Installed Gage Resistance Test: Quality Control

Installed Gage Resistance Test
Among its other capabilities, the Model 1300 provides for convenient, precise measurement of the gage resistance after installation. The instrument indication is displayed as the percent deviation from the nominal gage resistance, with full-scale ranges of either 5% or 1%. Since Micro-Measurements strain gages are manufactured to tightly controlled tolerances on gage resistance, the magnitude of the resistance shift due to installation provides a very sensitive index to certain aspects of gage installation quality. Shifts in gage resistance during installation should not ordinarily exceed 0.5% when using room-temperature-curing adhesives. With elevated-temperature-curing adhesives, on the other hand, the resistance shift may be greater due to the thermal stress in the gage after return to room temperature from the adhesive curing temperature. The magnitude of the shift depends on the curing temperature, the materials involved, and other variables, but should never exceed 2%, and should be uniform among identical gage installations to within about 0.5%.

Excessive resistance shifts are usually indicative of one or more flaws in the gage installation - poor installation technique, questionable solder connections, damage to the gage grid, leadwire damage, etc. A unique feature of the Model 1300 is that the completely wired strain gage can be connected to the instrument in a 3-wire circuit, just as it would be to a strain indicator. Thus, when the Model 1300 is used in its deviation mode, the effect of the normal resistance in matched leadwires is automatically canceled, and the instrument will therefore detect resistive faults not only in the gage, but also in the leadwires and solder joints.

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