Understanding The USE OF DOUBLES??????

[TSandM]

I think it's been covered, but here it is in bullet point form:

1. Increased total gas supply for longer or deeper dives

2. Redundancy, because you have two first stages separating two second stages, and two inflation sources, so if one fails, you still have another one.

As dives get deeper or go into overhead environments, both of those things become increasingly important. You can provide yourself with both either by diving backmounted doubles, or by using a sidemount setup.

BTW, doubles DIVE wonderfully. They're just a PITA wherever gravity is relevant. I'd dive them all the time, everywhere, if somebody else would get them into and out of the water for me

 

[covediver]

All good reasons if that is the kind of diving that you want to do.

Others just like to dive with doubles. Some do it because they have to (like when I did it when I briefly dived at club med decades ago as a quest instructor).

Others do it because they think it is what the "cool kids" do, and that is perfectly fine. Satire is the highest form of flatery. But for those motivated because it is what the "cool kids" do, get the training the cool kids have and understand the rigors of doing that type of diving.

 

[Doc Harry]

They're just a PITA wherever gravity is relevant.

Well said.
Especially steel doubles.

 

[kwinter]

Two tanks and two regs gives you redundancy. And it gives you redundancy.


iPhone. iTypo. iApologize.

 

[DaleC]

Large volume.. redundancy..?
I'ma gonna return these. They must be defective.

 

[Storker]

1. Increased total gas supply for longer or deeper dives

2. Redundancy, because you have two first stages separating two second stages, and two inflation sources, so if one fails, you still have another one.

3. Better balance and a COG closer to your back compared to a big single, which may be needed even on rec dives if you're just a little bit of an air hog. At least in drysuit country, where normal sac rates are a bit higher than 0.5 cuf/min.

A "normal" rec single around here is 10L 300 bar, or 12L 232 bar, both with a capacity of ninety-something cuf. Some prefer e.g a 12L 300 bar or a 15L 232 bar to have a little extra gas. I have mates who prefer a double 6L, double 7L or double 8.5L to those singles. They make for a really nice little rig, with better balance and a COG closer to the back than any of the singles I mentioned.


--
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Typos are a feature, not a bug

 

[Akimbo]

Here is a cutaway image just in case there is any confusion over how a doubles isolation manifold works. Note that the left and right valves control flow to the regulator but don't stop flow from the cylinders to the manifold.

Higher res version: http://www.xsscuba.com/graphics/valves/manifold_lg.jpg

 

[Diver0001]

I don't know why but I am having a hard time trying to understand doubles. Is the use of doubles strictly for wreck diving?
Is it because God forbid you get lost?
Is it so they can stay down longer?
But then wouldn't they have to deco.

I'm sorry if its a dumb question.
Just looking for someone to explain it in basic terms.

You're asking two questions. Doubles are intended for two purposes. (1) to give you more time and (2) to provide redundancy. Let me handle them one at a time:

1) Your first question is about #1... more time. If you get lost in a wreck then the fact of having doubles is only going to delay the inevitable if you can't find a way out. It won't prevent it except to the extent that you had more time to figure it out. Having more time, however, means having more options and it delays the point at which "problems" become "emergencies".

2) Doubles will allow you to go WAY over the NDL's.
Yes. And this is the point in most cases. There are divers who dive WELL beyond the NDL's, which can be done safely with the right equipment and procedures. Doubles allow for "redundancy" which means that problems like a free flow can be solved under water without having to go to the surface. In fact, once you're over the NDL's you CAN'T just go to the surface, so problems MUST be solved under water. Having redundancy means you always have a "plan-B".

And in general, doubles usually means that you have boatload of GAS which means you have a boatload of TIME to solve problems.

R..

 

[Akimbo]

You're asking two questions. Doubles are intended for two purposes. (1) to give you more time and (2) to provide redundancy...

Just to expand on Diver0001’s fine summary: There is a relatively rare exception the “more time” component and that is there are a few “baby doubles” in use. The prime motivation to use double 45 or 50s with an isolation manifold is redundancy and generally riding better on your back — even though they pack less gas than a large single.

An even less common application of doubles of any size is using a technique often called Progressive Equalization. The link explains it but the intent is to provide one or more “hard reserve” events, even when you forget to look at or can’t see your SPG due to zero visibility.

 

[FinnMom]

In addition to what other have said, I consider doubles an obvious choice for our local diving because we have to wear the weight anyway and might as well carry it in tanks with more gas. We need the weight because we wear drysuits and very thick undergarments that produce a great deal of bouyancy. For example, in winter I wear wool blend long underwear, a wool sweater over that, the thickest undersuit on the market and 2 wool socks on each foot. In summer I leave out the sweater and sometimes 1 of the sock pairs (BTW not joking).

Doubles are also really neat for carrying one big weight held securely between the twin tanks. It's great having that weight spread across your back instead of hanging on or near your waist. If you want ditchable weight on your belt, 3-4kg is more than enough to make the different between neutral and distinctly positive bouyancy. All the rest of the weight feels much better if you carry it elsewhere. The lighter belt is also less likely to slip and is vastly easier to manipulate if you do need to remove and replace it underwater.

That said, the single biggest reason I like doubles in cold water is that they feel great. It's like carrying the weight in a high-quality pack held securely against your back instead of some huge tube of weight hanging away some distance behind you and freely shifting its weight from side to side.