Strain Measurement        (5 of 7)
 
The Wheatstone Bridge
    This is the basic 4-corner Wheatstone Bridge circuit. It is a voltage divider where E is an applied voltage across two opposite corners and eo is the voltage read by an instrument across the other two corners. If all four resistances are equal, the voltage difference, eo, will be zero.
    This is a two-wire strain gage bridge circuit. Here a strain gage is placed in the circuit as one of the resistances. As the gage is strained, its resistance will change, and hence cause a voltage difference eo to exist. This voltage is proportional to the resistance change and the strain. Note that the lead wires to the gage also have resistances, RL. The strain measurement will include their resistances and introduce an appreciable (>1%) error if they are long (>20 ft.) Also any thermal strains in the wires due to temperature changes will be included in the strain indicator's output.
    The three-wire strain gage bridge circuit neutralizes the lead wire and temperature effects. In this case the third wire is used to "move" one corner of the bridge to the gage. Hence, one lead wire resistance is added to one arm of the bridge and the other to another arm. Thus, the length and temperature effects add the same amount to each side of the bridge and effectively cancel. The third wire (in the center of the diagram) is attached to the voltage meter and has no significant effect on the reading.
 
To read more about these three bridge circuits, vivit the Measurements Group page:
http://www.measurementsgroup.com/guide/tt/tt612/612a.htm