Pressure changes applied to the gage cause the elastic element to expand and contract. The movement of the element is translated into movement of the pointer through links, levers, and gears. Calibrating a pressure gage includes adjustment of these components until the gage reading accurately represents the input.
Input and Output Measurement Standards
The instrument under test ( in this case, the gage ) determines the calibration standards. First you need a source of pressure, which is provided best by a regulator. An input standard to measure the pressure applied is also needed. An appropriate input measurement standard for this calibration is precision.
Use a tee to connect the precision gage to the source of pressure and the gage under test. Be sure the gage under test is mounted in the same orientation as it is in the process.
Five Point Check
Determine the five test points used for the upscale and downscale checks of the gage under test. With any link and lever instrument it is important that your entire upscale check be done in an upscale direction and your entire downscale check be done in a downscale direction.
Accuracy of the Instrument
The test results should be checked for an accuracy within the manufacturer’s specifications. If the results are outside the manufacturer’s specifications, determine the type of errors present. On most motion balance instruments, try to adjust linearity first. Linearity is corrected at the midpoint of the range, apply 50% PSI. Use a template to check the 90 Deg. angle, matching the linkage angle with the template. With linearity adjusted, position the pointer so the gage reads midscale. You may need to remove the pointer and reposition it on the shaft. Lower the input to 10%, and adjust the zero so the gage reading equals the 10% input value. Now correct the span error. The input pressure is increased to 90%. Adjust the span until the gage reads this same 90% value. Repeat the zero and span adjustments until the readings at 10% and 90% are accurate. Zero and span interact, rechecking is required for best results.