NI USB-6009

[inyangau]

Would someone help me figure out why I am not getting accurate expected thermocouple readings from this device (NI USB-6009). I connected the thermocouple legs to the A0+ and AI0- and all the other input ports, but still getting readings between (488-900) V.

Thank you. 

[santo_13 · Accepted solution]

Without digging deeper, it could be because you're using hardware incapable of reading the thermocouple.

The USB-600x series is a low-cost DAQ, so they are not fully capable. Now to the technical reasons, USB-6009 has a 14-bit ADC with +/-1V range as the smallest, which translates to 122uV resolution but the absolute accuracy is +/-37.5mV

Now, typically thermocouple voltages are <80mV in the best-case, with a 37.5mV error for a 80mV measurement, needn't tell more.

[inyangau]

Thank you for the answer. It has cleared my issues with this box.

[inyangau]

Would it be correct to say that this box is for measuring mostly differential and Reference Single Ended voltage, and current signals? If you can. It does not have much documentation on what can be measured on it. Thank you.

[crossrulz · Accepted solution]

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@inyangau wrote:

Would it be correct to say that this box is for measuring mostly differential and Reference Single Ended voltage, and current signals? If you can. It does not have much documentation on what can be measured on it. Thank you.

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The USB-6009 was designed for basic voltage measurements, differential and RSE.

I'm trying to be careful here since a thermocouple measurement is technically a differential voltage measurement, but it is at such a low level (millivolts) that it will just show up as noise.  Find a DAQ that is specifically designed for thermocouples if that is what you are trying to measure.

[santo_13 · Accepted solution]

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@inyangau wrote:

Would it be correct to say that this box is for measuring mostly differential and Reference Single Ended voltage, and current signals? If you can. It does not have much documentation on what can be measured on it. Thank you.

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I would simplify it as low-cost DAQs are not designed for small signal measurements.

Typical applications would be to measure signals from sensors whose output is typically 0-5V or 0-10V or differential where the mV level errors are not significant.

NI has dedicated modules to read thermocouples https://www.ni.com/en-us/shop/category/temperature.html and the cheapest being https://www.ni.com/en-us/shop/model/usb-tc01.html with very good accuracy.