Other Dimensions: Tell Me Why
July 31, 2009
Science attempts to tell us how and why things work. Provable science is something that evolves over time, with each discovery adding to our knowledge. Thus, it would seem that yesterday’s questions no longer need to be asked. Alas, it is not always so.
I still get asked questions that were answered many times years ago and wondered why this could be. The answer is simple: Years ago, the people involved in calibration or product inspection got their jobs after serving time as machinists and toolmakers. Today, people are plugged into these areas without the same kind of experience so they’re starting from square one. If you find yourself in such a situation, the following questions and answers may be of help.
Why are the numbers on this year’s calibration report different than last year even though we haven’t used these blocks or gages since then?
The short answer is that the items involved may not be the same as they were last year. Despite advertising about stability, steel gage blocks, for example, will change over time whether you use them or not. The cal lab’s uncertainty may explain differences as well.
Why don’t you include calibration reports with all gages? You have to measure them to make them.
The main reason is that most gage buyers won’t pay the extra costs for this to be done correctly-in the lab by someone other than the person who made the gage.
Your calibration report doesn’t say if this gage is good or not. Why not?
This is usually because what is “good” varies between gage users, unless you want them checked to “new” gage standards that don’t apply to used gages. If you’ve got data on the report, you can make the call based on your needs. That’s the data the gage lab would use.
Why can’t you calibrate a couple of gages while I wait?
Most labs can do this, but they will have to adjust the measurement uncertainty value shown on the report if their temperature varies from the standard. This means that the actual uncertainty involved could be much greater than the tolerance on the gages.
Why are the uncertainties shown on my report different than the ones on your scope?
Usually this is because the scope uncertainties apply to items that are in mint condition, while used gages are not. The uncertainty has to be adjusted to reflect the state of the items calibrated.
What’s the difference between ISO 9000 and ISO 17025 when it comes to calibration labs?
The ISO 9000 standard relates to a quality system. The quality of the product or service from that system can be anything agreed to by the customer and the supplier. ISO 17025 includes all the requirements of ISO 9000 plus special elements that relate to calibration activities exclusively. These requirements include proof of performance. Because of this, ISO 17025 is the de facto standard around the world for calibration laboratories.
We calibrate our own gages. Lately, the numbers seem way off. Why would that be?
I assume your current numbers are “way off” previous calibration numbers. Assuming your equipment is up to speed, what’s changed lately? New personnel? It takes skill to do this work so perhaps your new person is not as skilled as the previous one. Conversely, the previous technician may have been out to lunch and you didn’t know it. After your new person has calibrated a gage, have them do it a couple of days later and compare the numbers. Of course, repeatability doesn’t mean accuracy, so if the numbers are still in orbit, it’s probably time to have a calibration specialist check things out.
What’s your question?
Everyone has questions they’d like answered but never get around to looking them up. You know, questions such as: How come every time the government does something my taxes go up? That’s a subject for another day, but you get my drift. However, if you have questions regarding metrology, I’ll do my best to provide answers in a future column. Send them to me at email@example.com.