Recalculating Thirds

[twaldo]

I never dive thirds. I always cut back by at least 100 psi, usually 200-300 psi. A small adjustment like that provides 3 times more gas in your reserves. And most people won't get that much farther breathing another 100-200 psi.

Again,

Thank you....

my dive computers are set to the most conservative option they have and I turn my dives before 3rds.... Ok.. so I only have 3 cave dives to date.... I still plan on keeping that profile.

Tim

[Nitrogenius]

A small adjustment like that provides 3 times more gas in your reserves.

That is a little exagaration unless you changed the laws of math..
but it does have significant Impact. At least it most likely keeping you in the area of the real usable gas in case of a gas emergency and Exit as Long as you can controll your SAC.


If you start with a 3600 psi cave fill a third yould be 1200 psi.. if you go 100 psi less you turn at 2500 psi and have used 1100.. if you losse your gas and are on your buddies (Hogarthian BM) or you loose one tank being solo (worst case Scenario) than you have 2500 psi/2 1250 giving you 113% over the 100% reserve which you would have diving exactly thirds.. however if you consider that with most regs you can't even breath the tank down to 0 you are pretty much at a real 100% reserve diving 100 psi less..but you still would have to 100% maintain the same SAC (assuming no flow conditions) to safely make it. This is why a simply 100 psi Feeling based additional reserve doesn't cut it for me as the real risk and reserve Situation strongly depends on the number of Independent cylinders and no of divers.. I usually tend to plan my gas reserves from that perspective as if I was solo diving though unless I know the Partner really really well.. But SM with a buddy already gives you plenty of reserve if your f** up Scenario is one tank failure (being the rule of third s Basis all along) Even diving thirds then you almost should make it to your Exit yourself and you have another 100% reserve with your Buddy.. Same applies for diving solo on thirds with 3 tanks.. So in some circumstances I am perfectly happy with thirds whereas in others 100 psi would not have me comfortable (as described in teh example above).
Now you mention sometimes cuttingt it 300 psi short, well that of course is a different Story as this already puts you at fourths so giving you an additional 50% of reserve (150% total) vs the 100% on thirds.. The latter (fourths) is the absolute Minimum reserve I would use in the constellations mentioned above (2 BM Hogarthian or solo with 2 SM tanks)
Having said that along my first 50 Cave dives doing backmount with Hogarthian I did in fact thirds being in a Team of 2, but I had never thought that entirely through, as with manifolded doubles the worst case really is that you might not be able to preserve any gas (failure at the manifold valve)..

[RN]

That is a little exagaration unless you changed the laws of math..
but it does have significant Impact. At least it most likely keeping you in the area of the real usable gas in case of a gas emergency and Exit as Long as you can controll your SAC.

How so. Starting with 3600, 1200 is thirds. Cut back by 100. That's 2200 for the dive and 1400 for reserves - 1100 plus 300 or 3 times the 100.

Cut back by 200. That's 2000 for the dive and 1600 for reserves - 1000 plus 600 or 3 times the 200 you cut back.

300. 1800 for the dive and 1800 for reserves - 900 plus 900, 3 times 300.

[Nitrogenius]

How so. Starting with 3600, 1200 is thirds. Cut back by 100. That's 2200 for the dive and 1400 for reserves - 1100 plus 300 or 3 times the 100.

Cut back by 200. That's 2000 for the dive and 1600 for reserves - 1000 plus 600 or 3 times the 200 you cut back.

300. 1800 for the dive and 1800 for reserves - 900 plus 900, 3 times 300.

Ok I guess I misunderstood what you meant in regards to the 3 times.. I was refering the 3 times towards the absolute reserves. Of course if your Basis was thirds any amount for itself you could back you would have a triple reserve... Has to be that way..