posted 30 January 1999 12:29 AM               

Date: Mon, 4 Jan 1999 09:46:07 -0800
From: "Nielsen, Larry E"
To: iso25@quality.org, 'Greg Gogates'
Subject: RE: M&TE Temp. operating range vs. specifications (fwd)

Scott,

We frequently run into this question at Southern California Edison where M&TE calibrations are performed in our metrology facility under more or less ideal environmental conditions, whereas the instruments are utilized under field conditions at our customer's facility.

Per our contract, we maintain a database of equipment specifications including both nominal and environmental (i.e., temperature-humidity compensation) specs for use in the field, as specified by the OEM. As far as actually calibrating under these conditions is concerned, we adopt the same philosophy as when we're asked why we don't always test at 100 percent of full-scale, or in between test points:

"Calibration actually occurs at the time the relationship between the unit of measure and scale reading of an instrument are established by the OEM. >From time to time, M&TE are re-tested to ensure that the basic relationship remains in tact, within specified limits. Test points are selected on the basis of an economy of effort with respect to performance characteristics of the unit. Where roll-off or other undesirable characteristic is a factor near the extreme points on a scale, procedures are modified accordingly. Otherwise, it is reasonable to assume that performance at selected points may be projected between test points and to the useful ends of the scale."

Performance specifications for M&TE are derived by the OEM by way of environmental testing of their products. We don't feel that it is the responsibility of a second-party metrology laboratory to second guess these specifications, or to attempt to reproduce the tests. Of course we realize there are special circumstances where nothing other than a special test will do. We treat these on a case for case basis.

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Larry E. Nielsen
So. Cal. Edison - Metrology
7300 Fenwick Lane
Westminster, CA 92683
(714) 895-0489; fax (714) 895-0686
e-mail: nielsele@sce.com
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