Pressure Gauge Calibration
As with any measurement device, pressure gauges need to be calibrated at regular intervals. A Beamex Whitepaper looks at the most common considerations when calibrating pressure gauges.
Pressure gauges are commonly used instruments. As with any process measurement device, they should be calibrated at regular intervals. When talking about pressure gauges, it is normal to refer to analogue pressure indicators which are provided with a pointer needle and a pressure scale and are often built with a Bourdon tube, diaphragm or capsule - a mechanical structure that moves the pointer across the scale as pressure increases.
Digital pressure gauges have a numeric pressure indication instead of an analogue pointer. While this article focuses on analogue gauges, most of the principles are also valid for digital gauges.
Put simply, a pressure gauge calibration requires a known accurate pressure input to be input to the gauge and read and compared with the gauge reading these. The difference in the values is the error and this should be smaller than the required accuracy for the gauge.
Some of the most common things you should consider when calibrating pressure gauges include:
Accuracy classes: Pressure gauges are available in many different accuracy classes, 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. It is important to know the accuracy class of the gauge being calibrated, as this will naturally specify the acceptable accuracy level.
Pressure media: When calibrating pressure gauges, the most common pressure media will be 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 air/gas, but as the pressure range gets higher, it is more practical and also safer to use liquid as the media.
Contamination: While installed in a process the pressure gauge uses a certain type of pressure media, which 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 back to process. Also, the other way around, sometimes the process media could be harmful to your calibration equipment.
Height difference: If the calibration equipment and the gauge to be calibrated are at different heights, the hydrostatic pressure of the pressure media in the piping can cause errors. This should not be an issue when gas is used, but when liquid is used as media, errors can occur. If it is not possible to have the calibrator and gauge at the same height, then the effect of the height difference should be calculated and taken into account during the calibration.
Leak test of piping: If there are leaks in the piping during calibration, unpredictable errors can occur so a leak test should be done prior to calibration. A simple leak test is pressurising the
system and letting the pressure stabilise. Some systems can maintain the pressure even with a leak if it has a continuous controller adjusting the pressure. So the controller should be closed. Adiabatic effect should also always be taken into account in closed system, especially with gas a media.
Adiabatic effect: In a closed system with gas as the pressure media, the temperature of the gas will affect its volume, which has an effect on the pressure.
When pressure is increased quickly, the temperature of the gas will rise, making it expand, resulting in a bigger volume and higher pressure. When the temperature cools, the volume of the gas becomes smaller and causes the pressure to drop. The faster the pressure is changed, the bigger the effect. The pressure change caused by this effect will gradually get smaller as the temperature stabilises. So, if you change the pressure quickly, make sure it is stabilised before deciding whether there is a leak in the system.
Calibration/mounting position: Because pressure gauges are mechanical instruments position will affect the reading so the gauge should be calibrated in the same position that it is used in the process.
Generating pressure: To calibrate a pressure gauge, you need to source the pressure applied to the gauge. This can be done using a pressure hand pump, a pressure regulator with a bottle or even a dead weight tester. A dead weight tester will provide an accurate pressure and you don't need a separate calibrator to measure the pressure, but it is costly, not very mobile, requires a
lot of attention, and is sensitive to dirt. It is more common to use a pressure calibration hand pump to generate pressure and an accurate pressure measurement device to measure the pressure.
Pressurising/exercising the gauge: Due to its mechanical structure, a pressure gauge will always have some friction in its movement, and may change its behaviour over time, so exercise it before calibration. To do this supply the nominal max pressure and let it stay for a minute, then vent the pressure and wait a minute. You should repeat this process 2-3 times before starting to do the actual calibration cycle.
Reading the pressure value (resolution): The scale in 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 recommended that the input pressure is adjusted so the needle is exactly at an indication mark, and then the corresponding input pressure is recorded. It is also important to look at the indication perpendicular to the gauge scale. Many accurate gauges have a reflecting mirror along the scale, behind the needle pointer. This should be read so that the mirror reflection of the needle is exactly behind the actual needle.
Number of calibration points: The different accuracy classes of gauges will determine the number of calibration points. For the most accurate gauges (better than 0.05%) you should use the 'comprehensive calibration procedure' and the calibration should be performed 11 calibration points across the range (zero point plus 10% steps) with three cycles in rising and falling pressure. For the medium accuracy class gauges (0.05 to 0.5%), use a 'standard calibration procedure' with 11 points, but less repeated cycles. The less accurate gauges (class equal or greater than 0.5%) are to be calibrated with the 'basic calibration procedure' with six calibration points (zero point plus 20% steps) with rising and falling pressure.
Hysteresis (direction of calibration points): Hysteresis will result in an indication that is not exactly the same when a pressure point is approached with an increasing pressure compared to a decreasing pressure. To find the amount of hysteresis, calibrate the gauge with increasing and decreasing calibration points.
Number of calibration cycles (repeatability): The calibration cycles are repeated several times to determine the repeatability of the gauge under calibration. If the gauge to be calibrated has bad repeatability, it will give different results during different calibration cycles. If you only calibrate it with one cycle, you will miss the repeatability information. The most accurate gauges should be calibrated with three calibration cycles.
Adjustment/correction: If the 'As Found' calibration is not within the accuracy requirements, something needs to be done. In most cases the gauge should be adjusted so that it will be within the allowed tolerance levels. After adjustment, the gauge needs to be calibrated again. If it is not possible to adjust the gauge, then a correction coefficient can be calculated and this coefficient must betaken into account in normal usage. If the gauge has a big error, then it is best to repair/replace it.
Calibration certificate: The calibration certificate should document the applied pressure and the indication of the gauge as well as an error calculation. It must contain other information also, as stipulated with standards/ regulations, including calibration uncertainty.
Environmental conditions: Most gauges have temperature effect specified and this should betaken into account. Environmental conditions (temperature and humidity) during the calibration should be recorded in the calibration certificate.
Metrological traceability: As with any calibration, the reference standard used to measure the applied pressure to the gauge must have a valid calibration certificate and its calibration must be traceable to the appropriate standards.
Uncertainty of calibration (TUR/TAR): With any calibration, you should be aware of the total uncertainty of the calibration measurements, otherwise the result will not have much value. The awareness of calibration uncertainty seems to be rising and it is also now included in more relevant standards and regulations. In some areas the TUR (Test Uncertainty Ratio) or TAR (Test Accuracy Ratio) is something that is used instead of the uncertainty calculation.
From a White Paper produced by Beamex
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