NI9213 Thermocouple measurement AC noise

[jarcTek]

I am using a cRio with various NI9213 thermocouple C-Modules. For my application I require the thermocouples not to be shielded and be very close (even thouching) to AC cables and sources. When I read the thermocouple values that are close to the AC cables, I see noise in sinusoidal patterns in various frequencies (depending on my sampling frequency). I am using Scan Engine set at High Resolution (High Speed mode yielded a more noisy signal). I would appreciate any help regarding this issue. Should I ground my cRio in a particular way? My last resource will be to sample at a higher frequency and average the values (assuming 60Hz noise, I would sample at at least 120 Hz).

Thank you!

[johnsold]

1. Make certain that whatever you do, you do it safely!

2. If the thermocouples may be touching the power line conductors, anything connected to the thermocouples (cRio, host computer, your hand) must be rated to withstand the peak voltage on the line.

3. It may be almost impossible to clean up the thermocouple signals sufficiently to avoid power line frequency interference.

4. Sampling the thermocouple signals synchronously with the power line and then averaging samples taken at or near the zero crossings of the power line might be a way to get reasonable results. At the zero crossing instants the interferring voltage is zeor or close to it.  Sampling at 60 or 120 Hz is not sufficient.  The sample clock must be synchronized or phase locked to the power line frequency.

5. Remember safety.

Lynn

[jarcTek]

Thank you for your reply.

I am working with very low voltages, but high electric currents. The problem I have with high sampling rates is that I am acquiring data from the 16 channels of 8 NI9213 Modules. According to the module's specs, the sampling rate should be 1200 Samples/s aggregate.

Are there any considerations regarding ground loops that I should be taking into account?

Thanks again.

[johnsold]

You should try to avoid ground loops.  With high currents you will have large magnetic fields. Those fields will induce current in any conductive loop around them. Any thermocouple which makes contact with one of the high current conductors may become part of a ground loop.  Orient the T/Cs at right angles to the high current conductors as much as possible.  This reduces coupling. The thermocouples themselves form loops.  Keep the T/C conductors as close together as possible and twist them if you can. That minimizes the enclosed area of the loop, helping to minimize induced currents. Shielding typically does not help much for inductive coupling.

I looked briefly at the NI 9213 specs.  It appears that there is no isolation between channels, 1.2 V between channels and COM, and 250 V between COM and earth ground.  It also appears that you have no option for synchronous sampling with those devices.

Lynn

[jarcTek · Accepted solution]

Thank you for your answers.

I had to settle for using the "High-Resolution" mode on the NI9213s and its 60db 60Hz noise rejection with some "best practices" when cabling the AC power lines and the thermocouples. Nevertheless, the noise is not completely filtered. Looking around for other data acquisition devices (not from NI particularly) I found thermocouple modules with at least 120db 60hz noise rejection. I am personally using another data acquisition provider with a thermocouple module that filters 60Hz noise like a charm. I wonder why the limited filtering on the NI9213.