Pressure measuring instruments, due to the different critical factors of their process, must offer high reliability, sensitivity and accuracy in their measurements. Therefore, periodic preventive maintenance, including calibration, is required.

The time interval at which any measuring instrument should be calibrated depends on its use. Calibration periods should 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.
• Stability history.
• 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 from 0.1 to 5% of scale range) and EN 837 (accuracy classes from 0.1 to 4% of scale range).

Depending on whether your pressure gauge is of better 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 locations in your process have the same need for accuracy. The specifications of the installed equipment, versus 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 high accuracy need, it can be calibrated less frequently.

The criticality of a measurement location is an important factor related to the time interval at which a pressure gauge should be calibrated. Instruments installed in more critical locations naturally need to be calibrated more frequently than a less critical location.

Workload or operating conditions: The workload and operating conditions of use can also affect how often a pressure gauge should be calibrated. If a measuring instrument is used very frequently and/or is installed and operating under very demanding or aggressive operating conditions, it is good to calibrate it more frequently.

Stability history: A record of periodic intermediate verifications of a pressure gauge should be kept in order to establish its stability history, as this is a very important aspect to establish the calibration time interval. If the gauge has a long history and is found to be very stable, it can be calibrated less frequently. On the contrary, if the history shows that the instrument shows systematic deviations in its measurement and often fails to recalibrate, it should logically be calibrated more frequently.

Standardized methods and applicable calibration software are available to show historical trends in behavior and will therefore help to perform the analysis.

Drift: Knowing the deviations of a pressure gauge can help us in the analysis of calibration frequencies. The drift is the difference of the immediate previous deviation of a pressure gauge in a calibration minus the deviation found in the current calibration.

Drift is calculated by performing repeated calibrations and if it is greater than expected, the calibration frequency should be increased in order to decrease 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 gauge.

Also in some areas of the companies there are regulatory requirements, standards or quality systems that specify how often the instrument should be calibrated.

In some applications, the costs and/or losses of a measurement failure are so high that it is more economical to calibrate 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 in any critical location.

In many industries, the quality of the final product can be tested either by measuring the product or by measuring the product itself.