I hate to be the bearer of bad news, but I bet if you are involved in a calibration program, you are doing one or more of these things. I’ve been in the calibration industry for 20 years now, and this list compiles some of the most common issues I come across on a regular basis. I’ve put together this list to help everyone involved with calibration to understand better some of the things you need to be thinking about.
1. your calibration cycles are arbitrary
How have you established your calibration cycles? Most instrumentation manufacturers will have a recommended calibration cycle time, typically annually. But, does this cycle time fit your requirements or use of equipment. I just spoke with a customer that has humidity probes that we calibrate. They send them in once a year, and nearly every time they do, they require adjustment. These are suitable quality probes that I am very familiar with. It turns out the customer uses them to check humidity chambers they have at 90% Rh. The units are subjected to this high humidity environment, which causes condensation on the probe on a regular basis and is aging the sensors faster than the manufacturer anticipated. I asked the customer if he had considered shortening his cycle times and his response to me was that “he didn’t know he was allowed to do that.”
2. you’re not considering your measurement uncertainty
Every time you send your instrument to me, I calibrate it and provide you with a measurement of uncertainty. If you send your device to someone else, they may even charge you extra just to have that measurement uncertainty included on your certificate. Why? Are you using this data? Measurement uncertainty is an essential part of calibration, without it, there is no traceability.
Metrological traceability is defined as the property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.
If you are using your instrument to make measurements, and you are not considering uncertainty, you are doing it wrong.
3. you’re making promises You can’t keep
I see this one frequently. These typical knee-jerk reactions to auditors and audits. “From now on everything we calibrate will conform to…” I have a client that wants all of his calibration certificates to reference a heat-treating standard (AMS 2750 E). About one-third of his equipment is relevant to the standard. But that’s what he put in his calibration program. All calibrations will comply with AMS 2750, current rev. Think about broad generalizations before you write them into your quality system.
4. you’re using the phrase “NIST traceable.”
Sorry to tell you this, but not everything that gets calibrated in the USA is traceable to NIST, and even if it were, who would NIST be traceable to? Part of the concept of calibration is to ensure an “inch” is an “inch” no matter where that “inch” is located. Metrological Traceability should link back to the International System of Units (the SI), not just any one National Metrology Institute (NMI).
To further complicate things, in the world of metrology, NIST traceable is a code name that means an “unaccredited” calibration. Accredited calibrations are assumed to be traceable and have the further assurance of being assessed by a recognized accreditation body (AB). NIST traceable is frequently used to describe a calibration that is not on the organization’s scope of accreditation.
5. you don’t understand your calibration certificates
Do you know how to tell if your calibration certificate is for an accredited calibration? Hint, it should have the logo of an AB on it somewhere. Do you know what data in the document is from your device and which is from the calibration standard? Do you see how a statement of compliance is being made on your certificate? Do the test points on the certificate make sense for how you use the device?
These and other questions like them are frequent issues that come up in audits. It’s not good enough in this day and age to just say “it’s calibrated” and have an auditor accept that.