I think what Mark is talking about is the baseline for the computer to know when the tank has 80 Cuft of air in it, which would be at 3000PSI for an aluminium 80.
ScooterGirl and Michael, just for the record, I'm sure you both know an Al80 doesn't actually hold 80cf, right? (at rated pressure, at least) Al80 = 77.4 cf at 3000psi. So if people are entering 80cf for the tank size, they're not getting accurate calculations out of it. (And SG, what is a 102? I've never seen a tank rated to 102. HP100 and LP104, yes, but no 102's.) I assume both of you are speaking shorthand for ease of discussion, but I wouldn't want people who are unaware of this to think that's the way to do it.
Full disclosure: I don't own a Uemis, although it's high on the list of serious contenders for my next computer. I've read the manual (once) and read many reviews (end-user and otherwise) with great interest, however.
Does the Uemis ask for the
working pressure of the cylinder and the
rated capacity (in cf) at that working pressure? Because that's all it needs (other than the pressure readings it gets itself) to calculate the proper SAC. Doesn't matter if a tank is overfilled or not. If the fill pressure is higher than the working pressure, the math still works the same to determine actual gas in the tank. (Ignoring a discussion of ideal gas laws)
If a tank is rated to 95cf @ 2640 psi, it's easy math to see how much gas it holds at 3000 psi. (3000/2640*95 = ~108) Rated pressure is simply 2640/2640*95=95.
If the Uemis is NOT doing a calculation to determine how much gas is actually in the tank, and always assumes a 95cf tank is holding 95 cf at the start, that's (a) retarded, and (b) completely wrong. Marc Bryan would be 100% correct to be suspicious. I find that unlikely, however, otherwise in the all-too-common scenario of starting off with an underfilled (or partially-used) tank, the Uemis would again be wrong in all of it's calcs unless you calc out (by hand) the actual volume of gas in the tank and enter
that as the "tank size" instead of the rated size. (Let's say 2200psi in an LP95, giving us 2200/2640*95=~79cf) It would be absurd to have to enter 79cf into the computer to get an accurate SAC rate out of it, so why would Mark need to enter 133cf to get an accurate calculation?
Assuming the Uemis is properly calculating tank size/capacity/volume based on entering the appropriate numbers, Mark would be completely messing with the computer by entering 133cf for the tank size, if he's diving an LP104 overfilled to hold 133cf (but still enters the correct working pressure for LP tanks, 2640), and getting incorrect SAC calculations out of it as a result. Otherwise he's basically inventing custom tank sizes on the fly by entering a mythical tank that holds 133cf with a 3500 psi rating. The math will work out in the latter case, but only because he's forcing it to by doing a lot of (what I assume to be) unnecessary hand calculations before entering it. That must get old pretty quick if there's any variation in fills on a day-to-day basis.