Purpose and scope The calibration process begins with the design of the measuring instrument that needs to be calibrated. The design has to be able to "hold a calibration" through its calibration interval. In other words, the design has to be capable of measurements that are "within
engineering tolerance" when used within the stated environmental conditions over some reasonable period of time. Having a design with these characteristics increases the likelihood of the actual measuring instruments performing as expected. Basically, the purpose of calibration is for maintaining the quality of measurement as well as to ensure the proper working of particular instrument.
Intervals The exact mechanism for assigning tolerance values varies by country and as per the industry type. The measuring of equipment is manufacturer generally assigns the measurement tolerance, suggests a calibration interval (CI) and specifies the environmental range of use and storage. The using organization generally assigns the actual calibration interval, which is dependent on this specific measuring equipment's likely usage level. The assignment of calibration intervals can be a formal process based on the results of previous calibrations. The standards themselves are not clear on recommended CI values: :
ISO 17025 ::"...shall be calibrated or verified at periodic intervals established and maintained to assure acceptable reliability..." :
ISO-9001 ::"... shall be calibrated at periodic intervals established and maintained to assure acceptable accuracy and reliability...Intervals shall be shortened or may be lengthened, by the contractor, when the results of previous calibrations indicate that such action is appropriate to maintain acceptable reliability."
Standards required and accuracy The next step is defining the calibration process. The selection of a standard or standards is the most visible part of the calibration process. Ideally, the standard has less than 1/4 of the measurement uncertainty of the device being calibrated. When this goal is met, the accumulated measurement uncertainty of all of the standards involved is considered to be insignificant when the final measurement is also made with the 4:1 ratio. This ratio was probably first formalized in Handbook 52 that accompanied MIL-STD-45662A, an early US Department of Defense metrology program specification. It was 10:1 from its inception in the 1950s until the 1970s, when advancing technology made 10:1 impossible for most electronic measurements. Maintaining a 4:1 accuracy ratio with modern equipment is difficult. The test equipment being calibrated can be just as accurate as the working standard. to connect the gauge under test to a reference master gauge and an adjustable pressure source, to apply fluid pressure to both reference and test gauges at definite points over the span of the gauge, and to compare the readings of the two. The gauge under test may be adjusted to ensure its zero point and response to pressure comply as closely as possible to the intended accuracy. Each step of the process requires manual record keeping. An automatic pressure calibrator is a device that combines an electronic control unit, a pressure intensifier used to compress a gas such as
Nitrogen, a
pressure transducer used to detect desired levels in a
hydraulic accumulator, and accessories such as
liquid traps and gauge
fittings. An automatic system may also include data collection facilities to automate the gathering of data for record keeping.
Process description and documentation All of the information above is collected in a calibration procedure, which is a specific
test method. These procedures capture all of the steps needed to perform a successful calibration. The manufacturer may provide one or the organization may prepare one that also captures all of the organization's other requirements. There are clearinghouses for calibration procedures such as the Government-Industry Data Exchange Program (GIDEP) in the United States. This exact process is repeated for each of the standards used until transfer standards,
certified reference materials and/or natural physical constants, the measurement standards with the least uncertainty in the laboratory, are reached. This establishes the
traceability of the calibration. See
Metrology for other factors that are considered during calibration process development. After all of this, individual instruments of the specific type discussed above can finally be calibrated. The process generally begins with a basic damage check. Some organizations such as nuclear power plants collect "as-found" calibration data before any
routine maintenance is performed. After routine maintenance and deficiencies detected during calibration are addressed, an "as-left" calibration is performed. More commonly, a calibration technician is entrusted with the entire process and signs the calibration certificate, which documents the completion of a successful calibration. The basic process outlined above is a difficult and expensive challenge. The cost for ordinary equipment support is generally about 10% of the original purchase price on a yearly basis, as a commonly accepted
rule-of-thumb. Exotic devices such as
scanning electron microscopes,
gas chromatograph systems and
laser interferometer devices can be even more costly to maintain. The 'single measurement' device used in the basic calibration process description above does exist. But, depending on the organization, the majority of the devices that need calibration can have several ranges and many functionalities in a single instrument. A good example is a common modern
oscilloscope. There easily could be 200,000 combinations of settings to completely calibrate and limitations on how much of an all-inclusive calibration can be automated. To prevent unauthorized access to an instrument tamper-proof seals are usually applied after calibration. The picture of the oscilloscope rack shows these, and prove that the instrument has not been removed since it was last calibrated as they will possible unauthorized to the adjusting elements of the instrument. There also are labels showing the date of the last calibration and when the calibration interval dictates when the next one is needed. Some organizations also assign unique identification to each instrument to standardize the record keeping and keep track of accessories that are integral to a specific calibration condition. When the instruments being calibrated are integrated with computers, the integrated computer programs and any calibration corrections are also under control. ==Historical development==