If you're using an AL80, your ideal redundant air supply would be a 26.4 cf tank...which doesn't exist, so you would carry an AL30.
.
The 'rule of thirds' as defined and as others have mentioned, means 1/3rd in - 1/3rd out - 1/3rd reserve. It's used as a safety factor for overhead environment diving, as others have mentioned. However, as others haven't mentioned, it is
also a standard practice in general technical diving, for both back-gas and deco gas.
In this debate we are talking about redundancy and reserve - because we aren't using doubles, we are using single+pony. Because of that, it is important to differentiate between the
reserve (of back gas) and the
redundancy.
- In technical diving (double tanks), that 'rule of thirds' is calculated by multiplying the calculated gas requirement by 1.5. (50% of total reserve)
i.e. divide your tank by 2 and add on that 1/2 as reserve.
- It is
not calculated by dividing your tank by 3 and adding on that extra 1/3rd - that would provide an
insufficient amount. (25% of total reserve)
If applied to this scenario, it'd apply it to mean a
redundancy 50% of your primary tank. So, for an AL80, your 1/3rd
redundancy would be
an AL40.
The
reserve (of your primary) would be a simple 1/3rd tank, or rock-bottom...
You have a routine 1/3rd reserve in your primary cylinder (single) and your
redundancy accounts for 1/3rd of your total gas. See the nuance?
1. "Rule of Thirds" does not apply to open water buddy diving. It is dangerous to do so, and will not give you enough gas to get out of an emergency in many situations.
If 'rule of thirds' provides sufficient gas for a pair in an overhead environment, I'm perplexed why it would be dangerous in an open-water environment. Given that an immediate abort/ascent remains the luxury of open-water diving. That does, of course, assume that the buddy pair 'turn' and ascend based on the fastest consumption of gas (the person with the highest SAC initiates the turn/ascent) and/or any variation in tank volume is accounted for.
There is no extra requirement in open-water diving. Whether open-water or overhead environment, the diver must carry sufficient gas to return, with buddy, to the surface or the next gas source (stages, deco etc). Either way, the 1/3rd rule applies.
More conservative fractions (i.e. rule of 1/4 or more) tend to be applied in circumstances where more insulation is required because the equal relationship between speed of entry and exit is less guaranteed; such as through restrictions, against flows on exit, or where there is a predictable increased risk of lost viz etc...
2. "Rule of Thirds" applies only to buddy diving in an overhead environment.
As mentioned, 'rule of thirds' has been taught for technical/decompression open-water 'buddy' diving as a standard, by most agencies - both for back-gas and deco gas.
3. "Rule of Thirds" also has no logical basis in OW solo diving. Your gas reserve for solo diving is based upon your calculated needs, not some arbitrary "Rule of Thirds."
In this respect, you are only considering the needs of an unimpeded ascent on demand. It's impossible to 'calculate your needs' for interrupted ascents. Solo divers should account for delayed ascents; due to the risk on entanglement/entrapment etc etc. They should also account for such scenarios imposing unforseen decompression obligations...
To use your example,
for the solo diver, with reserve and redundant gas, here's how I would do it:
OW Buddy Diving on the wreck of the Spiegel Grove
Total Gas = AL80 at 3,000 PSI + AL40 at 3000 PSI
"Rock Bottom" Gas = 1,200 PSI
Sorry.. I can't "think imperial" and don't want to make a maths error... so here's metric:
AL80 volume is 11 Litres / Pressure is 200bar = 2200 litres of air - primary
AL40 volume is 5.5 Litres / Pressure is 200bar = 1100 litres of air - redundant
Total gas volume carried is 3300 litres of air.
Turn point is calculated on backgas/primary cylinder only, as the redundant is not factored as a reserve. That turn-point is 1/3rd of backgas, so: 200 bar/3 = 65bar (rounded down for safety).
Diver 'turns' the dive at 135bar of the AL80, having used 715 litres of air (2585 litres remains in both tanks)
Diver 'surfaces' the dive at 65bar of the AL80, having used 1485 litres of air (1815 litres remains in both tanks)
This provides a very healthy contingency. In the worst case scenario; gas loss at the furthest point, the diver is assured of a 50% redundant reserve volume (1100 litres) in relation to their primary gas - more than enough to return and ascend. It also covers unforeseen issues, such as the level of deco obligation that would realistically present from a moderate overstay, or that unforeseen interruption to ascent.
The upshot of ALL of this discussion is that double tanks is far simpler and safer, compared to single+pony. This is because the reserve and the redundancy is integral and subject to a singular calculation.
