GRATICULES AND CHOCOLATE: STEPHEN BECKETT’S LEGACY DR. TREVOR OGDEN
Only a handful of members will remember Steve Beckett from his days in BOHS, but hundreds will have benefitted from the microscope eyepiece graticule he co-designed when he was a member, which made airborne asbestos measurement easier and more accurate. Sad to say, Steve died on 9th January. Steve arrived at the Institute of Occupational Medicine in Edinburgh in late 1971, after a physics degree at Durham, and a doctorate at York which involved maintenance and manipulation of high vacuums. His main job at the IOM was to generate steady clouds of asbestos at various concentrations in rat inhalation experiments, which was good exercise for his experimental skills, but over the next few years he also published 11 papers in the Annals of Occupational Hygiene and 8 elsewhere, which had a permanent effect on the way asbestos is measured worldwide. Steve collaborated with the laboratories of the three major British asbestos manufacturers and the Factory Inspectorate, who were all using the membrane filter method for measuring airborne asbestos concentrations. They all believed that they used the standard method, but Steve conducted slide exchanges, which showed that the mean results that the laboratories obtained on the same samples differed by a factor of three, with larger differences on individual samples. Henry Walton, Steve’s boss, knew well the errors in microscope counting of coal dust particles collected with thermal precipitators, and the acknowledgements to the Beckett and Attfield paper say that he suggested the trial of asbestos counting. Similar big differences were found in later exchanges between major laboratories in
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nine countries, and later still huge differences between laboratories in this country. It was clear that the intense public debates about what a safe standard for asbestos might be were of no point if the concentration depended so much on who measured it. Different laboratories were found to be using small variations of the standard method, but a large part of the differences in results were due to unexplained differences in counter perception and behaviour. In 1978 the Government’s Advisory Committee on Asbestos strongly recommended interlaboratory quality control checks, and this led in 1982 to the RICE scheme, which became available to all laboratories in 1984. Participation in RICE remains virtually compulsory for British laboratories using the membrane filter method in the workplace. Asbestos fibres are only counted if they are longer than 5μm, with diameter < 3μm, and length:diameter ratio >3:1, so it is necessary to use an eyepiece graticule, which superimposes a scale on the field of view. The graticule often incorporates a square or circle, and this can be used to define a countable area. Steve’s results showed, to everyone’s further surprise, that laboratories which just counted the fibres within the graticule area found almost three times as many fibres/mm2 (for chrysotile) as laboratories which tried to count fibres in the whole field of view. This was true for graticule areas smaller than about 100 μm in diameter, and the smaller the area, the
greater the effect. It demonstrated again the big differences produced by subtle psychological effects of perceiving and counting fibres. Laboratories were using graticules of various designs, but all intended for compact particles. The linear scales on these had to be used to measure the length and diameter of curly and intersecting fibres, to see whether they fitted the definition. Surely only a perfectionist with a very patient employer would go to this trouble of measuring 100 fibres per sample – there would be a lot of estimate of dimensions by eye. Steve and Henry Walton got together and designed a more suitable graticule, with one scale with 5μm divisions and a perpendicular one with 3μm divisions. Round the perimeter were examples of rectangular shapes of various sizes with 3:1 aspect ratios. To get these sizes right, each graticule had to be individually made for the microscope for which it was intended. This design rapidly found favour, and today is recommended or required all over the world. (Later, John Cherrie investigated the graticule area effect, and showed that it was not significant if the number of fibre ends in the graticule area was counted and halved, as in the modern method in HSE’s “Asbestos: The Analyst’s Guide…”) Gradually the asbestos-measuring world woke up to its deep problems. From 1977 I was responsible for these measurements in HSE, and attended a meeting with
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