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How do you calibrate a strain gauge for accurate stress measurement?

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Strain gauges are devices that measure the deformation or strain of a material under stress. They are widely used in mechanical engineering to test the strength, durability, and performance of various components and structures. However, to obtain accurate and reliable stress measurements, strain gauges need to be properly calibrated before and after each experiment. In this article, you will learn how to calibrate a strain gauge using a simple method that involves a known weight and a digital multimeter.

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1 Why calibrate a strain gauge?

Calibration is the process of adjusting or correcting the output of a measurement device to match a standard or reference value. Calibration ensures that the device is functioning correctly and eliminates any errors or drifts that may affect the results. For strain gauges, calibration is essential to determine the sensitivity or gauge factor of the device, which is the ratio of the change in electrical resistance to the change in strain. The gauge factor varies depending on the type, material, and configuration of the strain gauge, and it may change over time due to environmental factors, fatigue, or damage.

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Johnathon Hunt

Engineering Project Manager / Friction Stir Welding Specialist at CTC - Mechanical Engineering PhD
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One thing that I have found helpful is to calibrate the gauge across a wider window than the window the gauge will serve in. This provides not only robustness to the calibration but will also allow the engineer to see any nonlineararities that the gauge may have.

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2 How to set up a strain gauge?

To calibrate a strain gauge, you need to mount it on a specimen or a test rig that can apply a known stress to the material. The strain gauge should be aligned with the direction of the applied stress and attached securely with an adhesive or a bonding agent. You also need to connect the strain gauge to a Wheatstone bridge circuit, which is a simple arrangement of four resistors that converts the change in resistance of the strain gauge to a voltage output. The Wheatstone bridge can be powered by a battery or a power supply, and the output can be measured by a digital multimeter or an amplifier.

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3 How to apply a known stress?

To apply a known stress to the material, you need to use a weight or a load cell that can exert a precise and constant force on the specimen. The weight or load cell should be hung from a point on the specimen that is close to the strain gauge, and the force should be perpendicular to the surface of the material. You can use a hook, a clamp, or a wire to attach the weight or load cell to the specimen. The magnitude of the applied stress can be calculated by dividing the force by the cross-sectional area of the material.

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4 How to measure the voltage output?

To measure the voltage output of the Wheatstone bridge, you need to connect a digital multimeter or an amplifier to the terminals of the bridge. The digital multimeter or amplifier should be set to measure direct current (DC) voltage and have a high input impedance to avoid loading the bridge. You should also zero the multimeter or amplifier before taking any readings. The voltage output of the bridge is proportional to the change in resistance of the strain gauge, which is in turn proportional to the change in strain of the material.

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5 How to calculate the gauge factor?

To calculate the gauge factor of the strain gauge, you need to record the voltage output of the bridge and the applied stress for at least two different values of force. You can use a spreadsheet or a calculator to plot the voltage output versus the applied stress and find the slope of the linear relationship. The slope is equal to the product of the gauge factor and the bridge excitation voltage. Therefore, you can divide the slope by the bridge excitation voltage to obtain the gauge factor.

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6 How to verify the calibration?

To verify the calibration of the strain gauge, you need to repeat the calibration procedure with a different weight or load cell and compare the results. You should also check the consistency and stability of the voltage output and the applied stress for each measurement. If the results are similar and within a reasonable margin of error, you can conclude that the strain gauge is calibrated correctly and ready for use. If the results are significantly different or erratic, you may need to troubleshoot the strain gauge, the Wheatstone bridge, or the multimeter or amplifier.

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7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Muhammad Shehryar Khan

Banting Postdoctoral Fellow at Massachusetts Institute of Technology | Adjunct Assistant Professor at University of Waterloo | Assistant Editor at Materials Today Communications | Governor General's Gold Medalist
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Temperature Compensation: Strain gauges are sensitive to temperature changes, and this can affect their accuracy. To compensate for this effect temperature sensors or ubridge completion networks can be used. Environmental Factors: Strain gauges should be shielded from humidity, vibration, and electromagnetic interference. Calibration Frequency: Strain gauges should be calibrated regularly to ensure accurate measurements, depending on the type of strain gauge, the environmental conditions, and the level of precision required. Error Analysis: When calibrating strain gauges, it's important to analyze the sources of error (i.e., measurement noise, electrical interference, mechanical stress, and other factors) and minimize them.

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How do you calibrate a strain gauge for accurate stress measurement?

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7

1 Why calibrate a strain gauge?

2 How to set up a strain gauge?

3 How to apply a known stress?

4 How to measure the voltage output?

5 How to calculate the gauge factor?

6 How to verify the calibration?

7 Here’s what else to consider

Mechanical Engineering

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How do you calibrate a strain gauge for accurate stress measurement?

1

2

3

4

5

6

7

1 Why calibrate a strain gauge?

2 How to set up a strain gauge?

3 How to apply a known stress?

4 How to measure the voltage output?

5 How to calculate the gauge factor?

6 How to verify the calibration?

7 Here’s what else to consider

Mechanical Engineering

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