For people wanting more background information on the problems of gas usage in diving I thought I'd add what I've learned.
On Mon, Sep 9, 2024 at 5:36 PM Robert Helling via subsurface < [email protected]> wrote: > > One is if you measure the gas consumption in units of > > a) pressure drop per time > > or > > b) volume of gas at some reference pressure (typically surface pressure) > consumed per time > > the other is which term or abbreviation you use for a) or b). > In my area (Ontario, Canada) divers I know use SAC to refer to volume per minute at 1 ATA. Many new tech divers will be told it's really called RMV during training and will at some point will proclaim "we really should be calling it RMV" and then we all keep calling it SAC. > But the first issue is not up to discussion, the correct way do express it > is b)! > I agree. We mainly use this gas consumption when planning a dive to ensure we have enough back gas, travel gas, and deco gas. A diver may use different sized bottles often with different working pressures and needs to calculate gas use within each. As well, a dive team may be using dissimilar tanks and needs to do calculations to ensure there is enough contingency gas. To convert pressure/minute@1ATA to work in different types of bottles would be harder. Harder means more chance for error. Typcailly we rely on computers to do the calculation for us though. Tech diving teachers talk about "tank factors" which can be used to calculate how much PSI in a given tank equates to 1 cu ft. I do that kind of math in my head when I use my dive computer's pressure change/minute@1ATA display as feedback to monitor my consumption on on the fly. For people who are curious as to what tank factors are it's Working_Pressure / Storage_Capacity, e.g. 3000 PSI / 80 cu ft for a Aluminum 80 which results in 37.5 PSI/cu ft. > Once you are out of the water and write your log you can let your computer > do the calculation to convert this into volume (amount of gas) units which > is a much more meaningful way of expressing things. Others have mentioned > that it makes no sense to compare pressure units if you you have different > cylinder sizes. But even if you say you only have a single cylinder and > will always use the same you should realise the conversion between pressure > drop and amount of gas used is only simple in a world that knows only about > ideal gases. Subsurface takes great proud in the fact it is aware this is > only an approximation and takes into account the pressure dependent > compressibility of breathing gases. And if you do that a pressure drop only > translates to an amount of gas when you also know the starting pressure. > My ego likes that Subsurface approximates real gas when calculating volume/minunte@1ATA. I care mostly about usage when cave diving when I generally only use the top part of the tank my volume used is always less than ideal gas calculations, a lot less. When diving a cave one will use gas pressure consumed or remaining to determine at which point to turn the dive. To do this when team members have dissimilar tanks we convert back to pressure and record that as our turn pressure. For deco dives, you plan ahead to calculate how much gas volume you need for a dive profile then make sure you have enough gas for the dive. Sometimes you go back to the start and replan the dive profile until it matches the tanks you have. On the dive you work mainly on time and, if all things go to plan, reading tank pressure is just a comfort. If you follow proper tech training you only have to start calculating needed gas in the water when more than two or more things go wrong, like a gas loss + loss of buddy or a dive plan gone very awry. So the difference in SAC (amount of gas used) is more than 10% if you start > from a full cylinder or from a half empty cylinder and drop by the same > amount of pressure. This difference in invisible if you measure it in > pressure units but it is probably much higher than what you worry about > when you look in trends of your gas consumption over time. (The reason is > that at higher pressure air is significantly less compressible than at > lower pressures). > One's gas consumption rate is variable based on a lot of factors, This is one of a lot of presumptions made in diving. We might find that we are compounding errors in some cases. This is another reason why we do things like use "Rule of Thirds," "Gradient Factors," and even "Safety Stops." These take care of the slop in our measurements, usage, physiological changes and environmental differences. One example of slop in planning is relevant here. When we determine our gas usage rate we might be using the top, middle, bottom, or all three parts of tank. In the water we use pressure because, like you said, that's the gauge we have. Our gas usage rate varies a lot and our measurement is inaccurate because of problems measuring real gas. Like you said we can be off by 10% easily. So when we plan deco dives, we round-up, add to, or multiply our consumption rate to provide a safety factor that, among other things, compensates for slop. This is not to say we shouldn't try to be accurate and use real gas calculations. If we get more accurate in more things maybe we can improve our dive planning. You will not be able to seriously monitor your gas consumption over time if > you use pressure/time units unless you always use the same cylinder and > always start your dive with an identical starting pressure. > I suspect pressure/minute@1ATA was more convenient when everyone dived LP72s. Now we have a slew of different sizes and working pressures. > > For this reason I am convinced that option a) is simply wrong (or for a > less aggressive term: uninformed) and I will veto using it in Subsurface or > even offering it as an option to the used (just as I would veto offering to > turn off the real gas corrections even though we get many complaints about > Subsurface getting gas calculations „wrong“). > > Agreed. -- John Van Ostrand At large on sabbatical
_______________________________________________ subsurface mailing list -- [email protected] To unsubscribe send an email to [email protected]
