Collecting temps with USB-6251

[deadstick]

I'm trying to configure a VI for a student lab project reading up to a dozen thermocouples. I've been told (by a source of unknown reliability) that I can wire the TC's directly to the inputs of our USB-6251 (as opposed to using an SCXI chassis with TC module). I tried running a task this morning with the following settings:

Type K thermocouples

Constant CJC at 25C

Input range 0-150C

The TC's are exposed to temperatures in the 40-50C range, but are reading temps of 800-1000C.

Is there a way to correct this, or will we be forced to dedicate our SCXI-1000/1102 combination to this apparatus?

[JÞB]

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@deadstick wrote:

I'm trying to configure a VI for a student lab project reading up to a dozen thermocouples. I've been told (by a source of unknown reliability) that I can wire the TC's directly to the inputs of our USB-6251 (as opposed to using an SCXI chassis with TC module). ....Is there a way to correct this?

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Yes, find sources of known reliability.  The correct connection depends on the USB-6251 configuration.  If you have the screw-terminal termination flavor you are good to wire the thermocouple wires directly.  Mass terminated devices  need an external Signal conditioning device.  BNC terminated devices may not support thermocouple measurements.

Now you ask why!  The Screw terminal type has a cold junction reference at the terminals.  The mass terminated version does not have a CJC- it expes=cts a CJC at the breakout end of the cableing.  A BNC connector often involves nickle plated aluminum outer conductors and copper, or gold plating on the inner conductors (all sorts of bi-metal junctions.)

From your post I'm betting that the metalicity of the thermocouple is not maintained to the cold junction. 

[deadstick]

OK, there are no BNC's involved. The TC's are connected via TC wire to a screw-terminal block which I take to be the cold junction; via hookup wire from there to a 68-pin connector; and via a 68-conductor cable to the USB-6251.

[deadstick]

OK, our USB-6251 is the mass-term version. But with the cold junction occurring at the breakout point, we should be able to input a fixed CJ temp, no? This is for a student lab apparatus, and high precision isn't a requirement.

[NIquist]

Are you configured for differential inputs?  If you think you have everything set up correctly post a daigram of your connections and your VI.

[deadstick]

Thanks...I'll be in the lab this weekend checking the students' wiring, and I'll diagram what we have.