Thermocouple and captor on SCB-68

[BenBeauchamp]

I have a PCI6013 and I use a SCB-68 terminal block. I have to aquire 5 thermocouples and two captors (flow rate meters). I have to use differential mode. Here are my questions: Why do I get different voltage value from the thermocouple when i use different values of bias resistance? I used 10k and 75k and I get different values. How to choose the right value of resistance? Can't I just use a wire instead of a resistance?. Is it necessary to read the zero offset for ground reference? I yes, I do I do that (between which channels? Next: When I connect my captors, the voltage value from the thermocouple changes. Do I need to use a bias resistance also for those captors?

Thanks

[RIO-Todd]

The value of bias resistors needed will depend on the impedance of the sources you're using. Thermocouples have a very low source impedance and so you could just tie the negative side of the amplifier to ground via a wire and you shouldn't see any problems.

The user manual of the NI 6013 has a very good explanation of when to use bias resistors and what size of resistors will be needed (Chapter 4). Usually, bias resistors over 100 kohm are used. I've linked the manual below.

NI 6013/6014 User Manual
http://digital.ni.com/manuals.nsf/webAdvsearch/7FCCC893F141E3F586256C5A006CDDF9?OpenDocument&vid=niwc&node=132100_US

With regards to the flow-rate meters affecting the thermocouple readings -- it sounds like the flow-rate meters may have a high source im
pedance. Any source with an impedance over 1 Kohm may cause the problem of cross-talk. I would suggest to determine the impedance of the sensor and try decreasing the impedance with a unity-gain buffer if the impedance is too large. The documents linked below will help you out with this. I hope this helps.

Data Acquisition: Troubleshooting Unexpected Voltages or Cross-talk in Analog Input Channels
http://digital.ni.com/public.nsf/websearch/B9BCDFD960C06B9186256A37007490CD?OpenDocument

How Do I Create a Buffer to Decrease the Source Impedance of My Analog Input Signal?
http://digital.ni.com/public.nsf/websearch/CF83426BC3AC514A86256C10005A4771?OpenDocument

Regards,

Todd D.