5 5 strain gage calibration each strain gage attached to the specimen must be calibrated prior to running the test the gages are actually fine wire which stretch or compress with the specimen and thus increase or decrease in diameter this changes the electrical resistance of the wire and when calibrated can be related to strain in the gage by changing one of the normally constant resistors in the measurement system a known amount by interpreting this resistance change as though it were occuring at the strain gage calculations can be made to determine the amount of strain the resistance change represents the exact procedure is as follows a a 10 000 ohm resistor will be used for shunt calibration b determine the elongation range needed for practical strain measurement by noting the expected elongation at failure note also the gage factor and resistance of the gage c convert this expected elongation at failure to strain in inches per inch by dividing by 100 n number of active arms variable resistors this will normally be one 1 the resistance gage l l selected strain inches per inch expected elongation divided by 100 d from the formula below determine the strain that this selected resistance represents e set the recorder pen to read this strain directly on chart thus if the calculated strain is 0 00126 inches per inch 0 126 then pen is set to 1 26 inches on the chart a one inch deflection on the chart would then represent a 0 001 inch inch strain and a direct readout of strain is possible f it may be in some cases desirable to set the pen at some multiple of the calculated strain for a 0 00126 inch per inch calculated strain the.