How to Measure Displacement by a Strain Gauge
When force is applied to an object, a deformation called strain results. A strain gauge measures the deformation in units of distance deformed per unit of distance placed under strain. By measuring deformation with a strain gauge and knowing the length of the object placed under strain, you can determine the amount of total displacement placed on an object. You'll also need to know a bit about electric circuits, since you'll need to build a Wheatstone Bridge.
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Things You'll Need
- ohmmeter
- voltmeter
- DC power source
- various resistors
- strain gauge
- calipers or micrometer
Instructions
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1
Bond the backing of a strain gauge to the object over which you want to measure displacement. Use an adhesive recommended by the gauge manufacturer, or, if there is no recommendation, use Cyanoacrylate cement or epoxy. When bonding, lay the gauge's coils lengthwise so that they are in the axis of the strain to be measured.
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Measure the length of the object that will be subjected to force, or strained. Use micrometers or calipers for optimum accuracy.
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3
Use an ohmmeter to measure the resistance of the strain gauge with the object being measured experiencing no strain.
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Build a Wheatstone Bridge. Select a resistor (or several resistors in series to serve as the equivalent of a single resistor) that have the same resistance as the unstressed strain gauge. Place this resistor in series with the gauge. Place two other resistors in series with each other in a separate leg that is parallel to the first resistor and strain gauge. Connect one lead of a voltmeter between these two resistors. Place the other lead of the voltmeter between the first resistor and the gauge. Add a DC power source across the two parallel legs.
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Read the voltmeter with the object under no strain. Verify that it is zero.
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Calculate the relation between the object's strain and the bridge. Per eFunda.com, the voltage across the voltmeter is approximately equal to on quarter of the change in resistance of the gauge divided by its zero-strain resistance. For example, if a force applied to the object being measured creates a strain (deformation) of one thousandth of an inch per inch of object, the zero-strain resistance is 1000 ohms, and a one thousandth inch of deformation causes a 10 ohm decrease in resistance, and the power source to the bridge is 10 volts, then the voltmeter will read: V = ((10ohms/1000ohms)/4)*10 volts = 2.5mV.
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Convert the voltage reading to displacement. Per the step above, a 2.5mV reading correlates to a deformation of one thousandth of an inch per inch of object. If you have an object that is a foot long and the Wheatstone Bridge circuit reads 2.5mV, then the object is undergoing a 12 thousandths of an inch of displacement.
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