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I have to measure stuff for a living. Generally, those cheap ones aren't reliable. If you wang them about they can jump about and then you don't know where you are. Like @mdcc_tester mentioned, you can pick up a manual vernier caliper for not a lot and it'll always read correctly. I got my Mitutoyo ones second hand on eBay for about £15 delivered and they looked like they'd never been used. They'll always have resale value as well.
i keep seeing vernier calipers in this thread. do people get those cheap digital ones off ebay? are they good?