Not even the most accurate pressure instruments will hold their accuracy indefinitely, all are prone to drift over time.
Any device that is constructed to measure pressure is going to have some complexity in its construction incorporating an assortment of materials with different physical properties. When exposed to many pressure and temperature cycles these materials will expand and contract and drift by varying degrees from their original state.
Accuracy is defined in many different ways but typically there is no time factor incorporated into it. For example when a manufacturer builds a pressure sensor it will be tested at the end of the production process to ensure it is inside specification. This is a controlled environment which the manufacturer can guarantee the accuracy, but as soon as it leaves the factory it is impossible to predict what it will be subjected to during its service life.
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Some manufacturers will include long term stability in the overall accuracy statement or as a separate specification parameter. This is most common with calibration equipment where accuracy over time is more critical. Calibration equipment is less likely to be subjected to extreme process temperatures or a high number of pressure cycles, so it is easier to include a stability figure that is realistic for most applications. However, predicting the likely drift of pressure instrumentation that is fitted to a process is more difficult since the amplitude and frequency of process temperatures and pressures will vary from one application to another.
So that a pressure instrument does not drift outside its specification limits too quickly, a manufacturer will either use pressure sensing technology that is inherently stable over time or will widen the accuracy to a more conservative figure, so that even if it is used at the extremes of its specification it will not drift outside the stated accuracy limits over time.
The main component of drift for many pressure sensing technologies is the stability of the zero point. For a gauge reference pressure measuring instrument this is very easy to trim out on a regular basis since exact zero pressure is easily achieved by venting the pressure port to atmospheric pressure.
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Adjusting the upper or full scale point is more difficult since you need an accurate way of setting pressure. This is achieved by using a pressure calibrator which is able to apply the required pressure and provide a very precise pressure reading that is ideally 10 times more accurate than the pressure sensing device you are calibrating.
As a rule of thumb if the accuracy of your pressure measurement instruments should be calibrated at least once a year.
The only way to truly understand what calibration interval is necessary to keep the long term stability in check is to start calibrating on a frequent basis, possibly once per month if you have your own pressure calibration equipment in-house and record how much drift there has been since the last calibration.
After a few months there will be an emerging pattern in the stability of the pressure instrument and you can start to stretch out the re-calibration interval to a period where the required accuracy will not be compromised. It is important to note that if the typical environmental or process conditions change significantly the re-calibration interval should be reduced again to determine whether the changes have made any impact on the long term drift of the pressure instrument.
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