Tonight I connected my gauges to a test system we had at school, and I was reading about 175 on the high side. Well this other guy wanted to connect his gauges to the system so I disconnected my gauges and he connected his.
His gauges read 195! That meant one of us was pretty far off the mark... just to make sure the system hadn't changed dramatically I reconnected mine and again they read 175. So another guy connected his up ( these were all yellowjackets btw ) and he read 195 as well.
So as the final nail in my coffin this guy with glycerin filled gauges hooked his up and read 195, and then tried mine once more and they read 175.
I bought them at the AC Supply in Downtown Ft. Worth... so do I just tell them what happened, or do I have to prove they are outta whack?
Who Zero them
Maybe yours are -20 when there 0 pressure, maybe their's are + 20 when when there's 0 pressure. Maybe the instructor was a messing with the air flow on the Indoor unit. Thunder storm, naw you would have noticed that.
Connect your gauges and everyone elses to a bottle of known refrigerant, say R22, that has been in the same temperature for quite a while. Then look on a pressure/temperature relationship chart to find out what the R22 refrigerant bottle is. Compare your gauges to that chart temp and you have calibrated your gauges to THAT temperature.
Gauge calibration should be done as close as possible to the actually working pressures you generally use them for. Zeroing a guage is not calibrating them. All mechancial gauges will be off at some pressures. The idea is to calibrate them at the pressures you will generally test for.
And the refrigerant bottle should be liquid filled with lots of time to become the same temperature where you have it at.
"The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
- Alexis de Toqueville, 1835