Let's discuss some different types of calibration. Do you always need the accredited calibration? What are some alternatives? Are there different types of accredited calibration? What happens if you your particular spec is the best in the world and there isn't a standard that is better than yours? Let's get started.
Alright, you get it. Calibration is important. My last post generated a good bit of discussion on the Test and Measurement World forum on LinkedIn. A good point from the discussion is to properly label your instruments if they are not in a normal calibration cycle. This way they wouldn't accidentally be used in a critical application where calibration accuracy is important. That's a best practice for those of you who don't calibrate all of the equipment in your lab.
A traceable calibration by an accredited facility is expensive. Not only do you need to pay for the calibration to be performed, you don't have your instrument for a period of time and this affects your productivity. To top it off, your instrument might go out of spec in between calibration cycles which then puts all of your measurements between that calibrations in question. If your equipment needs to be traceable, however, maintaining a regular calibration cycle is the only way to prove to your auditors that you are traceable.
One best practice that we implemented in our production test systems is a nightly system self test. The goal is to catch malfunctioning or drifting working standards before they impact our customer's products. We design our ATE systems to have a higher or equivalent accuracy instrument to check our working standards. The standards are then checked against this "self test" instrument nightly to determine if either one has malfunctioned or drifted out of tolerance. If the self test fails, then production needs to follow up and determine which instrument is at fault. If the working standard is at fault, all products from the previous day are recalled for re-calibration. It is unlikely that both the self test instrument and working standard drifts out of tolerance the same amount and in the same direction. This method helps us to minimize our exposure to shipping out of tolerance products. Rather than having a year's worth of questionable product, we have limited it to a day's worth and often these products are still in our warehouse and haven't shipped to customers.
Often your lab or auditors might require more than just a calibration certificate to prove traceability. Most test and measurement manufacturers offer calibrations with data at extra cost. These are often called ISO17025 standard calibrations or Z540 standard calibrations in the US. Not only do these calibrations offer data, but the calibration providers have documented their uncertainties and methodologies to a higher level to ensure that they meet the respective standards. These calibrations are especially useful for those that need the uncertainties for their individual pieces of equipment and don't rely solely on the datasheet. The reports from these calibrations should include measurements of all specifications for your unit. They should also include the supporting test equipment's measurement uncertainty used to calibrate your instrument.