FEATURE PRESSURE MEASUREMENT
10 - 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, therefore you should exercise it before calibration. This is especially the case if the gauge has not been applied with pressure for a while. To exercise the gauge, 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 two to three times before starting the actual calibration cycle.
11 - READING THE PRESSURE VALUE (RESOLUTION) The scale in pressure gauges have limited readability. Gauges have 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. Therefore, it is recommended to adjust the input pressure so that the needle is exactly at an indication mark, and then record the corresponding input pressure. If you just supply a certain accurate input pressure and then try to read the indicator, it will cause errors due to limited reading accuracy. Also, it is 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 mirror helps you read it, and you should read it so that the mirror reflection of the needle is exactly behind the actual needle. Then you know that you are looking perpendicular /straight at the gauge. If the gauge has a digital indicator, then the
resolution (reading accuracy) is totally different. You can read the digital indicator equally accurate at any point of its range.
12 - NUMBER OF CALIBRATION POINTS The different accuracy classes of gauges will determinate the different number of calibration points. For the most accurate gauges (better than 0.05
per cent) you should use the “comprehensive calibration procedure” and the calibration should be performed 11 calibration points across the range (zero point plus 10 per cent steps) with three cycles in rising and falling pressure. For the medium accuracy class gauges (0.05 to 0.5 per cent), use a “standard calibration procedure” with 11 points, but less repeated cycles. The less accurate gauges (class equal or greater than 0.5 per cent) are to be calibrated with the “basic calibration procedure” with six calibration points (zero point plus 20 per cent steps) with rising and falling pressure. In practice, gauges are sometimes calibrated
with less calibration points. Hysteresis is discussed later on, but in order to find out the hysteresis, the calibration should be done with increasing and decreasing pressure points. Naturally the number of calibration points and cycles depends also on the application, criticality and accuracy requirement.
24 DECEMBER/JANUARY 2018 | INSTRUMENTATION
13 - HYSTERESIS (DIRECTION OF CALIBRATION POINTS) Again, due to its mechanical structure, a pressure gauge may have some hysteresis. This means that the indication is not exactly the same when a pressure point is approached with an increasing pressure compared to a decreasing pressure. In order to find out the amount of hysteresis, you should calibrate the gauge with increasing and decreasing calibration points, i.e. to go first up and then down with pressure. While doing this, it is important to make sure that the pressure moves only in the desired direction. For example, when you calibrate with increasing pressure you must make sure that you do not decrease the pressure at any point when fine adjusting the pressure, as this will cause you to lose the hysteresis information. If you overshoot the target point with increasing pressure, you need to come way back down and then increase the pressure again to the target point.
14 - “TAPPING” THE GAUGE Sometimes a mechanical pressure gauge may need a gentle tapping in order to make sure that it is released from any friction or lost flexibility, especially if it has not been exercised in normal use. During the calibration, once the input pressure is stabilised, you can gently tap the gauge to see if the indication changes. Of course, you need to be careful not to damage the gauge.
15 - NUMBER OF CALIBRATION CYCLES (REPEATABILITY) During calibration, the calibration cycles are repeated several times in order to determinate the repeatability. 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. As mentioned earlier, the most accurate gauges should be calibrated with three calibration cycles. In practice the repeatability is often tested as a type test for certain instrument types (make/model) and once the typical repeatability is known, the actual calibration is carried out, in practice with just a one calibration cycle, taking the typical repeatability into account.
16 - ADJUSTMENT / CORRECTION If the As Found calibration shows that the gauge 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 (As Left) to verify its condition. If it is not possible to adjust the gauge in
question, then a correction coefficient can be calculated and this coefficient must be taken into account in normal usage. This will, of course, make the usage more difficult. If the gauge has a big error, then it is best to
repair/replace it and not try to adjust it, as most likely it will not stay stable in the future.
17 - DOCUMENTATION One crucial aspect for calibration is, of course, to document calibration results in a calibration certificate. The certificate should document the applied pressure and the indication of the gauge as well as an error calculation (difference of applied pressure and indication). Also, the certificate needs to contain other information as stipulated with standards/regulations, including calibration uncertainty. If you make the certificate manually, it means
that you write the gauge’s indication and the applied pressure on paper and then calculate the error manually. You can also use automated calibration equipment that will perform the documentation and calculations automatically and also transfer the results to the computer for calibration software to store/print the results.
18 - ENVIRONMENTAL CONDITIONS Most gauges have temperature effect specified and this should be taken into account. Calibration is most often conducted at room temperature, but the gauge may be used at a different temperature in the process. This difference in temperatures may cause differences in gauge accuracy between calibration and process usage. Environmental conditions during the calibration should be recorded in the calibration certificate.
19 – METROLOGICAL TRACEABILITY As with any calibration, you must ensure that the reference standard you are using to measure the applied pressure to the pressure gauge has a valid calibration certificate and that its calibration is traceable to the appropriate national standards (metrological traceability).
20 - UNCERTAINTY OF CALIBRATION 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 increasingly included in relevant standards and regulation. In some areas the TUR (Test Uncertainty Ratio) or TAR (Test Accuracy Ratio) is something that is used instead of the uncertainty calculation. The concept of this is to make sure that you have a calibrator (or reference standard) that is several times more accurate than the instrument to be calibrated, and if you know this, you do not need to calculate uncertainty. One of the most commonly used ratios is 1:4, meaning that the calibrator’s specifications are four times better than the specifications of the gauge to be calibrated. Using this TUR/TAR ratio method, you are not
aware of all the relevant uncertainty components of your calibration process and you do not know how good the calibration really is. Therefore, calculating the total uncertainty of the calibration is the recommended method. Find out more about calibration here:
https://blog.beamex.com/how-to-calibrate- pressure-gauges
Beamex
www.beamex.com
/ INSTRUMENTATION
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