Formula for PSIG
#1
I have a Trane air conditioner, model #TTD736B100A1 with electric heat. I have an automotive set of gauges which reads in PSI. Does anybody know how to convert to PSIG from PSI? The low side reading I get from my automotive gauges is 75 PSI and I want to know what the equivelent is for the PSIG. Is the system undercharged? Also the high side reading is 285.
#2
The g in psig indicates "gauge", meaning whatever the gauge reads. This is to distinguish from, say, psid, which indicates differential pressure, i.e. pressure difference between two sources, or psia, i.e. actual pressure. For example, you might have an oil pressure indicator on your car that is run from a sending unit. The pressure indicated on your instrument panel would be psig, whereas if you hooked up an actual gauge straight to the pressure source, i.e. the engine block fitting, you would have have psia (assuming the gauge was properly calibrated). Sometimes the actual pressure and the indicated (gauged) pressure are the same, sometimes not. Whatever the gauges are reading is your pressure, the only time you would need to convert would be if there was a known calibration error.
#3
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In most engineering disciplines psia is referred to as pounds per square inch, absolute. psig is gauge pressure as the_tow_guy says. The difference is psia is pressure above absolute vacuum, so atmospheric pressure would be about 14.97 psia, or 0 psig. psia has many uses and can be read on some gauges.
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After re-reading your post I realized I didn't do a good job of answering your question. If your gauge is reading atmospheric pressure and it says 0 (zero), it is going to give you psig. If the gauge reads about 15 psi when reading atmospheric pressure the reading will be psia.