Pressure measurement instruments, due to the different critical factors of their process, must offer high reliability, sensitivity and precision in their measurements. Therefore, periodic preventive maintenance is required, including its calibration.
The time interval in which any measuring instrument must be calibrated depends on its use. Calibration periods must be appropriate to ensure confidence in its measurements.
There are many variables on which this period will depend:
- Accuracy class.
- The criticality of a measurement location.
- The workload or operating conditions.
- The history of stability.
- Drift
- International Standards or specific quality systems.
- Overload outside its allowable overload limit.
- Incorrect handling.
- Uncertainty or tolerance required.
- Manufacturer’s recommendation.
Some international standards recommend fixed recalibration intervals, but deciding when to recalibrate a measuring instrument is the responsibility of the user. Only the user and his quality system know the importance of the quality and reliability of the results or what effect a systematic error will have from a safety, production or financial point of view.
Accuracy class: Pressure gauges are available in different accuracy classes. These are specified in ASME B40.100 (accuracy classes 0.1 to 5% of scale range) and EN 837 (accuracy classes 0.1 to 4% of scale range).
Depending on whether your pressure gauge is of better precision accuracy or not, the calibration interval will also depend. A more accurate pressure gauge should be calibrated more frequently than one of lower precision accuracy.
The criticality of a measurement location: When installing a pressure gauge in your process, it is important to know what the accuracy requirements are for that specific location. Not all places in your process have the same need for precision. The specifications of the installed equipment, compared to the required need of the installed location, will affect the calibration period. If you install a pressure gauge with a high accuracy class in a location that does not have a need for high accuracy, it can be calibrated less frequently.
The criticality of a measurement location is an important factor related to the time interval in which a pressure gauge must be calibrated. Instruments installed in more critical locations should naturally be calibrated more frequently than a less critical location.
Workload or operating conditions: Workload and operating conditions of use can also affect how often you should calibrate a pressure gauge. If a measuring instrument is used very frequently and/or is installed and operating in very demanding or aggressive operating conditions, it is good to calibrate it more frequently.
Stability history: A record of periodic intermediate verifications of a manometer must be kept in order to establish its stability history, since it is a very important aspect to establish calibration time interval. If the gauge has a long history and is found to be very stable, you can calibrate it less frequently. On the contrary, if the history shows that the instrument has systematic deviations in its measurement and frequently fails to recalibrate, logically it should be calibrated more frequently.
There are standardized methods and calibration software applicable to show historical behavioral trends and will therefore help in performing the analysis.
Drift: Knowing the deviations of a manometer can help us analyze calibration frequencies. Drift is the difference of the immediately previous deviation of a manometer in a calibration minus the deviation found in the current calibration.
The drift is calculated by performing repeated calibrations and if it is greater than expected, the calibration frequency must be increased in order to reduce the risk of non-conforming work or product between calibrations.
International Standards or specific quality systems: Some international standards recommend calibration periods depending on the type of manometer.
Also in some areas of companies there are regulatory requirements, standards or quality systems that specify how frequently the instrument must be calibrated.
In some applications, the costs and/or losses of a measurement failure are so high that it is more economical to calibrate instruments more frequently than to wait for the instrument to fail. This is especially the case in the Pharmaceutical, Food and Beverage and other regulated Industries, or anywhere critical.
In many Industries, the quality of the final product can be tested both by measuring the final product and by making various measurements during the manufacturing process. These measurements must be maintained with precision and reliability and this can only be achieved with periodic calibrations.
Overload outside its permissible overload limit: An overload outside the limits established by the manufacturer for a certain pressure gauge can cause a deviation in its measurements since the sensing element would not withstand said overload and could deform. In this case, it is essential to place the instrument out of service until it is calibrated again, to determine if the damage suffered by the instrument is high or can be ignored and calibration alone will suffice. If the damage is considerable, it should be analyzed whether it is worth continuing to use it or replacing it with another pressure gauge.
Incorrect handling: Correct handling of a measuring instrument is a very important part of its correct functioning. The manufacturer’s recommendations regarding correct handling and assembly must be taken into account.
Poor handling or assembly could lead to erroneous readings.
Uncertainty or tolerance required: A manometer with very good technical specifications that is used for measurements with much lower accuracy than its tolerance, should have longer calibration periods. The problem with this is the costs. Equipment with better technical specifications is more expensive, so it may be more economical to calibrate an instrument more suitable for the desired measurement accuracy more frequently. This should be analyzed before purchasing the pressure gauge. On the other hand, if equipment is calibrated or verified very frequently it could be used with less uncertainty under certain conditions of periodic verifications.
Manufacturer’s recommendation: Many manufacturers have a recommendation for the calibration period of their equipment. This case occurs mainly for standards and reference calibrators. Manufacturers know best how their equipment performs and how it behaves over time. The quality systems of the Industries that manufacture this type of instruments carry out tests on their equipment before making them available to the market. These studies and quality tests allow them to specify a typical long-term stability over a given period of time, usually over a year.
Therefore, the manufacturer’s recommendation is a good starting point for deciding the initial calibration period for a pressure gauge. Of course, over time, the stability of the device must be monitored and the calibration interval adjusted accordingly.