Floating thermocouple measurements with NI 9205 moduleS, one does measure fine

[Driessen1]

We currently bought a CDAQ-9172 8 bay USB station and 4 NI9205 modules for use of measuring 64 channels of T-type thermocouples.

The problem is as follows:

One module plugged in the chassis. Freshly booted PC, DAQ turned off.

Chassis is turned on. Computer recognizes the chassis and all the modules installed (in this case one). The wanted module is selected and SignalExpress is started.

Standard signalexpress openes with the voltage readout on all channels. The measurement is stopped. The voltage measurement is deleted and with Add Step, a Thermocouple Type T anaolgue input is selected.

CJC is set to channel one as was indicated in the other forums. We start the measurement.

Now two things might happen.

1. The values read out fine for about 2 to 3 minutes and suddenly decide to run off to -inf.

2. All readings are fine and will run for more than 30 minutes

Even more troubling is that when we install the other modules, reactivate the chassis and restart signal express all values immediately run through to -inf.

We've tried each module seperately at all the different places and the issue described above stays ...

Does anybody know what to do or should we return the equipment to the company we've purchased it from ?

[RikPrins]

Hello,

This is just to notify others that we are allready working on this one. As we disscussed on the phone this morning the problem is most likely occuring due to bias current from the module. It looks like some load is building up because there is no return path to ground for the BIAS current. I advise to try a bias resistor from the negative lead of your thermocouples connected to the COM pin of the module. A second option you can try to see if BIAS current is indeed the problem is connecting the thermocouples in Single Ended mode.

Hope this helps, let me know if there are any other issues.

Regards,

RikP - National Instruments Applications Engineering