USB 6259 Lagging Response

[ddrop]

Hi,

I'm having problems with my USB 6259. I've connected a thermocouple to ai0 through a banana clip, and I'm trying to get accurate temperature readings from it. Unfortunately, when I try to test out my device by pinching the thermocouple with my fingers, it reads the temperature increases much more slowly than my Fluke thermometer (which I'm pretty sure is right). Also, there is a solid 8-10 second lag time between me releasing the thermocouple and the temperature readings capping off, which should definitely NOT be the case. Could this be hardware problem? Or is it a software problem? Any suggestions on how to fix it?

Thank you

[JÞB]

It is a set-up problem.

I've connected a thermocouple to ai0 through a banana clip

So, what is the cold junction reference? 

[ddrop]

Doesn't a problem with the cold junction only result in an error of about 1-2 degrees? Why should this mean that my device reads increasing temperature well into 10 seconds after heat has been removed from the probe?

Also, what do you suggest I do to fix this?

[JÞB]

The problem is you created two new thermocouples at the metal transitions that are causing you all kinds of odd behavior.  the probe wires are different metals than your connectors.  there are specific considerations you need to take (that the fluke input connector addresses)

[ddrop]

Ok, thanks... Do you have any idea if putting the thermocouple ends straight into the pinout would allow the device to read fine?

[JÞB]

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@ddrop wrote:

Ok, thanks... Do you have any idea if putting the thermocouple ends straight into the pinout would allow the device to read fine?

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Yes I do- no, it will not help.

I actually created a simulated task an tried to select a cjc source "built in" and got the -20077 error so we know that there is no on-board cjc referance.  I'll wave my hand for an app engineer!

Oh, NI~~~

what devices have onboard cjc referances?

[JÞB]

Get yourself one of these NI USB-TC01

[Aaron O]

Jeff,

The NI 9211, 9213, 9214, or the 9219 would be the modules that support built-in CJC (other than some SC Express, PXI and SCXI modules). You could also use the USB TC-01 for a simple USB solution.

ddrop,

I would take a look at the Temperature Measurements with Thermocouples: How-To Guide for a detailed description of making thermocouple measurements. Another good resource is the Taking Thermocouple Temperature Measurements. There is a section on performing these measurements directly and compensating for CJC in software. NI's modules will make thermocouple measurements more straightforward. 

Regards,

Aaron

[JÞB]

Thanks Aaron