Pressure gauges are very common instruments in the process industry. As with any measurement device, pressure gauges need to be calibrated at regular intervals to assure they are accurate. There are many things to consider when calibrating pressure gauges. This article lists 20 things you should consider when calibrating pressure gauges.
1. Accuracy classes :Pressure gauges are available in many different accuracy classes. Accuracy classes are specified in ASME B40.100 (accuracy classes from 0.1 to 5 % range) as well as in EN 837 (accuracy classes from 0.1 to 4 % range) standards. The accuracy class specification most often being “% of range” means that if the accuracy class is 1% and if the scale range is zero to 100 psi, the accuracy is ± 1 psi.
2. Pressure media :When calibrating pressure gauges, the most common pressure media are gas or liquid. Gas is most often regular air, but in some applications, it can also be different gases, such as nitrogen. Most commonly, the liquid is water or oil. The pressure media during the calibration depends on the media that is used in the process that the gauge is connected to. Media also depends on the pressure range. Low pressure gauges are practical to calibrate with air/gas, but as the pressure range gets higher it is more practical and also safer to use liquid as the media.
3. Contamination :When calibrating pressure gauges, the most common pressure media are gas or liquid. Gas is most often regular air, but in some applications, it can also be different gases, such as nitrogen. Most commonly, the liquid is water or oil. The pressure media during the calibration depends on the media that is used in the process that the gauge is connected to. Media also depends on the pressure range. Low pressure gauges are practical to calibrate with air/gas, but as the pressure range gets higher it is more practical and also safer to use liquid as the media.
4. Height difference :If the calibration equipment and the gauge to be calibrated are at a different height, the hydrostatic pressure of the pressure media in the piping can cause errors. This normally is not an issue when gas is used as the media, as gas is light compared to liquid. But when liquid is used as media, the liquid in the piping will have a weight due hydrostatic pressure and can cause errors. The magnitude of the error depends on the density of the liquid and the difference in height, as the gravity is pulling the liquid inside the tubing. If it is not possible to have the calibrator and gauge at the same height, then the effect of height difference should be calculated and taken into account during the calibration.
5. Leak test of piping:If there are any leaks in the piping during the calibration, unpredictable errors can occur. Therefore, a leak test should be done prior to calibration. The most simple leak test is to pressurize the system and let the pressure stabilize for some time, and monitor that the pressure does not drop too much. Some calibration systems (pressure controllers) may be able to maintain the pressure even in case of a leak, if it has a continuous controller adjusting the pressure. In that case, it is difficult to see a leak, so the controller should be closed to enable a closed system for a leak test.
6. Adiabatic effect:Adiabatic effect
In a closed system with gas as the pressure media, the temperature of the gas effects the volume of the gas, which has an effect to the pressure.
7. Torque：Especially for torque sensitive gauges, don’t use excessive force when connecting pressure connectors to the gauge, as it may damage the gauge. Follow manufacturer’s instructions for the allowed torque force. Take the time to use proper tools, appropriate adapters and seals.
8. Calibration / mounting position：Because pressure gauges are mechanical instruments, its position will effect to the reading. Therefore, it is recommended to calibrate the gauge in the same position as it is used in the process. Manufacturer’s specifications for the operation/mounting position should also be taken into account.
9. Generating pressure：To calibrate a pressure gauge, you need to source the pressure applied to the gauge.
10. Pressurizing / exercising the gauge Due to its mechanical structure, a pressure gauge will always have some friction in its movement, and may change its behavior over time, therefore you should exercise it before calibration：
11. Reading the pressure value (resolution)The scale in analog pressure gauges have limited readability. It has major and minor scale marks, but it is difficult to accurately read the pressure value when the indicator is in between the scale marks. It is much easier to see when the needle is exactly at a scale mark.
12. Number of calibration points
13. Hysteresis (direction of calibration points)
14. “Tapping” the gauge
15. Number of calibration cycles (repeatability)
16. Adjustment / correction
17. Documentation – calibration certificate
18. Environmental conditions
19. Metrological traceability
20. Uncertainty of calibration (TUR/TAR)
Chengdu Yuanjiyou Technology Co., Ltd. is a professional supplier of all kinds of instrument products, such as pressure gauges,Level Gauge,Speedo gauge and so on .Any questions,please visit us at :jugauge.com