How do you compensate for temperature effects in strain measurement?

In a perfect world that exists at room temperature with no variations, temperature effects are a moot point. There would be no variance due to changing temperature effects in leadwires or in the strain gages used for measurement. That world does not exist, and temperature variation is a reality most of us must deal with. The extent of that variation and what effect that has on your measurement are then your responsibility to navigate.

What are temperature effects?

There are two major ways to compensate for strain gage temperature affects.

 First, all materials expand and contract as a result of changing temperature and is commonly referred to as the Coefficient of Thermal Expansion. It is measured in parts per million per degree temperature. You may remember from the definitions for strain measurement that one microstrain (1 με) is 0.000001 ε or one part per million. With that being the case, we can redefine the Coefficient of Thermal Expansion as με per degree (F or C). Micro-Measurements treats gage foil to respond similarly to this with a Self-Temperature Compensation (STC). In the part number:

This number, when properly paired with the CTE minimizes the apparent effects of temperature over a specified range. An important note here is the minimizing and not removal of thermal effects. Two different metals cannot be forced to respond the same way since CTE is a material property. In the given gage number above, this would commonly be paired with Aluminum substrates (CTE 12.9) or Tin substrates (CTE 13.0). A common misconception is that these gages can only be paired with the matched CTE material. Each gage can be bonded down to any substrate with the proper surface preparation procedure. Self-Temperature Compensation can be intentionally mismatched to mathematically rotate the STC curve in the testing range to limit the change in thermal output over that range. By this mitigation of the variance in thermal output during the test, the intention would be mathematically accounting for the offset or configuring your instrumentation to eliminate a DC shift. More information on this topic can be found in “Strain Gage Thermal Output and Gage Factor with Temperature: Tech Note 504” from our Knowledge Base | Micro-Measurements. 

Secondly, each material has a temperature coefficient of resistance (TCR). Similar to the CTE, this number represents the percentage by which a material changes resistance over a temperature range. Some materials, like copper at 22% per 100°F, have a very high TCR, while others, like manganin at 0.11% per 100°F, have a very low TCR. Inevitably, there will be some length of wires in any strain gage-based measuring system and that, aside from the gage itself, is where this effect is seen most prominently. Depending on the configuration of a Wheatstone Bridge, this can be a devastating problem, or almost entirely removed. Several caveats are required for the removal to take place:

1. All leadwire lengths within the bridge should be of the same length.
2. Active gages must be from the same foil lot - preferably the same package.
3. Active gages must all be exposed to the same temperature regime.

When these conditions are met, like changes occur in all arms of the Wheatstone Bridge which means that no voltage potential is generated across the signal leads from the changing temperature.

One last note on this topic:

The discussions here are relevant to applications under 550°F long term and 700°F short term as things change when higher temperatures are reached. High-temperature gages will have the STC portion replaced with “NC” which means “Non-Compensated”. The thermal outputs at high temperatures are so excessive that no amount of compensation will account for them. Beyond that, the results will be different for each temperature cycle and non-repeatable. That is not to say strain measurement cannot be done at high temperatures, but it takes an entirely different approach to be successful.

Do you have questions about compensating for temperature effects in your strain measurement projects? Looking for detailed guidance or specific solutions? Reach out to our Applications Engineering team at Micro-Measurements for personalized support and expert advice. Email us at mm@vpgsensors.com or call 919.365.3800, opt. 1 for Micro-Measurements, opt. 2 for Applications Engineering. Let us help you ensure the precision and accuracy of your measurements.

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