How Do I Calculate the Maximum Strain I Can Read with the NI 9237 Module?

Updated Jul 29, 2019

Reported In

Hardware

  • NI-9237

Driver

  • NI-DAQmx

Issue Details

The NI 9237 Operating Instructions and Specifications states a typical input range of ± 25mV/V, but my strain gage datasheet gives me a strain information in terms of % or µStrain. How do I calculate maximum strain for the 9237 module?

Solution

Strain is the amount of deformation of a body due to an applied force. More specifically, strain (ε) is defined as the fractional change in length, as shown in Figure 1. 
Figure 1. Definition of Strain
 
Strain can be positive (tensile) or negative (compressive). For each type of bridge configuration, the calculation formula will be different. 

Sample Calculation: 
Using a quarter bridge circuit, you can use the formula below to calculate the strain ε where the Vo/Vex ratio is the input voltage (-25mV/V and +25mV/V) to calculate both maximum positive and negative strains and GF is the Gage Factor given by the sensor manufacturer.
Figure 2. Quarter-Bridge Circuit
 
In this example, for a given GF of 2, it will result in both equations :
And after a few manipulations: