After I wrote the last column on this subject a number of queries popped up so I figured a sequel would be worthwhile.
A few customers are requesting that reports show the actual lab humidity during calibration of their equipment. I realize that this can be a critical consideration for some types of calibration but when it comes to fixed limit gages, I have never heard the rationale for such a request other than it’s what their customer expects. Most dimensional labs I know of keep humidity between 40% and 50%. It’s true that in days gone by humidity was more critical due to the electronics used at the time. This concern fell by the wayside with the development of solid state electronics. However, if lasers are being used humidity can have a detrimental effect but most of these devices incorporate sensors and electronics to automatically condition their readings to compensate if needed.
The primary reason dimensional labs keep a watch on humidity today is to keep static electricity under control and to prevent rust forming on precision lapped surfaces typically found on gage blocks. Of course, the comfort of those folks working in the lab is also a good reason to keep an eye on it.
CALIBRATION DUE DATES
We still get customers asking us to choose recalibration due dates for equipment that we’ve calibrated. I understand why this is needed in the grand scheme of things. However, I think the primary reason for customers or their customers in turn wanting it on a calibration report is because there is an air of believability or authority to it.
As I’ve mentioned in previous columns, calibration facilities are in no position to determine such dates as they are rarely aware of all of the elements that go into arriving at them for each customer. One reason they may do so—even without being asked—is to drum up business. This was foreseen by the creators of ISO 17025 who forbid the practice unless the customer has specified the intervals.
Since the lab’s customer must specify the intervals if they are to be copied on an accredited report, they don’t really need to be there. Even when customers do state what they want to see on the report, some of them are so far from the norm the reputation of the laboratory issuing the report could be brought into disrepute. To get around this, the lab may put a qualifier with the due date stating it has been supplied by the customer.
When you think about it, the lab is being asked to predict the future for the item calibrated and while calibration personnel are tested for all manner of things, psychic ability isn’t one of them.
Quality auditors will usually not question due dates shown on reports and this may be the real reason why customers want them there.
EQUIPMENT DUE DATES
Some quality standards require calibration labs to show the calibration dates of the equipment they use for a given application on the reports they issue. I can’t speak to other areas of metrology, but on the dimensional side, I have yet to understand why this request pops up. Yes, it is important that lab equipment is calibrated at regular intervals but this is checked on location during technical assessments by accrediting agencies. Doing so in this way means the assessor can put the date(s) in context and better evaluate their suitability. If necessary, the assessor may review supporting documentation as well. Such an evaluation can’t be done by looking at a date alone.
Showing a NIST report number on calibration reports can show traceability to a national or international standard but the number on its own can be meaningless. Unless you see the actual NIST report, you have no way of knowing who the report was issued to which could be someone several links down the food chain rather than the lab that issued the report on which it appears. Accrediting agencies know this and their assessors will check the original NIST reports during an on-site assessment to confirm its validity. If it is not relevant to the work being done, the lab will not be accredited so having every report decorated with NIST numbers is not required by ISO 17025 or particularly helpful on its own.