twin bail out pressure

[cecilb63]

I am curious about when divers use twins...whether for redundancy or for depth, either manifold or two regs.

Do you follow your same rules for two tanks as you do for one? That is, if you by rule always exit/return at 500 psi with a single, do you do the same when with twins or maybe something less?

 

[cecilb63]

oops, I realize this isn't an issue with a manifold!

But for two reg - no manifold setup?

 

[DA Aquamaster]

I dive with independent doubles (one reg on each tank with no gas transfer connection between the two) and use a rule of thirds for the reserve.

What this means in practice is that I breathe 1/3 of the first tank (down to 2000 psi with a 3000 psi tank or down to 1600 psi wqith 2400 psi tanks). I then switch to the other tank and breathe 2/3rds of the contents of this tank (to 1000 psi or 800 psi) and then switch back to the first tank until I reach the surface (or first deco stop if using deco bottles) at which point I should have no less than 1000/800 psi left in this tank as well.

This management strategy limits the gas switches required to two, uses 1/3 rd of each tank on the way in, uses 1/3 rd of each tank on the way back out and saves 1/3 sin each tank as a reserve. It also means that if at any point in the dive either regulator fails or the contents of either tank are lost, the other tank will have enough gas to get me back to the surface (or to the first deco stop on a deco dive with deco bottles).

 

[roakey]

That is, if you by rule always exit/return at 500 psi with a single...

You might want to do a search on this board for the term "Rock Bottom". You'll find that the 500 psi rule isn't a rule, it a cruel guessing game that the recreational dive industry foists upon unsuspecting students.

In other words, "500 psi at the exit" it isn't gas management at all.

Roak

 

[Sea-Diver]

The real "rule" is the rule of 3rds!

 

[Diver0001]

I am curious about when divers use twins...whether for redundancy or for depth, either manifold or two regs.

They're useful when you need to take a lot of gas or when you need full redundancy.

Do you follow your same rules for two tanks as you do for one? That is, if you by rule always exit/return at 500 psi with a single, do you do the same when with twins or maybe something less?

Well, the most important thing in any dive--singles, doubles, recreational or technical--is making sure you get there and back without running out of air. There are several strategies for planning it all depending on the parameters of the particular dive at hand (depth, time, contingeny, navigation) but they all come down to matters of control, predictability and risk mitigation. The 500 psi rule is appropriate for certain types of dives. Other dives need other rules.

R..

 

[roakey]

The 500 psi rule is appropriate for certain types of dives.

As we've gone though in some of the rock bottom discussions, it's never appropriate. It tells you how much you should be "back on the boat" with, not when to turn the dive.

When to turn the dive then becomes a guessing game, since you have no idea, except by a guess, when to turn the dive to end up with "500 on the boat".

In fact, it's probably less than useless because folks who dive this way THINK they're gas planning when in fact they're not. It gives them a false sense of security.

It's akin to giving directions like: "Turn 1/2 mile *before* the big red barn."

Roak

 

[Diver0001]

As we've gone though in some of the rock bottom discussions, it's never appropriate. It tells you how much you should be "back on the boat" with, not when to turn the dive.

When to turn the dive then becomes a guessing game, since you have no idea, except by a guess, when to turn the dive to end up with "500 on the boat".

In fact, it's probably less than useless because folks who dive this way THINK they're gas planning when in fact they're not. It gives them a false sense of security.

It's akin to giving directions like: "Turn 1/2 mile *before* the big red barn."

Roak

I think if you swim a basic out-and-back pattern over either the same terrain or out at a slightly deeper depth and back at a slightly shallower depth if that's possible then turning the dive regardless of where you are at 1750psi is going to get you there.

The other assumptions in this are that:
- the return leg of the dive is shallow enough that 500psi is enough to do an air share ascent
- you have uninhibited access to the surface at all points during the dive
- no current to speak of.

In other words, for this, most basic of all scenario's, a basic turn pressure will work.

If 1750 is the wrong number for you, then use 2000 and say "turn on thirds" and then it will fall into a more comfortable paradigm. I don't think it's worth arguing about. The main message to this guy has to be that 500 doesn't always work and perhaps it only works for the most basic of possible scenario's. Think like a beginner here, Roak.

Having said that, I can appreciate that you don't like this and I will agree that this is not gas planning. It has nothing to do with gas planning and I wasn't trying to suggest that it does....I can see how you read that into my previous post but it wasn't my intention.

R..

 

[DA Aquamaster]

I think we need to separate the terms "reserve" and "gas planning" as the two are not the same.

Gas planning requires that you be able to conservatively anticipate how much gas you will need to complete the dive (including any deco required) as well as determine how much you may need for various contingencies due to deviations in depth, time, gas consumption, sharing gas with a buddy, etc.

Your reserve is what you need to have available to ensure you can meet the requirements for any contingency plans that become neccesary.

So, I for basic shallow water dives within the NDL's a 500 psi reserve may be adequate. But when you get into doubles you are almost always talking about longer dives, deeper depths and potentially decompression, all of which usually require a reserve significantly larger than 500 psi (and will also require much more thorough and complex gas planning).

 

[oxyhacker]

Remember that the 500 psi figure probably has as much to do with gauge accuracy/reliability as it does with air consumption.