John,
I think you guys know how to relate the pressure to the physical data of the gas. Ed's got some physical data posted at weldreality.com gas section for handy reference. Note Acetylene is dissolved in acetone and consequently has a unique pressure/temperature/volume formula somewhat different from Boyle's.
I use inches of water as a vacuum standard, and according to what I was told (not specifically TAUGHT) several decades ago, understand it to be an "absolute" scale, rather than a gauge value (PSIG) which (I WAS taught) is the difference between test pressure and atmospheric (which depends on the variable barometer readings) but I have no reference to convert it to PSI which I can find (gotta be here somewhere).
In reality the meter is probably reading gauge pressure anyway and merely calibrated in inches; there should be some internal compensation for barometric variable, however.
I believe your meters, according to the scale in which they are calibrated, are approximating absolute pressure. It provides greater precision for laboratory calculations but the vessell may actually draw a slight vacuum if readings are quite near zero and the barometer is high.
Regards,
d
BTW Scott, we typically use more O2 in our shop breathing than we do welding or cutting; it's not our mainstay and the scale of our work is rather small. Sorry our field isnt the right one to be able to help your research.