Dale:
Mechanical pressure gauges seem to be fairly rugged. I have a large dial 3-60 psi and a small dial 6-160 psi that are 15 years old. At a certain pressure I found the former read 54 psi and the latter 57 psi whereas some factory-checked gauges I purchased recently read 55 psi.
However, unlike a steel rule it does not matter for our application if pressure gauges do not read accurately in standard units. All that is necessary is that for a given pressure any one gauge gives exactly the same reading for at least one day.
I suggest one way to see if a given gauge is up to the job is to measure a course of about 300 meters with tire pressure at a particular gauge reading. Let a lot of air out of the tire, restore the initial pressure, and re-measure the course. Both course measurements should be the same.
If the calibration factor is always measured at the same pressure, any shift outside the usual tight range might alert to a shift in gauge characteristics.