Using Thermocouples on USB-6229

[dacad]

HI all,

I am setting up a USB-6229 M-Series multi-DAQ and intend on using a total of eight (8) type K thermocouples along with some pressure transducers. I discovered that I apparently have to use two of the analog inputs for each t-couple in order to get a reading.  When I attach the first thermocouple, it seems to read ok albeit a little noisy (I can deal with that).   I am doing my initial testing in Signal Express just to confirm connections, etc.

When I connect a second t-couple, it consistently generates an error and will not acquire a signal.  The first t-couple signal also either generates an error or transmits extremely slow.   I have not tried three yet but figure it would only compound the situation.

I have been advised by a 'consultant' that I should be able to read teh analog signal straight from the t-couples without any problems. This same person however also told me I would need only one AI for each t-couple so I now question his advice.

So I am now looking to the experts on the best / simplest way to connect my eight t-couples to my device.

Thanks

Doug

[JoeFriedchicken]

Hi Doug,

When connecting thermocouples to a DAQ card, my recommendation is to use referenced single-ended mode (as opposed to differential) [1]. This means that each thermocouple only requires a single analog input channel (whereas differential mode requires two channels per thermocouple). The physical connections you would make are: positive lead of thermocouple to an analog input channel and negative lead to the AIGND line.

Your consultant is also suggesting that you use referenced single ended. Differential mode will work, but it requires external resistors to bleed off bias currents in the system that are generated by unreferenced thermocouple.

In SingalExpress, be sure that you're properly setting the channel's terminal configuration mode. Since SingalExpress defaults to differential mode, this is most likely the source of your error.

[1] Field Wiring and Noise Considerations for Analog Signals
https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...