🚀 go-pugleaf

----------------------------Original message----------------------------


>I have a question about the calibration of test equipment.  Any feedback
>would be greatly appreciated.  The situation is this:  we calibrate all of
>the test equipment that we use for formal acceptance testing.  Most of this
>equipment we own, but some of it we rent or lease.  On the equipment we own,
>we keep track of what equipment is used on a specific test.  So if, when get
>that piece of equipment re-calibrated, we find that is was out of
>calibration, we can trace back to the tests we performed and determine how or
>if they were affected by the out of calibration condition.
>
>On the rental and lease equipment, we ask for the equipment to be calibrated
>before it is sent to us and require certificates of calibration to accompany
>the equipment.  So far so good.  But, when we are done with the rental
>equipment we send it back and if that equipment happened to go out of
>calibration while we were using it we have no way of finding out.  The rental
>agency does not calibrate the equipment when we return it, they calibrate it
>when they send it out and only if a customer asks them to.  The piece of
>equipment we used may not get calibrated for a long time or maybe even never
>again. So we can't just ask them to let us know if it is found to be out of
>calibration.  The only solution we can think of is that we ask for it to be
>calibrated when they send it to us and then again when they receive it back
>from us.  Obviously this is cost prohibitive.  Any ideas?  How have other
>companies dealt with this?  Our goal is to meet ISO-9001 and ISO 10012-1, and
>more importantly reduce our risk of using out of calibration equipment for
>testing.
>
>Thanks
>
>Darren Kent
>Ph: 306-931-3425
>Fx:  306-933-1486
>Inet: Kent@SEDSystems.ca


Darren,

See if your rental company maintains calibration records for the equipment in qu
 estion.  If so, you can estimate the calibration reliability from the mean time
  between out-of-tolerance occurences.  R=exp(-t/m), where t is the time since t
 he last calibration and m is the mean time between out-of-tolerance occurences,
  is a simple but reasonble estimate for your calibration reliability.  If this
 data is unavailable, see if the manufacturer or major user has a recommended ca
 libration interval for the item.


Also consider your accuracy ratio.  If your calibration equipment is 10 times mo
 re accurate than the equipment calibrated, then you should not be too concerned
 .  However, with accuracy ratios below 4-to-1, you will need to demand some lev
 el of reliability; and if your accuracy ratio is below 2-to-1, you probably wan
 t to spend the money to re-calibrate.

Hopefully, by prioritizing your calibration budget according to reliability, acc
 uracy ratio, and criticalness of measurement, re-calibration will no longer be
 cost-prohibative.


If you are leasing more than one calibration equipment model, then you can run a
 n intercomparison program.  If not, you may want to run inter-comaparisons with
  laboratories outside your organization.  You may want to bring up your problem
  to the NCSL Conference:


The (U.S.) National Institute of Standards and Technology is co-sponsoring the s
 emi-annual National Conference of Standards Laboratories, July 31 - August 4, 1
 994, in Chicago.  Contact phone number (303)440-3339, or write to NCSL, 1800 30
 th Street, Suite 305-B, Boulder, CO 80301, USA.


Hope this helps,

Steve Dwyer (not affiliated with the NCSL conference)

Calibration Intervals
Metrology Engineering Center
sdwyer@thor.nwac.sea06.navy.mil