Whether designing dedicated test stands or developing control systems, it may be necessary to calibrate the signals that are being measured. One of our test system specialists has written a detailed article for Control Design magazine, Process variables and the art of calibrating instruments.
In this article, Jim McCarty, walks us through the key elements to a good procedure.
- Accuracy of the test input
- Repeatability and reproducibility
- Stability of the test conditions
- Completeness in the test range
- Controlled test conditions (even for parameters not under test)
- Clear and accurate Pass/Fail criteria
Unfortunately, it is often troublesome to induce a specific measurement condition. For example how do you make your test input be an exact temperature? A discussion on creating a sensible test, understanding repeatability vs reproducibility, knowing short term vs long term stability, and of course, following the KISS principle (KEEP IT SIMPLE …) are reviewed in helping to navigate your calibration process design. Along the way, industry terms, such as "golden units" and "working standards", are defined.
The effort to develop a good calibration test is a smart investment. Jim writes…
"Building automated of semi-automated calibration and SPC capabilities into software is well worth the repeatability and reproducibility it buys you, and it is by far the best way to enforce the traceability required " (for your) " environment. Development costs can be justified by comparing them to the cost of extra labor spent troubleshooting test systems or, depending on the application, escapes and subsequent product recalls. Both the calibration and SPC software capabilities can be built in such a way that doesn’t require re-validation of existing code."
If you are in the mode of coming up with a calibration process with pass/fail results that insure that the performance of your device or system is reliable, the last step may be the most important. Learn what can be done to make a report with clear and accurate criteria as well as meaningful test results. Examples of linearity graphs, data plots, calibration plots, and SPC plots are provided. As Jim points out, this will keep you from wrongly passing or rejecting units based on the test and provide valuable insight if things do go awry.