Thermocouple feedback

[Olly85]

Hello,

Hopefully someone can help shed some light on a hardware/software problem I am having.

I am using a cDAQ 9174 chassis with an NI 9213 Thermocouple card and an NI 9375 DIO card.

The DIO card is triggering some relays which in turn are enabling multiple heating elements.

The elements are being heated up to temperatures of over 700C and have a thermocouple enbedded inside them.

The system should be quite straight forward. Enable elements, read temperatures. However we seem to be having some issues with feedback on the thermocouples. At certain temperatures (which can vary, currently it is happening repeatibly at 100C and 600C), the temperature reading will drift (by 100's of degrees C), usually reading lower than it actually is. The drift ONLY happens if an element is turned on. If the measurement drifts and you disable the element it will immediately return to it's correct reading. This temperature drift not only effects the temperature of the element that is turned on, but also any other temperatures being read via that card. Disconnecting the thermocouple (while it and other channels are reading incorrectly) will result in the other channels returning to their correct reading.

It's as if the thermocouple is getting some feedback, but when I plug the thermocouple into a hand held reader it displays fine with no issues.

Anyone have any ideas?

Many thanks

[Tom-S]

Hi,

Could you confirm how your have your thermocouple wired into the device?

Kind regards,

Tom

AE UK
NI

[Olly85]

Hi Tom,

Thanks for the reply.

I am using floating thermocouples that are connected (with an extension cable) straight into the cDAQ. The green wire going to the positive, the white going to the negative. The cDAQ chassis is grounded but the COMs on the NI9213 are not.

Kind regards,

Olly

[Tom-S]

Hi Olly,

Thank you for providing that information. As you are using floating thermocouples on a differential measurement system I would take a look at the following article - i think you may need to ground your signals.

The Need for Grounding Thermocouples When They Are Already Wired In Differential Mode

http://digital.ni.com/public.nsf/allkb/862567530005F09C86256522005CD1EF?OpenDocument

I hope this information helps,

Kind regards,

Tom

AE UK

NI