Have you given much thought as to where calibration tolerances come from? Unless you provided these to your calibration supplier (or internal cal lab), these should be solely based upon the specifications that the Original Equipment Manufacturer (OEM) expressly states regarding the expected performance that the instrument model will maintain over its calibration interval.Now that you've gone this far in the thought process, then consider this: If the performance specifications come from the OEM and are associated with the model number, then what happens for my instruments that don't have a model number? What specs are applied to the calibration in that situation? Well, there is usually still a model number, just not on the instrument. There are a number of ways to track down the correct model number but, if all possibilities are exhausted and you still can't find one, then there are only two options left: provide the cal lab with the tolerances that will fit your needs or go with a "Data Only" calibration; the latter option is not recommended.
Here's the breakdown:
Rely on the model number to get to the right performance specs for your instrument, unless superseded by your own custom requirements/specs
If no model number shown on the instrument, refer to OEM documentation to determine the correct model and specs
If no source documentation available, contact OEM
If OEM is no longer in business, default to model number "Unknown" (since one must have existed but can no longer be determined)
Assign specs for calibration based on process applications (and possibly test points)
As a last resort, request "Data Only" cal but understand the burden this places on you to review cal results for impact to processes
For more details, read our paper on this subject. And as always, contact us if you need more information or need help with your calibration program.