Measurement Error when Using the NI 9216/9226 DSUB with a Terminal Block

Updated Jul 4, 2018

Reported In

Hardware

  • NI-9216
  • NI-9226

Issue Details

I am using the NI 9216 or NI 9226 DSUB with a terminal block accessory to take resistance measurements in Three Wire Mode. I noticed that measurements had an additional error than what was expected. What can I do to prevent this and why is this occurring?

Solution

This behavior is occurring due to the mismatches in the lead resistance from the 9216/26 to the terminal block and inside the terminal block itself. This is not an issue when performing a four wire measurement. A three wire measurement has an additional error when there is a mismatch in the RTD+ and COM connection path resistance.

Feature 1. Connecting an RTD to the NI 9216

To compensate for this source of error, a calibration/adjustment may be preformed. Connect a 3-wire short on the channel of interest by using a wire to connect RTD+ to RTD- and RTD-to COM. Read the resistance value with the channel. This is the resistance offset error due the path resistance mismatch. Subtract this offset value from future readings to get the adjusted value.

Be sure to subtract this value before converting from resistance to temperature if taking measurements from an RTD. A function used to convert from resistance to temperature, Convert RTD Reading.vi, can be found in theĀ Functions Palette>>Numeric>>Scaling. An additional function to convert from resistance to temperature, RTD to Temp.vi, can beĀ found below.

This calibration/adjustment may be skipped and the additional error for three wire length mismatch specified in the NI 9216/9226 Datasheet can be added to the accuracy to account for the three wire resistance error.

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