Thermocouple measurement problems using NI9211

[AndrewFMills]

Hi

I'm getting strange readings on my NI9211 (used with USB carrier) when trying to take a measurement. I have the NI9211 and a Eurotherm 3216 controller connected to the same unsheilded grounded thermocouple (diagram attached). I realised this isn't best practice, but I have no alternative, and have used the set up successfully in other systems.

I'm using this set up for 3 thermocouples & 3 controllers sharing 3 channels on the same NI9211. With one TC connected to both the readings are fine, when I connect a second is when the readings are changed. I always get the correct reading on the Eurotherm.

It was suggested to me I should use resistors to link the negative legs of the thermocouples to the common on the NI9211. I've tried this with limited success. It seems to only work at certain temperatures.

Any ideas how I can correct it?

[Tori.M]

Hi Andrew,

Firstly, as you know, having an unshielded grounded thermocouple is not advisable.

To check that the other channels on your thermocouple module are working as expected with just the one thermocouple connected, please use the Measurement & Automation Explorer software. Click here for a tutorial on this.

This will work out whether your module is faulty. My main suspicion would be interference between the three thermocouples.

Have you made a connection to your COM port on your 9211? This acts as a ground.

Kind regards,

[AndrewFMills]

Hi Tori,

Thanks for the reply. It works correctly with only one connected not all three.

I haven't got anything connected to the COM terminal, as I understand the ground is made at the "hot end" of the thermocouple as it is directly in contact with a metallic object.

I should explain also, all three thermocouples are of the same type, and attached to the same object in different locations but would be electrically linked.

[Tori.M]

Hi Andrew,

As your thermocouples work fine in individual channels just not at the same time, there appears to be some issues with interference between channels.

Have a look at the manual for this module as it details quite a lot of information.

When using multiple thermocouples, a small offset error occurs, however this can be increased when thermocouples with a higher lead resistance are used. This offset error can be measured using the Autozero channel on your module so I would definitely recommend measuring this to find out the offset that you are seeing.

Kind regards,

[PeterKr]

Hello Andrew,

please, did you solve your problem after years? I have punctually the same problem and I don´t know how to solve it. I have connected 2 thermocouples. Below 600°C works everything ok, above starts problem. One thermocouple is connected to two controllers and 2-nd TC is connected directly to 2-nd port of NI9211 directly.

Thank you for your help.

Peter

[AndrewFMills]

Hi Peter,

Yes I did manage to resolve the problem. I installed a 10kOhm resistor between the negative thermocouple terminal and the COM terminal for all channels. Very occasionally the problem does still reoccur, but 99% of the time it is now working fine - still in use today.

Andrew

[PeterKr]

Hi Andrew,

thank you very much for your help. I´ll try this.

Peter

[ikaiser]

Andrew, you are awesome! +1 kudos from me for being responsive 6 years after you posted your last message in this thread (-:

[AndrewFMills]

Thank you! I have to admit I was a little surprised to see the email alert in my inbox!

[PeterKr]

Hello Andrew,

after years, Im back with the old problem. After your help started system to work well. They used it 2 years, than one year not. Now another employee had start this system again and the old problem is back. Resistors are connected. We use two thermocouples connected to channels 0,1. Resistors are connected also only to these two channels. Im reading again your recommendation. Please, should be connected all four channel to com port? It means also two empty ports? Thank you very much.

Kind regards

Peter