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Equipment calibration

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Definition of maximum permissible error for a monitoring & testing equipment

The maximum permissible error will be, at best, the accuracy specified by the manufacturer of the equipment. Of course, this specification should be confirmed by a calibration and before the end of warranty period. After that, if the equipment does not have the accuracy specified by the manufacturer, it is no longer his problem but yours. It is perfectly acceptable to consider a maximum permissible error higher than specified, taking in accounts 2 economical factors:

  • Calibrations costs: higher is the maximum permissible error, higher can be the calibrations interval. Measuring equipment drift with time and therefore its accuracy also degrades. This drift is the major factor when defining calibration interval. Now, if we increase the maximum permissible error of the equipment, its permissible drift is also higher and consequently calibration interval can be larger. The advantage is of course reduction of calibration costs. The increase of maximum permissible error can also allow cheaper calibrations, with less accurate references, possibly making internal calibrations instead of external ones.
  • Process Tolerances: any measurement made with an equipment will have to consider its accuracy (=maximum permissible error). For instance, consider that a caliper is used to measure screws with a tolerance of 10+/-1mm and the caliper has a maximum permissible error of 0,5mm. This means that any screw with a width higher than 10,5 mm or lower than 9,5 mm will be rejected. Consequently there is a reduction of the process tolerance. In our example if the probability of width of screws between 9 and 11 mm was uniform, half of it would be rejected.

However, when the equipment is electrical, like a multimeter, osciloscope or spectrum analyzer, it may be easier to simply follow the maximum permissible error specified in its service manual, because most of the time the calibration lab will have an automated calibration software that will print, in the calibration certificate, the comparison of obtained errors (+uncertainty) with the maximum permissible error specified by the manufacturer, which means less work for you.

In conclusion, the definition of the maximum permissible error will be a compromise between:

  • cost of equipment calibration;
  • cost of its accuracy reduction, meaning, in a production environment, the cost of rejecting materials or products that are actually good;
  • for electrical equipment, if calibration certificate includes comparison with maximum permissible error.
For more information: More information     

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