I have to disagree with your example here, Larry. Steve's example does require one additional switch but is much safer. Check out this article -
Sidemount Gas Management « Rob Neto - for an example of how your example might not be safe.
I certainly see your point, and it's a very good one so I won't argue the point - much.
But there is a larger systems issue to be considered and you have to look at the the whole package and approach.
In your example everyone goes away happy without buddy breathing, while the 1/3 rd differential is problematic. No disagreement there, but the solution was not to get there in the first place.
An advantage of the "one third" maximum differential approach is simplicity in terms of number of gas switches and minimum complexity in pressures to switch at - you only need to calculate and remember 2 numbers, so it's easy even at ENDs in the 100-130ft range.
The simple approach also makes it very easy to dive smaller "thirds" that will build in an extra gas reserve while maintaining balanced tanks with no real effort or mental gymnastics for the diver and still with only 2 reg switches. In your example the diver comes out 200 psi short of having enough gas to exit. However, if instead of diving 1100 psi thirds, the diver turned only 100 psi earlier, that extra 200 psi that is needed is now present, as there is 1300 psi in the lowest tank at the failure - and since the dive turned 100 psi sooner, that same failure is also 200 psi closer to the exit, so the gas pad in the "low" tank is now 400 psi. The only downside is that I probably got to see a little less cave - but I have a lot more reserve to deal with all manner of other delays and emergencies that could occur on any given dive, so I am ok with that.
As an aside, I also dive with a 5 foot hose on each tank, so I can give the larger tank to the probably excited OOA diver who probably has an elevated SAC. That avoids the potential of having to give the low tank to an excited diver and further complicating the gas crunch.
In that regard I agree with you that pushing a 1/3rd differential in full "thirds" gas management is problematic - *when diving mixed teams*, but requires multiple failures to become a problem on a side mount only team, and you address that indirectly in your example, so again I don't think we disagree on that point.
However, I'll go a step farther and argue diving all the way to thirds is potentially problematic on
any dive. Consequently, I'm pretty happy managing my gas with only 2 reg switches, but doing it with 100-200 psi less than a full third used on penetration, especially on a sidemount only team.
The concepts are all related as the degree to which you will burn time and gas dealing with and beginning the exit after a failure is dependent on the cave, the conditions, the configurations and the team members involved. Consequently, gas management in SM aside, with a teammate that I communicate and work well with under pressure 100-200 psi smaller thirds in a touristy cave is fine. But when I dive with a team mate of lesser ability or who is not as known a quantity, the reserve goes up accordingly. Similarly, the reserve goes up with unfamilair cave, tighter restrictions, more potential for silt or other delays, no flow/low flow, siphons, etc. SM versus BM team mates becomes just another factor to consider in planning an adequate reserve rather than defaulting to full "thirds" for penetration.
The point is that the focus is now on a very carefully considered and planned
reserve and switch/turn pressures, rather than the temptation that some divers may have of just switching every 500-600 psi and more or less winging it on what becomes a dive to full thirds gas management with an inadequate reserve.
---------- Post Merged at 09:06 AM ---------- Previous Post was at 08:50 AM ----------
Check SPG, switch reg, is easy.
Yes, it's easy and that's the rub - it invites (apparently) math challenged divers to do less than adequate gas planning and instead just check the SPG and switch on rote 500 or 600 psi increments with no real thought to whether the actual reserve is adequate.
In actual practice the above math in my post is very simple and involves small numbers like "100", "200" or "300" psi, multiplying by "2" and dividing by 3, all of which can easily be done in your head on the surface.
