SCXI - 1112 wrong value with two channels

[M. CHAUMONTET]

Hi,

I work with VI logger to acquire temperature with 1000DC chassis and SCXI1112. My problem is when I use more than 1 thermocouple, temperature is wrong : for exemple, I start an acquisition with two thermocouples connected and if I disconnect one, the value of the other change (not always same step).
My measures are made on PCB with potential : this can be the reason of my problem ?
Is there any solution to solve this ?

Thank you for your help
Regards
Michael

[Richard_KEROMEN]

Hello,

Your problem seems quiet strange cause you're using 2 (or more) different channels. It may be the consequence of the fact that you just touch the cold junction compensation when you unplug a thermocouple. Or it might also be linked to the fact that you bring cold air when you unplug it...
What's the temperature modification range? 0.5° ? 0.05°?

Regards

[M. CHAUMONTET]

First thanks for your help...

Delta temperature could be very important (near 15°C).

I will explain you my application :
I measure temperature of connection of relay soldering on a PCB (thermocouple is in bullet of tin). When I switch on the power supply, I have a decrease of temperature value. This delta depend of the tension of the power supply. I have the same problem when I unplug a thermocouple.


Regards
Michael

[Richard_KEROMEN]

Hey,

Do you do this with MAX? If you don't try it.

On which channel do you plug your themocouples ?

Regards.

[Logan_K]

There are a couple of options that I see for this issue:

1) It is possible that this issue is related to the default convert rate that DAQmx uses for SCXI tasks - and with a fast convert rate you may be seeing crosstalk between channels. What version of DAQmx are you using? This behavior was recently changed for DAQmx 8.5, and should be improved for your application. See this knowledgebase:
http://digital.ni.com/public.nsf/websearch/1BF39CC271CA7A9D8625728E0061EF95?OpenDocument

2) The channels of the SCXI-1112 are not isolated, so you may have a ground loop if you are connecting your thermocouples directly to a piece of metal that has a different ground potential than your system. If this is the case, then you should either electrically isolate your thermocouple, or use a isolated SCXI module for making your measurements.

Regards,
Logan K