Polymer Testing 6 (1986) 161-162

Editorial

Any laboratory which becomes registered under an accreditation scheme, such as NATLAS, The National Laboratory Accreditation Scheme, in Britain or its equivalent elsewhere, has been forced to take calibration of its instruments very seriously. The realisation that to be sure of accurate results there is a need to prove the calibration of every instrument by a traceable chain back to nationally recog- nised standards has in quite a short time caused an enormous increase in the effort which has been applied to calibration. As little as ten years ago it was not common in polymer laboratories to have traceable calibration for anything except the force scales of tensile machines. Enormous faith was placed in the manufacturers of weights, thermometers and dial gauges, etc., to have supplied accurate goods and an even greater faith placed in the goods not changing with time.

Accreditat ion schemes have been the main force which has changed these attitudes and practices, not only laboratory accredita- tion schemes but schemes which accredit the whole quality control operation of manufacturing companies because these also demand calibration of test apparatus. There are, of course, many laboratories who still assume rather than prove that most of their equipment is correct but these will surely diminish if the quality movement continues.

All this calibration is very expensive and in one sense a lot of it is 'wasted' because the majority of instruments do not require adjust- ment when they come to their annual or monthly check. The object of the calibration schedule is to be as sure as is reasonably possible that all the parameters of the apparatus which affect the result are always correct. It cannot provide an absolute guarantee because a fault or drift could occur at any time, but the calibration intervals are chosen in light of the best knowledge available and then adjusted in the light of experience according to generally recognised rules.

The more you get into calibration the more you think of what needs checking, and there is a definite danger of becoming paranoic.

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Polymer Testing (6) (1986)----~ Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Northern Ireland

162 Editorial

The calibration of ageing ovens started with a single temperature measurement in the centre; then it was realised that you need to scan and define a 'safe' volume. For rubbers at least, a calibration of the airflow was added and now the ovens are continuously monitored with several probes in case it wanders in the night. This needs computer control and for proof of a correctly calibrated oven you spend more per year than the price of the oven.

Comprehensive control is probably justified for some ageing work but there has to be a limit, and commonsense has to prevail in the conflict between perfection and economy. It is a complete waste to overcalibrate; if you are measuring in hours you do not check the clock to microseconds. The levels have to be suited to the measure- ment and you cannot transfer attitudes from one area to another. Length and temperature in a physical testing laboratory are very different from a full-scale fire test rig; +0.01 mm on dumbell thickness and +0.5 °C on ageing temperature is a different world from +50 cm from the flame and between 800 and 850 °C.

However, although I half expect my coffee beaker one day to be s tamped 'calibrated to grade A' it brings a great deal of confidence and peace of mind to have the calibration officer probing every little gauge and dial.

R. Brown