Calibration, Calibration process success factors

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Summary

Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device.

Details

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 extent of the calibration program exposes the core beliefs of the organization involved. The integrity of organization-wide calibration is easily compromised. Once this happens, the links between scientific theory, engineering practice and mass production that measurement provides can be missing from the start on new work or eventually lost on old work.

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.

Every organization using oscilloscopes has a wide variety of calibration approaches open to them. If a quality assurance program is in force, customers and program compliance efforts can also directly influence the calibration approach. Most oscilloscopes are capital assets that increase the value of the organization, in addition to the value of the measurements they make. The individual oscilloscopes are subject to depreciation for tax purposes over 3, 5, 10 years or some other period in countries with complex tax codes. The tax treatment of maintenance activity on those assets can bias calibration decisions.

New oscilloscopes are supported by their manufacturers for at least five years, in general. The manufacturers can provide calibration services directly or through agents entrusted with the details of the calibration and adjustment processes.

Very few organizations have only one oscilloscope. Generally, they are either absent or present in large groups. Older devices can be reserved for less demanding uses and get a limited calibration or no calibration at all. In production applications, oscilloscopes can be put in racks used only for one specific purpose. The calibration of that specific scope only has to address that purpose.

This whole process in repeated for each of the basic instrument types present in the organization, such as the digital multimeter pictured below.

Also the picture above shows the extent of the integration between Quality Assurance and calibration. The small horizontal unbroken paper seals connecting each instrument to the rack prove that the instrument has not been removed since it was last calibrated. These seals are also used to prevent undetected access to the adjustments 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.

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External Links

  • WikipediaMaster Calibrators Association - Industry Experts on Electrical Calibration, Adjustment and MetrologyElectrical Calibration VideoVideo which explains the relationship between Calibration - Traceability - Accreditation with flow measuring devices

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