The pressure gauge is a very common instrument. Like many measuring devices, pressure gauges need to be calibrated regularly to ensure their accuracy. There are many factors to consider when calibrating a pressure gauge. This article lists 10 factors you should consider when calibrating a pressure gauge. Before that, we can briefly learn the principle of pressure gauge calibration.
If we simplify the principle of a pressure gauge calibration to its minimum, we can say that when we calibrate a pressure gauge, we provide a known accurate pressure input and read the indication on the gauge, and then document and compare these. The difference in the values is the error and the error should be smaller than the required accuracy for the gauge.
So let’s take a look at the ten things to consider.
1 – Accuracy level
There are many different accuracy levels for pressure gauges. Accuracy levels are specified in the ASME B40.100 (accuracy level in the range of 0.1% to 5%) and EN837 (accuracy level in the range of 0.1% to 4%). The accuracy level specification is usually “% range” which means that if the accuracy level is 1% if the scale range is from zero to 100 psi, the accuracy is ± 1 psi.
Make sure you know the accuracy level of the meter you are calibrating, it may have other effects on the calibration process.
2 – Pressure media
When calibrating pressure gauges, the most commonly used pressure medium is gas or liquid. The gas is usually ordinary air, but in some applications, it can also be a different gas, such as nitrogen. The most common liquid is water or oil. The pressure medium in the calibration process depends on the medium used in the process where the measuring instrument is connected. The media also depends on the pressure range. Low-pressure gauges can actually be used to calibrate air/gas, but when the pressure range becomes higher, it is more practical and safer to use the liquid as a medium.
3 – Contamination
The dirt inside the instrument may enter the calibration equipment and cause harm. With pneumatic meters, you can use dirt/moisture traps, but for a liquid operated gauge, you should clean the gauge prior to calibration.
One of the most extreme process situations is if the instrument is used to measure oxygen pressure. If any grease enters the high-pressure oxygen system while calibrating the meter, it may be very dangerous and may cause an explosion.
4 – Height difference
If the calibration equipment and the instrument to be calibrated are at different heights, the hydrostatic pressure of the pressure medium in the pipeline may cause errors. When gas is used as the medium, this is usually not a problem because gas is lighter than liquid. However, when liquid is used as a medium, the liquid in the pipeline generates weight due to hydrostatic pressure, causing errors. The size of the error depends on the difference in the density and height of the liquid because gravity pulls the liquid in the tube. If it is impossible for the calibrator and the gauge to maintain the same height, the influence of the height difference must be calculated and considered when performing the calibration.
5 – Pipeline leak test
If there are any leaks in the pipeline during the calibration process, unpredictable errors may occur. Therefore, the leak test should be performed before calibration. The simplest leak test is to pressurize the system, then allow the pressure to stabilize for a period of time, and monitor that the pressure does not drop too much. Some calibration systems (pressure controllers) may be able to maintain pressure even in the event of a leak if it has a continuous controller to adjust the pressure. In this case, it is difficult to find leaks, so the controller should be turned off to enable a closed system for leak testing. In closed systems, especially in gaseous media, the adiabatic effect should also be considered.
6 – Calibration/installation position
Since the pressure gauge is a mechanical instrument, its position will affect the reading. Therefore, we should place the meter and the calibrator in a similar position during the calibration process. The manufacturer’s specifications for the operating/installation location should also be considered.
A typical specification for the installation position is that a 5-degree change in position should not change the pressure gauge indication more than half of the accuracy level (0.5 times).
7 – Generating pressure
To calibrate a pressure gauge, you need to source the pressure applied to the gauge. There are different ways to do that: you can use a pressure hand pump, a pressure regulator with a bottle, or even a deadweight tester. A deadweight tester will provide very accurate pressure and you don’t need a separate calibrator to measure the pressure, but the deadweight tester is expensive, not very mobile, requires a lot of attention to the use and it is sensitive to dirt.
8 – Pressurizing / exercising the gauge
Due to its mechanical structure, the pressure gauge will always have some friction in use and may change its performance, so you should exercise it before calibrating it. This is especially true if the pressure gauge has not applied pressure for a period of time. To exercise, provide the rated maximum pressure, let it stay for one minute, then release the pressure and wait for one minute. You should repeat this process 2-3 times before starting the actual calibration cycle.
9 – Adiabatic effect
In a closed system with gas as the pressure medium, the temperature of the gas affects the volume of the gas, which has an effect on the pressure.
When the pressure increases rapidly, the temperature of the gas will rise. This higher temperature causes the gas to expand, resulting in larger volume and higher pressure. When the temperature starts to decrease, the volume of the gas becomes smaller, which will cause the pressure to drop. This pressure drop looks like a leak in the system, but it is actually caused by an adiabatic effect due to changes in gas temperature. The faster the pressure changes, the greater the impact. As the temperature stabilizes, the pressure change caused by this effect will gradually decrease.
So, if you change the pressure quickly, make sure to let it stabilize for a while before judging whether the system is leaking.
10 – Torque force
Especially for torque-sensitive pressure gauges, do not use excessive force when connecting the pressure connector to the pressure gauge, as this may damage the pressure gauge. Follow the manufacturer’s instructions to obtain the allowable torque. Take the time to use the proper tools, proper adapters, and seals.
The above are ten issues that we need to pay attention to when calibrating the pressure gauge. So what are some other factors to be aware of? Let’s discuss it together.
