Thanks for the great advice. My add is a reminder (education for some) on both units–engineers are anal about units–it helps us pass exams and not screw things up) regarding absolute vs gage pressure. Seems like the distinction is useful in this case. psi, psig and psia. Here goes: If I have 65 psi in my tires, it is aka 65 psig. Easy to remember since if I put a gage on it, it will measure 65 psig. The g stands for gage. (actually it will measure less than 65 psi since some pressure is lost filling up whatever device one uses to measure with. This is measurement error. Unavoidable. Related error is an uncalibrated gage but that is off topic). If you let all the air out of the tire it will measure 0 psig. Ahem, however, it will still have 14.7 of pressure inside of it. Nature insists on everything equal and it will be the same pressure inside and out. In Denver it will be a little less than 14.7, In Death Valley a little more. Air pressure is literally the weight of the column of air (and this is literal, not a metaphor, not a “way of thinking of it”), above wherever one is. This column extends as high as there is air–roughly up to our atmosphere. BTW there are a lot of planets, (millions if not billions), out there but only 1 with an atmosphere and only 1 with life. Hmmmm…maybe, just maybe we should consider our atmosphere that supports life, (as far as we know and with our level of instrumentation/satellites we know pretty dang far!), is so unique and thus we should treat it with the utmost respect. Sacred. Forget about clean air and clean water, our atmosphere is our sole provider/protector of life. Where was I?. If you do some googling you’ll find out that pressure is pretty complex. Bernoulli and Dalton did the heavy lifting nailing this down. Static pressure, dynamic pressure, force of gravity even mach numbers enter into the formulas. Also amazing is not just the units types–psi to pascals (English to SI) but the unit forms: e.g.: psi to feet to Atms (or Bars) or inches of mercury (or water column (wc). Most units are made up of other units and thus have “straight” conversions. Not so with pressure, one needs to understand the phenomena…(or google). Dave.