# What is a strain gauge and how do I use one?

As the title says, I have no idea what these things are or how to get or use one. Can I receive a simple explanation or links to one? I'm a computer engineer so I have very little physics/mechanical engineering background.

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Did you check wikipedia? en.wikipedia.org/wiki/Strain_gauge –  Qmechanic Feb 1 '12 at 16:09
Okay that helps a bit but how do I use one? –  Unfortunate Assignment Feb 1 '12 at 16:10
You attach it to something that you want to measure the strain of! –  Martin Beckett Feb 1 '12 at 16:11
look at the second link on google! it has plenty of explanations and you can purchase everything you need from them. –  user2963 Feb 1 '12 at 16:14

Application:

A strain gauge is a device used to measured the strain (change in length as a proportion of the original length) in an object as a result of an applied load. Most strain gauges are designed to measure strains in only one direction.

How it works:

A common type of strain gauge consists of thin metallic foil cut into a pattern such that most of the current flow is oriented lengthwise along the object you are attaching the gauge to. As the object is subjected to a tensile strain, the gauge elongates longitudinally. This causes the lengths of conductor parallel to the strain to become longer and (as a consequence of Poisson's ratio) thinner.

For a fixed resistivity, the resistance of a conductor is inversely proportional to the cross sectional area (which is decreasing) and directly proportional to the length (which is increasing). This can be expressed as the relation:

$R=\rho\frac{l}{A}$

Resistivity ($\rho$) is a material property, so it remains constant. Thus, we know that the resistance of the gauge is increasing.

Measuring this change in resistance allows us to find out the applied strain, by the relation:

$\epsilon=\frac{\Delta R/R_G}{GF}$

where GF is the gauge factor (constant for a gauge). The gauge factor can be determined by measuring the change in resistance caused by a known strain.

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+1 for fine description of most common type of strain gauge. Comment just to point out that many other unconventional designs are possible as well. –  Mark Rovetta Mar 26 '13 at 2:07

I worked with strain gauges in a lab. They are snakelike traces etched onto a plastic substrate about the size of a dime, and you glue them onto your test surface with special glue. You solder leads on and wire it back to a wheatstone bridge circuit. I think the standard is 350 ohms, so you would have three fixed resistors and your strain gauge. If you want to get fancy you can get an extra strain guage and glue it down to a dummy piece of test material for temperature compensation, but plain resistors are fine for the other branch of the wheatstone bridge. It works out that with a 2-volt excitiation, I think, microstrains = microvolts, so you want a really good voltmeter that displays five or even six decimal places for accurate work.

(A microstrain is a one-part-per-million deformation.)

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Hmm I asked my supervisor if this is what they want and they want something that sits in between two ropes and produces and analog signal. They call it a strain gauge but they might be referring to something else. –  Unfortunate Assignment Feb 1 '12 at 16:28
@UnfortunateAssignment Well, you could put a radio between the ropes and produce an analog signal, but I suspect you’re trying to measure the pulling force. This would usually measured by a load cell, which may or may not use a strain gauge to give a reading. –  Psirus Feb 1 '12 at 18:26
Much of the detail given here is extraneous and applies only to a particular instrument. The interesting parts are the conductive trace affixed to the subject and the amplification of the resistive change by means of the Wheatstone bridge. The rest is just fiddly stuff that keeps students busy. –  dmckee Feb 1 '12 at 19:51
I find dmckee's comments to be pointlessly hostile to the point of being bizarre. The question asked how do you use strain gauges. I explained how to wire them up in a wheatstone bridge, and pointed out the need for a voltmeter that goes to five digits. What exactly is fiddly about that? –  Marty Green Feb 1 '12 at 23:25
dmckee, I con't object to your writing whatever you want to write, but I've read it again, and like I said the first time, it still seems pointelssly hostile to the point of being bizarre. The conversion between microstrain and microvolts is no accident, and I don't remember at which excitation it becomes exact, but my point was that you're not going to get anything useful out of a wheatstone bridge with a \$29 multimeter. I don't think that's fiddly stuff to keep the students busy, whatever that's supposed to mean. –  Marty Green Feb 2 '12 at 22:49

To add to Marty's answer up there, a strain gage (guage sp?) is typically used to measure applied load. You might wish to check out the wikipedia article on load-cell

The strain gauge has known electrical resistance that changes when it is strained (load applied to it). This change is a function of the load; hence the applied load can be determined by measuring the electrical resistance on loading. The principle is used in weigh-bridges at toll-booths, railway sidings, docks etc to translate the mass of a vehicle/body into a form that may be directly read by a computer.

(We used it to measure coal brought by railway wagons to the dump-chute by applying the output from the bridge to an ADC which was then processed by the application )

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This seems more like a practical mechanical engineering question than a physics question. There are a great variety of strain-sensors available commercially. What you need really depends upon your application. There are entire books written on the subject. An appropriate sensor for measuring the strain of a bridge or a concrete dam would be quite different than a sensor intended to be used inside a transmission electron microscope. If you need a practical answer about strain-gauges, your best source of information may be strain-gauge manufacturers.

There are some very interesting (IMO) physics questions that arise if you need an unconventional strain-gauge for a specialized application. Strain is essentially displacement divided by a length. I have seen original strain-gauge designs developed (with input from physicists) based on resistance, induction, lasers, fiber optics, and piezoelectricity to measure strain in everything from living muscle to glaciers.

The is also much opportunity for work here for computer scientists, because all the information returned by strain sensors must be numerically processed to be useful.

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