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USB-600x Accurate Thermocouple or Low Voltage Measurements

Updated Apr 18, 2019

Reported In

Hardware

  • USB-6002
  • USB-6001
  • USB-6003
  • USB-6008
  • USB-6009

Issue Details

I am using a USB-6001/6002/6003/6008/6009 and I want to take thermocouple or other low voltage level measurements with it, but my measurements do not appear to be very accurate. Why is this happening and what can I do to obtain more accurate temperature readings?

Solution

NI USB-6008/6009:
The NI USB-6008 has a 12-bit resolution with a minimum range of +/- 1 V, which translates to a minimum resolution of 480 uV. The NI USB-6009 has a 14-bit resolution and a minimum range of +/- 1 V, which translates to a 122 uV resolution. The poor voltage resolution of these input devices makes them non-ideal for low voltage measurements. Additionally, when acquiring in the +/- 1 V range, each device provides a typical accuracy of +/- 1.53 mV at 25 °C. The poor accuracy of these devices at low-level signals also makes them non-ideal for thermocouple measurements.

Here are some common thermocouple types and their associated voltage responses:
  • K type: 41 uV/°C
  • J type: 56 uV/°C
  • T type: 52 uV/°C
Due to the large minimum voltage resolution, the USB-6008 and USB-6009 can only detect changes of approximately 8-9 °C  and 2-3 °C respectively, and only accurate to within 30 °C.  


NI USB-6001/6002/6003:
The NI USB-6001/6002/6003 do have similar ADC resolutions (14 or 16-bit), however since their voltage input range is set to +/- 10 V the resolution is decreased. For example a 14-bit analog digital conversion of a signal ranging from +/- 10 V provides a resolution of around 1.22 mV.

A better alternative for measuring thermocouples is the NI 9211 and USB-TC01 as they are specifically designed for taking thermocouple measurements.  The NI 9211 has built-in signal conditioning, 24-bits of resolution, and +/-80 mV input range which is better for thermocouple readings.  The USB-TC01 is compatible with J, K, R, S, T, N, and B thermocouples and uses a standard miniplug for easy thermocouple connection. 

These resolutions would mean that these devices can only detect changes of approximately 23-29 °C.

Additional Information

For more information about the equipment referenced, please visit the respective links found in the related links section below.

Estimates of the responses for different thermocouple types can be obtained from the NIST thermocouple tables.

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