[Mr Anonymous]

My father sold industrial valves, thermometers, regulators, etc. Following his passing, I have been going thru the "stuff". I ended up building a pressure test rig because of concerns over which tire pressure gauge was correct. Using my air compressor, a good regulator, a tee to accept various NPT adapters, and so on. It was a fairly interesting rabbit hole. I have a highly accurate, 6" diameter, brass, 0.-100psi gauge. I tested it again a handful of 'very decent' quality gauges, and found about 3-4 that 100% matched the Big One. I accepted it as a standard, although technically they could all be reading the same wrong pressure. I considered that the law of averages would to mean that the Big One and these others are likely "correct" enough for my purposes.

I checked a gaggle of other pressure gauges (oil pressure, tire pressure, etc) on my "test rig". I ended up throwing out about 50% (I had a lot). The worst offenders are the small handheld round gauges with the 1" long tire valve attachment coming right off the gauge. The kind you get near the checkout counter at any parts store. At an observed 50psi test pressure, there wasn't one within 5 lbs, and one that was off by 12! There's a reason they are cheap!

Those "pencil-type" tire pressure gauges are closer than you'd think - stangely enough.

Later on, said "test rig" was reconfigured as a leakdown tester for a car I was diagnosing. 3 dead cylinders out of 8. Ugh.