable for Real time Data acquisition it starts showing very high measurement values instead of usual 45-50 deg C it goes to 300, 400 deg C and then to NULL.

Put a bit of tape under the probe.
Thermocouple probes are usually galvanic connected, using a DMM with PC connection to measure the thermocouple will also work (DMM PC connections are galvanic isolated).

[Ian.M]

I use to measure the temperature of RF amplifier by pasting the probes on the amplifier case with a tape which works perfectly but as soon as I connect it to a PC with a microUSB cable for Real time Data acquisition it starts showing very high measurement values instead of usual 45-50 deg C it goes to 300, 400 deg C and then to NULL.

Put a bit of tape under the probe.
Thermocouple probes are usually galvanic connected, using a DMM with PC connection to measure the thermocouple will also work (DMM PC connections are galvanic isolated).

*THIN* tape with good thermal conductivity.  If you have any Kapton tape handy try that.

You could also try a USB isolator to break the ground loop.

[Annamalai]

I use to measure the temperature of RF amplifier by pasting the probes on the amplifier case with a tape which works perfectly but as soon as I connect it to a PC with a microUSB cable for Real time Data acquisition it starts showing very high measurement values instead of usual 45-50 deg C it goes to 300, 400 deg C and then to NULL.

Put a bit of tape under the probe.
Thermocouple probes are usually galvanic connected, using a DMM with PC connection to measure the thermocouple will also work (DMM PC connections are galvanic isolated).

*THIN* tape with good thermal conductivity.  If you have any Kapton tape handy try that.

You could also try a USB isolator to break the ground loop.

Currently I have used a very thin paper tape underneath the probe which works but reduces the accuracy by ~ 0.5 to 1 deg C. USB Isolators also seems good which I think won't compromise that.

Thanks everyone for their replies.