Surely " CALIBRATION" would involve some sort of adjustment, but it doesnt. All they do is check your kit for accuracy, Why dont they call it an "Accuracy check"? Oh wait a mo.... then they would not be able to charge as much!!! Absolute CON.
Errors in Measurement?
No measurement is perfect. However, the imperfections of a measurement must be established and they are normally quoted as the tolerance or error of measurement in relation to a standard or absolute value. Thus a system of comparisons is established whereby any measurement made can be related to the standard value. “What is a working standard?” below gives a possible chain of relationships by which everyday measurements in electrical quantities may be evaluated in terms of the accepted standard values.
What is Calibration?
Calibration is the process of comparing the accuracy of instruments readings to known calibration sub-standards held at a calibration laboratory whose traceability and accuracy is derived and maintained by comparison with National or International primary calibration standards such as those held at the National Physical Laboratory (NPL) in UK and at the National Bureaux of Standards (NBS) in the USA.
National and International Primary Standards?
Organisations such as NPL and NBS and their equivalents in other countries have expended considerable effort in determining and maintaining the absolute value of electrical quantities. Quantities such as the Volt, Ampere, Ohm, Farad, Henry and Hertz etc, have been determined in terms of the fundamental quantities of mass, length, and time with the greatest precision possible although uncertainty still exists in their exact values even if only a few parts in 1000000000 (10[SUP]9[/SUP]
). All this effort so that 1 Russian Volt = 1 UK Volt = 1 USA Volt = 1 Australian Volt etc and performance of equipment and measurement of physical phenomena are made on a common basis.
What is a Working Standard? The calibrator that calibrated your MFT or meter, in metrology terms is called a working standard eg. it does the day to day work of calibrating instruments. The calibrator was itself compared with what is known in metrology as a sub-standard which is held at the calibration laboratory, this does not mean there is anything defective about it, merely that it has been derived from an absolute standard which is known in metrology terms as a primary standard, which itself has been derived from the fundamental standard quantities of mass, length and time.
Definitions from United Kingdom Accreditation Service (UKAS) website:
Calibration: - is defined as specific types of measurement performed on measurement standards, material measures and measuring instruments to establish the relationship between the indicated values and known values of a measured quantity. The term covers calibrations carried out using appropriate reference equipment at any location.
Adjustment: - defined as the operation of bringing a measuring instrument into a state of performance suitable for its continued use.
Sometimes, however, the word calibration is misused to describe the process of altering the performance of an instrument to ensure that the values it indicates are correct within specified limits (e.g. adjusting an instrument until its reading agrees with that of an instrument or standard of greater accuracy than the instrument calibrated). Although the nature and magnitude of the adjustment is determined by a pre-adjustment calibration, known in metrology as an “as found calibration”.
The procedural sequence:
- First calibration (to determine the approximate magnitude of the adjustment needed).
- Adjustment.
- Second calibration.
The results from the calibration are needed for the instruments reproducibility to be estimated (essentially its change in characteristics over a long time interval). This is what in metrolgy terms are called the calibrated uncertainties and was highlighted and questioned by Tony on his ludicrous calibration certificate.
In summary, calibration is checking the tester on every function and every range to see that it is within specification, certifying it as fit for continued use.
Adjustment is not calibration, this is the biggest misunderstanding of people outside the metrology environment.
However, when you send your MFT or meter for calibration and the as found calibration indicates that the instrument is outside its specification and that the change of adjustment required to restore it to within the specification is within the permitted adjustment range. The calibration engineer will adjust the instrument, re-calibrate it and certify that it now meets its specification. This, plus any initial firmware update available, will normally be included within the agreed calibration fee. If component changes are required or other repair or replacement parts you will be notified of the additional cost before final calibration, as this will usually be extra.
Statistics from UKAS:
Suggest that 65% of instruments do not require adjustment, that 10% need some adjustment, 5% need component changes to allow them to be brought back within limits, 5% require a semiconductor device, display or memory replacement, 5% require a firmware restore or upgrade, 5% require replacement leads, probes, batteries or fuses and 5% require repair or replacement of protective cases. Note:- These figures are for electrical and electronic instruments in general and not specifically for electricians MFTs and other meters which may well be different values.
