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# Measurement Computing Data Acquisition Knowledgebase

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# Are the StrainBook WBK16 Strain Gage Sense Wires needed?

The purpose of the sense line is to measure the excitation voltage at the strain gage and compensate for voltage loss over long lead wires. For example, let's say the wire has a total resistance of 3.8 ohms from the StrainBook out to the gage and the gage in use is a 120 ohm quarter bridge circuit. If the excitation is 5 volts then the current in the excitation wire is 5/120 or approximately 42mA. Multiply 42mA by 3.8 ohms in the wire and you will see that there is a 0.16 volts dropped across it. Without the sense line at the gage the excitation would be 4.84 volts instead of 5 resulting in an error of 3.2%. Note that this compensation method works because the sense line do not carry any significant current so there is no loss over the wire. Think of it as if the StrainBook has a built in digital volt meter to measure voltages at remote bridge circuits.