Calculating Absolute Accuracy of an NI Strain Module

Updated Nov 9, 2020

Environment

Hardware

  • NI-9237
  • C Series Strain/Bridge Input Module

I am using an NI Strain/Bridge Input module. How can I calculate the theoretical accuracy values of my system. What is the minimum increment value from this device?

The minimum increment will depend on the sensor you have connected as well as the accuracy from the NI 9237. The equation for calculating the absolute error for a strain device is dependent on the setup of the system and the specific NI module you are using. The example given here is for a NI-9237 C Series Strain/Bridge Input Module with a quarter-bridge setup. 

The following equation can be used to calculate the theoretical absolute accuracy of the module: 

Absolute accuracy = (Gain error * Reading) + (Offset Error * Range) + Noise + Half Bridge Tolerance + Quarter Bridge Tolerance 

where:
  • Gain Error is the portion of the accuracy due to how well the slope of the actual transfer function of voltage relates to the ideal transfer function. This is the full scale error of the module after accounting for the offset error.
  • Offset Error is the constant difference between the actual voltage and voltage read by the module across the input range of the module.
  • Noise is the error due to the system noise generated by the module.
  • Half Bridge Tolerance is the error due to the tolerance of the half bridge completion resisters. This error results in a constant offset error in the measurement.
  • Quarter Bridge Tolerance is the error introduced by the tolerance of the quarter bridge completion accessory. It will be composed of both gain and offset error. This value can be found in the 9944/9945 manual.

By convention all values are calculated in mV/V. The reason for this is explained in this Knowledge Base article.

It is important to note that the above calculation will give you the worst case absolute accuracy of the measurement before performing shunt and offset calibration. It is recommended that shunt and offset calibration are performed before taking measurements with the NI-9237. 

After performing shunt and offset calibration the new absolute accuracy equation can be given as the following:

T = Worst case temperature change from when shunt and offset calibration were performed

Accuracy = (Gain Drift * T) + (Offset Drift * T) + Noise + (Half Bridge Drift * T) + (Quarter Bridge Drift * T)

The accuracy after shunt and offset calibration is primarily dominated by the error due to temperature changes.