Instument of laboratory that request calibration


Montserrat P

One year ago, we had a QS 9000 audit where one of the finings was that we did not count on with trial tubes, precipitation glasses...without to be in our calibration system.
Because of it, we request to the company that sell them (Fisher) the certificate of calibration, because we can not calibrate the trial tubes or glasses here in the plant. As soon as they send us the certificates when our laboratory request new instruments, this certificates, the calibration area update that information in the system. We established a period of each 6 months, as soon as this period finish we destroy the tubes or glasses (again because we cant to validate the calibration). It represent a cost for the area and also, sometimes when the laboratory request new tubes, the supplier provide them with new instruments but with certificates that will be obsolets in less that 6 months according with our system.

Is requested to calibrate this kind of instrument?
Could not be better to validate if the instrument is in good conditions:not broken, not damage for chemicals, etc.? or at least to establish frequency of 2 years for each instrument?

M Greenaway

What is the basis for the 6 monthly scrapping of this glassware ?

I have had a similar situation with glass hydrometers, however all we did was request a calibration certificate when purchased, and then just check for damage at prescribed intervals - reason being it was felt that the instrument could not go out of calibration (or accuracy to national standards). Might be wrong but we got away with it !

Montserrat P

Inst. of Laboratory that request calibration

The reason is that or maximum interval calibration is one year. Because of the nature of this kind of instrument (we have hydrometers too) we did select a period of 6 months. We decided to make scrap them after that time, because we thought (maybe we are in an error) that only an visual inspection was not enought and maybe the glass could be suffer other kind of damages or change that could not see in a visual inspection.

I have thought about established the requirement of an visual inspection each 6 months and according with the results to make or not scrap the instruments.

M Greenaway

Well in the case of our hydrometers they were an enclosed glass vessel with a weight in the bottom and a scale etched on the stem at the top. The device works (as you know) by being placed into a fluid and sinking to a point where the density of the fluid supports the weight of the hydrometer. Clearly this can only become innacurate if the weight changes, and the weight can only change if something is added or taken away from the 'material' of the hydrometer. Clearly an enclosed glass vessel is very difficult to add weight or reduce weight, any significant change should be visible to the naked eye. Also any cracks in the vessel would possible change the accuracy as fluid could potentially enter the hydrometer causing it to 'sink' further. As such this rationale was used to decide that once 'calibrated' by the manufacturer a simple visual examination was sufficient.

A similar situation is with glass measuring devices that measure volumes of liquid. These work by filling the glass to a point on an etched scale. Again innacuracy can only occur if the volume of the glass changes. Assuming that the glassware is suitably stored and not subject to melting point heats, the volume of the glass cannot change as glass cannot tolerate any permanent deformation (non elastic). So a simply check that the scale etched on the glass is still visible should be enough.



I agree with Martin

IMHO - Unless your measurement accuracy requirements are extreamly tight (then you are probably using the wrong equipment anyway) the certification from the supplier should be all you need. If you verify the suitability of the equipment at perscribed intervals why would anyone argue?

One thing though - you will need to get rid of your internal calibration interval of one year max. If you say you are going to calibrate annually, you have to do it. Why shoot youself in the foot by setting a maximum limit?

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