Editing Calibration (section)

===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.<ref name="JablonskiBrezina2011">{{Citation
| editor1-last = Jabłoński | editor1-first = Ryszard
| editor2-last = Březina | editor2-first = Tomaš
| series = Mechatronics: Recent Technological and Scientific Advances
| last1 = Ligowski | first1 = M.
| last2 = Jabłoński | first2 = Ryszard
| last3 = Tabe | first3 = M.
| title = Procedure for Calibrating Kelvin Probe Force Microscope
| page = 227
| year = 2011
| isbn = 978-3-642-23244-2
| doi = 10.1007/978-3-642-23244-2
| lccn = 2011935381
}}</ref> 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.<ref>{{cite book|title=Military Handbook: Evaluation of Contractor's Calibration System|url=http://www.barringer1.com/mil_files/MIL-HDBK-52.pdf |archive-url=https://web.archive.org/web/20141204234336/http://www.barringer1.com/mil_files/MIL-HDBK-52.pdf |archive-date=2014-12-04 |url-status=live|publisher=U.S. Department of Defense|page=7|access-date=28 November 2014|date=17 August 1984}}</ref>

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.<ref name="JablonskiBrezina2011" /> If the accuracy ratio is less than 4:1, then the calibration tolerance can be reduced to compensate. When 1:1 is reached, only an exact match between the standard and the device being calibrated is a completely correct calibration. Another common method for dealing with this capability mismatch is to reduce the accuracy of the device being calibrated.

For example, a gauge with 3% manufacturer-stated accuracy can be changed to 4% so that a 1% accuracy standard can be used at 4:1.  If the gauge is used in an application requiring 16% accuracy, having the gauge accuracy reduced to 4% will not affect the accuracy of the final measurements. This is called a limited calibration. But if the final measurement requires 10% accuracy, then the 3% gauge never can be better than 3.3:1. Then perhaps adjusting the calibration tolerance for the gauge would be a better solution. If the calibration is performed at 100 units, the 1% standard would actually be anywhere between 99 and 101 units. The acceptable values of calibrations where the test equipment is at the 4:1 ratio would be 96 to 104 units, inclusive. Changing the acceptable range to 97 to 103 units would remove the potential contribution of all of the standards and preserve a 3.3:1 ratio. Continuing, a further change to the acceptable range to 98 to 102 restores more than a 4:1 final ratio.

This is a simplified example. The mathematics of the example can be challenged. It is important that whatever thinking guided this process in an actual calibration be recorded and accessible. Informality contributes to [[tolerance stacks]] and other difficult to diagnose post calibration problems.

Also in the example above, ideally the calibration value of 100 units would be the best point in the gauge's range to perform a single-point calibration. It may be the manufacturer's recommendation or it may be the way similar devices are already being calibrated. Multiple point calibrations are also used. Depending on the device, a zero unit state, the absence of the phenomenon being measured, may also be a calibration point.  Or zero may be resettable by the user-there are several variations possible. Again, the points to use during calibration should be recorded.

There may be specific connection techniques between the standard and the device being calibrated that may influence the calibration. For example, in electronic calibrations involving analog phenomena, the impedance of the cable connections can directly influence the result.