A pressure gauge measures the pressure within a system, and accurate measurement ensures consistency in applications. Accurately calibrating pressure gauges protects against leaks and over-pressurization of application requirements. In most cases, pressure gauge calibration is considered a non-issue because gauges are all over the plant.
Therefore, an operator will assume the accuracy of these devices despite being out of calibration. There is no pressure gauge calibration standard because it depends on the usage of the gauge. However, ensuring the pressure gauge measures pressure effectively is by accurate calibration. Here are some aspects to consider regarding calibration and how to calibrate your pressure gauge.
What is Calibration?
Pressure calibration matches the output of a pressure measuring device with that of another pressure measuring device. It may also involve comparing a measuring device against a standard measuring instrument for higher accuracy. The process involves plumbing the device under test (DUT) onto the standard device and generating standard pressure within the measurement circuit.
In a commercial setup, the calibration uses the manufacturer’s transmitter or pressure gauge calibration procedure. The reference standard should be at least four times the instrument’s accuracy under test or with a lower standard measurement uncertainty than the DUT.
Equipment for Pressure Calibration:
Deadweight Testers:
Deadweight testers use calibrated weights to test the accuracy of the pressure gauge readings. These pressure gauge calibration apparatuses are made up of fluids, connection ports, and weight pistons.
Pressure Comparator:
A pressure comparator works like a deadweight tester and generates pressure until the desired pressure reading is recorded.
Master Gauge:
A master gauge is a digital gauge for calibrating measurement tools. Its pressure gauge calibration offers a ±0.1% accuracy depending on the manufacturer.
Process of Calibrating a Pressure Gauge:
Ensure that the calibrator has recent calibration according to manufacturer specifications. If it is not up-to-date, perform calibration using this calibration procedure.
- Step 1: Set the scale pointer to zero before applying pressure to the gauge. This step is essential for ensuring that zero error does not peek during pressure gauge calibration.
- Step 2: Apply full pressure to the gauge while adjusting the linkage for the pointer to reach the maximum scale reading.
- Step 3: Completely bleed the pressure to zero and confirm that the pointer is reading zero on the scale. If the pointer does not read zero, adjust it accordingly.
- Step 4: Repeat steps 2 and 3 above until you achieve accurate readings.
- Step 5: Set the applied pressure to 50% of the maximum reading if you need to adjust the pressure for linearity. The pointer should read at 50% of the maximum scale reading when adjusting for linearity.
- Step 6: Confirm that the pressure gauge reads accurately at 0, 50%, and 100% scale readings. You could repeat the re-adjustment process multiple times to get a consistent, correct reading. However, this calls for patience and care.
- Step 7: After completing step (6), record the gauge readings for the applied pressure readings on a calibration sheet. For accuracy and traceability, the calibration document has information about the calibration of the measuring instrument.
- Step 8: If you are doing bench pressure gauge calibration and want to make a calibration certificate after the process, draw a graph of the readings and gauge pressures. In most instances, simple calibration is enough.
Factors to Consider When Calibrating Pressure Gauges:
Pressure Media
Gas and liquid are the two main pressure media. Liquid applications use oil or water, while gas flow meter calibration can use nitrogen. Calibrating low-pressure gauges may use air/gas, and higher pressures require liquid media.
Height Difference
When the gauge and calibration equipment are set at different heights, the hydrostatic pressure of the pressure media within the piping may create errors. These errors are common with gas flow meter calibration considering the gas is light.
Pressure Generation
You can utilize a pressure hand pump, a dead weight tester, or a pressure regulator with a bottle to generate pressure. A deadweight tester is more accurate, considering you do not need an extra calibrator. However, this procedure is expensive and immobile. A pressure calibration hand pump generates pressure, and a calibrator measures the pressure.
Calibrating pressure measuring devices take different approaches depending on the equipment. The procedure given above is applicable in calibrating most industrial pressure gauges and pressure transmitters. You can always reach out to industry experts to help with accurate Pressure gauge calibration.
