Common mode voltage on thermocouple DAQ's

Equipment, Tools & Supplies Test and Measurement
,
1

Hi,

I need some support on product selection for thermocouple (TC) modules. I need to measure temperature using TC’s on a test section that is energised with DC voltage. The TC’s will need to operate simultaneously on a test section with a voltage range of +/- 10 V. I have had trouble with the TC’s being saturated with common mode voltage when using a Picologger TC-08.

I am looking at getting a new NI module (9212 or 9213) or maybe a Redlion E2-16ISOTC-D. Im not sure the metric to qualify these units for this particular issue. The NI modules give two common mode voltage ranges CH-COM and COM-Earth. Can you offer any advice on which metric should be used to assess this?

I would have thought the CH-COM range would be the appropriate value but I have used a NI 9211 units on this same setup and it reads OK. It has a CH-COM range of +/- 1.2 V but the unit is seeing +/- 18 V so Im a little confused on that score. Any insights here? Else any other product suggestions for high common mode voltage range?

Thanks

Ben

2

Greetings,

With respect to what reference point is this ±10V measured? The answer should give you some clue as to what to look for.

The 9212 mentioned appears to incorporate isolation per-channel, the 9211 a shared isolation to earth ground, and the PC-08 seems to mention no isolation at all, and thus would seem a poor choice to me for applications where the TC elements are potentially in contact with an electrically conductive substrate. (Which strikes me as a situation I’d like to avoid if possible.) Thermocouples make tiny signals, and having currents flowing in unexpected places is a great way to get terrible results–isolation is one’s friend there… Of the modules suggested the 9212 looks like the preferable option to me.