Thermocouple Reads Excessive Noise on USB DAQ Devices

Updated May 3, 2018

Reported In

Hardware

  • USB-6008
  • USB-6002

Software

  • LabVIEW Base
  • LabVIEW Full
  • LabVIEW Professional

Driver

  • NI-DAQmx

Other

Windows 7, 8

Issue Details

I am measuring a K- or a J- Type thermocouple with a USB DAQ. I'm receiving noise on the range of +/- 10 degrees C. Why is this happening?

Solution

Using a USB DAQ for thermocouple measurements may be noisy for a variety of reasons:
  • The instrumentation amplifier input bias currents can move the voltage level of the floating source out of a DAQ input's valid range.
  • The low resolution of some USB DAQ devices (such as USB-6002 and USB-6008) may provide undesirable results when measuring the small voltage output of a thermocouple. There are potential workarounds for this:
    • Read the signal in as a voltage instead of as a temperature and convert it with the Convert Thermocouple Reading VI
    • Amplify the thermocouple output before it reaches the DAQ device
    • Obtain data with another, higher resolution device

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