A watchmaker's website explaining the most misread marking in the trade
"30 m" stamped on a case back does not mean the watch can be worn swimming three metres down — it means a new watch in still water withstood three atmospheres in a laboratory. An arm in water moves, and the gasket is rubber that hardens on its own, even in a watch that was never wet. The calculator on this page takes three things — the figure on the case, the years since the last pressure test and the type of crown — and places two markers side by side on a vertical depth column: a brass one where the manufacturer stamped something into steel, and a white one where the watch actually is today. At a hundred metres and six years since service, fifty-two remain; with an ordinary push-pull crown nothing above fifty is counted, however large the number on the case back, because water enters where the winding stem enters. Beside the column sit eight tiles with the real thresholds: a shower demands more than swimming, because hot water and soap destroy rubber faster than pressure does, and a sauna is a flat no regardless of any figure. The most interesting part, though, is how the calculator ends for a recently serviced watch: with the sentence "nothing needs doing here and we have nothing to sell you". That same sentence travels to the workshop inside the enquiry, with a tag in the subject line, so that nobody at the counter offers the customer a test the website has just talked them out of. The price list carries the workshop's current queue with the date it was checked, because a full movement service takes as long as the parts take to arrive.
