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  Measurement, Test and Calibration
  Calibration

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Author Topic:   Calibration
DAdams
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posted 24 January 2001 10:53 AM           Edit/Delete Message   Reply w/Quote
We are an old textile plant with a gazillion gauges on our machines. Currently we have the yardage clocks and temperature guages on all machines calibrated. Every 1000 yard roll we produce is tested in the lab for shrinkage, color, weight etc. before it is inspected and shipped to our customers. These test take place while the machine is running. All the test equipment in the lab is calibrated by an outside source. If a product fails a lab test the machine operator is notified and adjustments are made (speed, temperature, etc.) These guages on the machines are used as a guide or "starting point". Do we have to have all the guages in the plant calibrated for QS9000?

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Jerry Eldred
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posted 24 January 2001 11:33 AM     Click Here to See the Profile for Jerry Eldred   Click Here to Email Jerry Eldred     Edit/Delete Message   Reply w/Quote
No you don't have to have ALL gauges in the plant calibrated to satisfy QS9000.

***I SAY THAT VERY CAREFULLY***

You MUST, however, either calibrate or account in some way for assuring that any or all of the gauges that could in any affect the product quality or it's ability to satisfy specifications read correctly.

There are a number of variations on how this can be done, and many variations on the most intelligent way of doing, depending of course on the particulars of your processes.

If, for example, there are gauges which are purely for "convenience", such as a pressure gauge for a pneumatic function, which, if they were totally incorrect in their reading would still not have any possible impact on the product, those could be INDICATION ONLY. In semiconductors, there are tens of thousands of gauges every where you look. Only the trained professionals can determine wihch of them are important to the process.

I wouldn't dare speculate which of your gauges could impact product quality. But those engineers, etc. at your factory should be able to make that determine. My caution would be to make sure each decision is intelligently made such that you could convince an auditor. If you use the reading on a gauge to set any parameter in your control plan, it certainly ought to be calibrated. If a gauge measures a specified parameter, it needs to be calibrated. If there is a safety issue involved, it should be calibrated. If you have reading limits for proper operation of a piece of production equipment, it should be calibrated. The pneumatic example above-- Let's say your pneumatic cutting blade operates at 100 psi (I may not have a good number there). If there is a lower limit to where the cutting blade will function (let's say 80 psi) and an upper limit before hoses spring leaks (let's say 150 psi), it should be calibrated.

A way around calibrating a lot of the peripheral gauges is to use S.P.C. on the process. If there are 20 gauges on a production tool that you don't believe have a significant impact on the process output, set up a chart and plot their readings daily, every shift, every week, or whatever works well). Have a calibrated measurement of the output of the process. Plot that output on a control chart. Use those gauge readings as feedback. When the control chart drifts toward an out-of-control condition, check for a similar trend in the gauge readings. You have not only eliminated the calibration requirement for 20 gauges, but used them as a trending tool for improved process control. This can actually be more effective, and less costly than traditional calibration schemes (in specific circumstances). This will not eliminate the need to calibrate those gauges that truly do need it.

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