How often should you calibrate?
There is no recommended fixed period of time for re-calibrating pressure instruments since they all differ from each other in how accurately they measure pressure and how stable they are over time. Ideally if pressure measurement performance is critical to your business operation it is advisable to calibrate all pressure measuring devices as frequently as possible. Once at least 3 sets of calibration test results has been collected, the data can be compared to determine whether the re-calibration cycle can be lengthened.
How accurate does a pressure calibrator need to be?
This will depend on the required of the devices being calibrated. For example a process may only require a 1% accuracy for a pressure gauge which has an accuracy of 0.25% FS. So although the device is capable of achieving a 0.25% accuracy it only needs to be verified to be within 1%. Ideally you should try to aim for a 10 to 1 ratio between calibrator and device under test accuracy, but a lower ratio is often accepted, since many devices now offer much higher accuracies, making it more difficult to source a calibrator which is cost effective.
Can a master pressure gauge scaled in psi be used to calibrate a process pressure gauge scaled in bar?
Yes it can, first convert the pressure range of the master gauge to determine it's equivalent value in bar, e.g. a 30 bar range is approximately 450 psi and would be suitable for calibrating process gauges up to that range. How low a range can be calibrated will depend on the difference in precision between the two instruments. When converting pressure units for calibration purposes make sure it is an accurate conversion so that no additional errors are introduced.
How do you read the error of an analogue dial pressure gauge when the reading is between two dial marks?
Apart from precision test gauges the dial marker (cardinal point) can be separated by a large gap without any graduation making it difficult to determine the exact reading. When calibrating these types of gauges it is best to set the required pressure using the device under test rather than the calibrator since it is easier to set the pressure to a cardinal point. The error can then be read more accurately from the calibrator where you will be able to read the pressure with much better resolution.
e.g. A 30 bar gauge with 1 bar incremental marks is calibrated using a 30 bar calibrator with 0.1 bar incremental marks or resolution. To calibrate at the mid point of 15 bar, apply pressure to the gauge until the dial needle is lined up exactly with 15 bar cardinal point. Next note the pressure on the calibrator which might read something like 15.3 bar. If the calibration was performed the other way around you would have to approximate the 0.3 bar error since there is only a 1 bar reading resolution.
Why do calibration procedures include increasing and decreasing points?
Many pressure measuring instruments utilize a mechanical sensing device which relies on the deformation of a material when pressure is applied. When a material is flexed, it will change shape slightly differently when the stress is released and the material is allowed to return to its original shape. This effect is called mechanical hysteresis and is what causes the difference in readings between increasing and decreasing pressure points. When testing a pressure instrument the accuracy should be checked over both increasing and decreasing pressures to account for any hysteresis related errors. When considering hysteresis it is important to use a calibrator that has a much smaller hysteresis characteristic than the device under test.
What does "traceable" or "traceability" mean in relation to pressure calibration?
Most countries have a National Standards Laboratory which is tasked with providing and maintaining the countries most accurate measurement instruments which are the primary source of the countries measurement standards. In order to ensure all measurement devices are within the expected level of accuracy performance they must be regularly checked using a more accurate calibrator. The calibration is only truly valid if the calibrator accuracy has been verified. The validity of a calibrator's accuracy is determined by the quality of it's calibration, therefore a traceable calibration certificate should include some form of statement indicating that it can be traced back to national standards. If necessary it should be possible for a calibration certificate to provide enough information to enable a person to trace back the calibration of calibrators through the hierarchy of calibration equipment used by the manufacturer or service provider ultimately to the national standard. The actual cal cert does not have to include the complete trace back to the national standard, but it is expected that if a particular business was to be audited it should be able to prove traceability by producing calibration records.
In the UK what is the difference between a traceable calibration and a UKAS calibration?
A traceable calibration certificate may only include a statement such as "traceable to national standards" or the type and serial number of the calibration equipment. There is no way to verify this without auditing the company quality assurance system to validate the statement. Therefore without the benefit of a quality audit the authority of a traceable cal cert is based on the trust and reputation of the supplying company. To provide a more trusted calibration certificate, some companies choose to become UKAS (United Kingdom Accreditation Service) certified. This involves an assessment of the calibration equipment and staff to make sure they both meet a required standard and subsequent assessments are carried out to maintain accreditation. If a company is UKAS approved it is able to issue its own UKAS approved calibration certificates.
What is the difference between a primary and secondary standard?
A Primary Standard uses technology that measures pressure using fundamental parameters such as mass and area in the case of a Dead Weight Tester or a head of fluid (e.g. water, mercury) in the case of a Liquid Column Manometer. A Secondary Standard is one that measures pressure indirectly via a gauge or sensor and should be calibrated on a regular basis using a more accurate secondary or primary standard.
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