Originally Posted by FBinNY

I manufactured bicycle pressure gauges for a number of years until cheap imports made it impractical.

Gauges are calibrated against known pressure. In our place we had an air line kept at constant pressure and having a check gauge plumbed in-line. The line also had a port for checking with a "master gauge" which was built to a higher accuracy spec. and kept off line except for our weekly check. Once a year the master gauge was sent out for checking which was done using a "dead weight" device which basically was an inline pneumatic cylinder supporting a known weight, and so could be known to maintain a very accurate pressure.

In short, air gauges, like all measuring instruments, and calibrated against masters, which in turn are calibrated against better masters, and at some point can trace their calibration to the masters, or weights kept at the National institute of standards.