PRESSURE MEASUREMENT FEATURE


CALIBRATING PRESSURE GAUGES P


ressure gauges need to be calibrated at regular intervals to assure they are accurate.


There are many things to 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 as well as in EN 837 standards. The accuracy class specification most often being “per cent of range” means that if the accuracy class is one per cent and if the scale range is zero to 100psi, the accuracy is ±1psi. Make sure you know the accuracy class of the gauge you are going to calibrate, as this will naturally specify the acceptable accuracy level, but it will also have other effects on the calibration procedure.


2 - PRESSURE MEDIA The most common pressure media are gas or liquid. 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 gas, but as the pressure range gets higher it is more practical and also safer to use liquid as the media.


3 - CONTAMINATION While installed in a process the pressure gauge uses a certain type of pressure media, this should be taken into account when selecting the media for the calibration. You should not use a media during the calibration that could cause problems when the gauge is installed. Also, sometimes the process media could be harmful to your calibration equipment. There can be dirt inside the gauge that


can get into the calibration equipment and cause harm. With gas operated gauges, you can use a dirt/moisture trap, but for a liquid operated gauge, you should clean the gauge prior to calibration. One of the most extreme process situations is


if the gauge is used to measure the pressure of oxygen. If any grease goes into a high pressure oxygen system during the calibration of the gauge, it can be very dangerous and could cause an explosion.


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 the 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


Beamex highlights 20 things to consider when calibrating pressure gauges to guarantee accuracy


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 pressurise the system and let the pressure stabilise 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. Adiabatic effect should also always be taken into account in a closed system, especially with gas as a media, as explained in the next section.


6 - 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 on the pressure. When pressure is increased quickly, the


temperature of the gas will rise, and this higher temperature makes the gas expand, thus creating a bigger volume and higher pressure. When the temperature starts to cool down, the volume of the gas becomes smaller and this will cause the pressure to drop. This pressure drop may seem like a leak in the system, but it is actually caused by the adiabatic effect due to change in the gas temperature. The faster the pressure is changed, the bigger the effect is. The pressure change caused by this effect will gradually get smaller as the temperature stabilises. So, if you change the pressure quickly,


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make sure you let it stabilise for a while before judging that there is a leak in the system.


7 – TORQUE FORCE Especially for torque sensitive gauges, do not use excessive force when connecting pressure connectors to the gauge, as it may damage the gauge. Follow the manufacturer’s instructions for the allowed torque force and 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 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. A typical specification for a mounting position is


that a change of five degrees in position should not change the gauge indication more than half (0.5 times) of the accuracy class.


9 - 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 dead weight tester. A dead weight tester will provide a very accurate pressure and you do not need a separate calibrator to measure the pressure, but a dead weight tester is expensive, not very mobile, requires a lot of attention to use and it is sensitive to dirt. It is more common to use a pressure calibration hand pump to generate pressure and an accurate pressure measurement device (calibrator) to measure the pressure. A pressure controller can also be used to supply the pressure.


INSTRUMENTATION | DECEMBER/JANUARY 2018


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