A compression tester is a good example of the "measure with a micrometer- - - -mark with chalk- - - - -cut with an axe" principle. Its only practical use is to get a percentage variation of pressure readings from one cylinder to another. The actual pressure indicated on the gauge is irrelevant. If you want an accurate indication of how much pressure a cylinder can hold, do a cylinder leakdown test with compressed air. Why would anyone be concerned if it takes 3 PSI instead of 1 PSI to unseat the Schrader valve? Try using three different compression gauges to check the same cylinder. I'll be surprised if they read within 10 pounds of each other.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!