Thermocouple Values Fluctuating? Lots of noise.

[tharding247365]

First thing, I'm not an electrical guy, but I'm trying to figure this out.

I have 2 Type T thermocouples. One connected to an Omega USB-UTC that is connected to a laptop, the other connected to a Heater Controller that uses a PID controller

Both of these thermocouples connectors are fastened to a piece of steel to measure temperatures. As the heater heats up, I wanted to record the values in an excel sheet over time.

Taking readings from my laptop, I get ambient temperature 70F (heater controller off)

The minute I turn the heater controller on (But the device is not actually heating anything) the values fluctuate from 80-100F. I'm getting a lot of noise for some reason.

The noise seems to be coming from the PID Heater controller itself. Grounding I don't think is the issue...Could be wrong.

Things I've tried:
Unplugging power cord, running on batteries
Using a Desktop
Different thermocouple

What in the world is this witchcraft???

 

[danw2]

>What in the world is this witchcraft???

Answer: The official name is "thermocouple thermometry". Witchcraft is the layman's everyday term.

Does the noise happen if you take the thermocouple off the steel entirely, and measure air temp? If so, the noise is not coming through the T/C, but through the PC/USB)

If the noise disappears when measuring air temp, then the question is, is the thermocouple grounded or ungrounded? If you're using the 'beaded wire thermocouple' that comes with it, it probably has an exposed junction, right? And the exposed junction is touching the steel? Probably a ground loop with the USB's ground.

Or, if the thermocouple is metal sheathed, the thermocouple can be grounded to the sheath or ungrounded, not touching the sheath.

Grounded is quicker responding to temperature changes, but is subject to ground loop problems.
Ungrounded is slower responding to temperature changes, but is less subject to ground loop problems.

To test a sheathed thermocouple, find an ohmmeter (DVM on the 20 ohms/resistance scale) and check from either lead to the sheath to see if there's a low resistance, an ohm or two. The resistance will either be 500K ohms or more (ungrounded) or low resistance (grounded).

The spec sheet doesn't say whether the unit is isolated. But the device is using the USB power, so it could be isolated, but it probably isn't.

 

[hacksaw]

you need to be more specific about the sensor installations such as are they grounded, ungrounded, etc.