How do you connect thermocouples to an NI USB-6221 board to measure temperature?

It depends on how designed the USB-6221 is to deal directly with thermocouples.

Thermocouples output a very small voltage in response to temperature, which could be directly read by the analog input in the 6221, but you wouldn’t get great range.

What I’d recommend is connecting the thermocouples up to an analog devices AD595-AQ (you can buy them here: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=AD595AQ-ND )—datasheet is here: http://www.analog.com/static/imported-files/data_sheets/AD594_595.pdf

It’s a CDIP package, so you should be able to breadboard them up to test, and then solder to a perfboard (this one: http://www.sparkfun.com/commerce/product_info.php?products_id=8814—which I know it fits on b/c I have one on it—sparkfun also sells the AD595, but for about $6 more than digikey, but if you’re already ordering from SFE, then you’ll save on shipping, etc.)

Connect the AD595 as shown on page 3 of the datasheet, and you’ll get a consistent voltage (10mV/ degree C) with 0C = 0V on a K-type.

So, you should be able to get a +5VDC rail out of the 6221, and a ground rail, and then you can just connect the V_out (pins 8 + 9) to each of your analog inputs on the 6221, and you’ll be all set.

Thanks for the advice, but I actually solved the problem – we’re using thermocouple analog converters (TAC80-K to be specific) to autocorrect for the reference temperature that a thermocouple circuit needs and to amplify the signal from the thermocouple. They fit nicely in a connector that in turn fits into some of the BNC analog input ports.

Brilliant! Glad you made it work =)