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How a Strain Gauge Fits into a Load Cell

In the last two weeks, we explored strain and then strain gauges . Next in the series, we’ll discover how that strain gauge fits into a load cell.

 

We have this strain gauge and we know that subjecting it to force causes it to deform and its electrical resistance to change. Last week, we briefly mentioned that a Wheatstone bridge is used to help us read this change. According to Wikipedia, a Wheatstone bridge is “an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, on leg of which includes the unknown component.” A basic Wheatstone bridge circuit contains four resistances (four strain gauges), a voltage input*, and a voltmeter. This Wheatstone bridge configuration is used because the use of four strain gauges amplifies the amount of change in the resistance, making it easier to read and more accurate, as well as less sensitive to thermal changes.

*The voltage input into the Wheatstone bridge could be constant or not. If the amplifier is ratiometric (able to measure the input voltage relative to the output voltage) then constant input voltage is not necessary.

 

The four strain gauges are configured in such a way that the current from the power source splits, flows through the sequence of resistors, then recombines into a single conductor (image). Three of these resistors have known values. The value of the fourth resistor is not known. As the strain gauge is loaded, the bridge becomes unbalanced producing an indication at the voltmeter. So a known voltage is input into the circuit and then the output voltage should be slightly different due to the change in electrical resistance of the strain gauge when loaded.

(Click here for cool pictures of Wheatstone bridges.)

 

So when these elements, the strain gauges configured as a Wheatstone bridge, are bonded to an engineered physical structure, the overall package is called aload cell. Wikipedia defines a load cell as “a transducer that is used to convert a force into electrical signal.” Although other types of load cells exist, strain gauge load cells are the most popular and surpassing all other types because manufacturers continue to increase their accuracy and lower their costs.

 

As always, if you have any questions related to this material, our support staff at Cooper Instruments is available to help. Contact them by calling (800) 344-3921 or emailing sales@cooperinstruments.com.

 

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