No matter how careful we are at any given task, mistakes often are made; the calibration of gages and instruments is no exception. Sometimes the mistakes are hidden by a quantity of repeatable numbers. At other times, the fact that two or more laboratories offer the same calibrated values is used as proof that the numbers are good when both could be making the same mistake.
While it will be assumed that newcomers to the calibration field are the greatest source of such problems, this assumption itself is a mistake. Many experienced hands in the game are prone to making mistakes as well, but because they’ve been at it for so long, it is assumed they know what they are doing.
There is a lot of “how to do it” literature out there but little “how not to do it” information-a void I’ll attempt to fill in the next few columns. Where will the information come from to fill this gap in our knowledge? Mistakes made by others-a very reliable source. Equipment makers and experienced calibration laboratories encounter these mistakes all the time, particularly in the heat of battle over measurements.
This column will deal with some general mistakes, while future columns will look at mistakes often encountered in the calibration of specific gages and instruments.
Many companies purchase software to avoid buying the standards, but in the real world, at this time, the printed specification is the law-not the software. And there can be significant differences between the two. In a similar vein, some folks rely on a general purpose handbook for such information. In both cases, either source could be out of date since the source documents are usually reviewed every five years.
Another mistake made by calibration facilities is to use the markings on gage handles as the “standard” to which they are calibrated. They can be in error in some cases, or where foreign specifications are involved, may be marked with product dimensions, not gage dimensions.