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Vishay Measurements Group
Interactive Guide
Strain Gage Accessories
Precision Resistors
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Precision Resistors |
IntroductionFixed resistors have two primary uses in strain gage circuits: shunt calibration of strain-measuring instrumentation, and bridge completion. For shunt calibration, a fixed resistor is temporarily shunted across a bridge arm to produce a known resistance change in the bridge circuit. The resulting instrument indication is then compared to the calculated strain corresponding to the resistance change. For bridge-completion applications, a fixed resistor may be used in the adjacent arm of the bridge to complete the external half-bridge circuit when a single strain gage is connected in a quarter-bridge arrangement. Similarly, when it is necessary that the full-bridge circuit be formed outside the instrument, a matched pair of fixed resistors can serve as a balanced half bridge.In each of these applications, the accuracy of the strain measurement is affected, directly or indirectly, by the accuracy and stability of the fixed resistors used in the circuit. It is important, therefore, that only precision, high-stability resistors be selected for these purposes. Bridge Completion Modules Modern strain gage instrumentation is readily available with built-in bridge completion resistors and "dummy" gages to accept quarter- and half-bridge strain gage input circuits. However, if the instrumentation at hand is not provided with these components, or if the measurement application does not permit their use, external bridge completion must be provided, and MR-Series Bridge Completion Modules can be an excellent choice in these applications. Completing the bridge circuit at the strain gage site provides for a symmetrical, balanced leadwire system between the strain gage circuit and the instrumentation. They can reduce effects of noise pickup in the leadwire system in some environments. Where switch contacts, slip rings, or other mechanical connections are employed between the strain gages and measuring instrumentation, or when leadwires will be periodically disconnected from the measuring instrument, accuracy can be improved by completing the bridge at the measurement site. Bridge completion modules can be designed to meet special circuit requirements. 
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