doubles worth it???

[DA Aquamaster]

I also normally use double 72's on dives that could be done single tank. In part because I still want the redundancy and if I sling a tank it just as well be a deco gas rather than a pony. I also like diving the same basic configuration all the time. So about the only single tank dives I do anymore are shore dives where I have to haul the tanks up/down/to/from the water, or ice dives where getting out of the water with doubles can be a pain.

I do agree that there is more drag with doubles, but less than you think. Most divers saddle themselves with more drag than that through poor trim, poor positioning, and poor streamlining of their equipment. So in practice, with attention to all the other details a double tank diver can still cruise along in relative ease compared to their single tank counterparts.

 

[mempilot]

I agree with you. I like the convienence, safety, and familiarity with SOP by diving the same configuration on every dive no matter the depth. The only change is in the actual cf volume and not the configuration.

I, too, only use a sling tank for deco or travel gas purposes, and the emergency reserve comes in the form of extra volume and redundancy in the doubles.

Smaller doubles have smaller diameters in the 7" or less range which are fairly compact against the body. Total width of this system can be as little as 15" wide compared to 17" or more for larger doubles. The weight of two small LP steel tanks isn't much more than a large AL single and is quite manageable both in and out of the water.

I find it much easier to shut down a valve on a manifold than on a h-valve due to the wider spacing. The h-valve only solves 2/3 of the potential failure in it's system, while a manifold does a more thorough job of securing the failure possibilities.

I also normally use double 72's on dives that could be done single tank. In part because I still want the redundancy and if I sling a tank it just as well be a deco gas rather than a pony. I also like diving the same basic configuration all the time. So about the only single tank dives I do anymore are shore dives where I have to haul the tanks up/down/to/from the water, or ice dives where getting out of the water with doubles can be a pain.

I do agree that there is more drag with doubles, but less than you think. Most divers saddle themselves with more drag than that through poor trim, poor positioning, and poor streamlining of their equipment. So in practice, with attention to all the other details a double tank diver can still cruise along in relative ease compared to their single tank counterparts.

 

[mweitz]

I don't know, I think it is a valid question. I know a few divers that use doubles for recreational dives. My wife tried mine after her 15th dive or so and really liked them and has been diving them ever since.

Nice things about doubles for recreational dives:

1. You don't have to swap out tanks between dive 1 and dive 2
2. You get more bottom time out of a set of doubles than you would with two single tanks (I'm going to simplify this, but if you save 500 PSI on the first dive you lose it. With doubles you can use it on the next dive, or make your first dive a little longer.)
3. You have some added redundancy compared to single tanks

Things to be cautious of:

1. Getting REALLY bent
2. Knowing all about the manifold, how it works and when to turn what knob. Your diving buddy needs to understand it as well so they can assist you if needed.
3. Your fill station needs to know not to close the isolator when filling the tank, especially with Nitrox fills
4. A doubles rig is fairly expensive. Figure a couple grand for the doubles, with a BP / Wing, a couple first stages and a couple second stages.
5. If you are diving steel doubles it is recommended to use a drysuit.

With all that said, the best thing to do is after you have been diving a while, try to find someplace that rents doubles and hire one of the staff to go out with you and show you how to use them.

Best of luck and keep asking questions!

Mark

 

[Xman]

...snip...
2. You get more bottom time out of a set of doubles than you would with two single tanks (I'm going to simplify this, but if you save 500 PSI on the first dive you lose it. With doubles you can use it on the next dive, or make your first dive a little longer.)
...snip...

Whilst I agree with most of your post, I don't agree with number two (quoted above).
If you save a 500PSI reserve in your doubles at the end of the second dive, then this is no different to saving 500PSI in each tank if used seperately. The 500PSI in the manifolded doubles = twice the actual gas that 500PSI is in a single tank.
If you mean that you will go lower than 500PSI in the manifolded doubles, then you are defeating the point of the redundancy.
This is the confusion that arises when you discuss PSI(or BAR/mPa) as though it is a measure of gas volume. It is entirely dependant (as you know already I'm sure) on the internal volume of the specific cylinder/s being discussed.

 

[mweitz]

With a single tank you need to come back with some gas at the end, sometimes quite a bit depending on the dive. In doubles, if you use them for two dives, and you do the more aggressive dive in the beginning, you can safely fully use half the volume of the tanks. So say you have double 12L tanks. You could use all 12L on the first dive, and then finish the second dive with a bit of air in your tanks (doing it safely of course). Does that make sense? My example wasn't very good, for the reason you stated, but it does hold true for most of the recreational dives I do...

Mark

 

[Xman]

I think I understand what you're getting at, but I don't agree that its a good idea. From what I understand, you are saying that you run your doubles down to less than the 500PSI reserve on the second dive. Whilst this is certainly doable, it is not in my opinion a good idea as I stated above. The same reasons for keeping 500PSI in your cylinder remain the same for doubles. If you are using them for redundancy (the only sensible reason I can think of), then you are not leaving yourself with 500PSI (worth of a single tank) minimum available gas if you do this. If something fails on one of your regs or valves, then you are left with less than 500PSI in the remaining cylinder. This is optimistic as it assumes that you isolate instantly and conserve all of the remaining cylinder's gas.
Whilst this level of redundancy is probably not required for the sorts of dives that we were discussing, I cannot see how you can sensibly suggest running your doubles down to less than 500PSI intentionally during a dive.
By my way of thinking, if you end up with 500PSI minimum in your doubled up 12l cylinders, this is NO DIFFERENT than diving two dives with single 12l cylinders, ending with no less than 500PSI.
I will use BAR in my reasoning rather than PSI, as I am far more familiar with metric.
You do dive 1 with a 12l cylinder initially filled to 232Bar, and end the dive with a minimum reserve of 50Bar. You have 2784l total in your cylinder at the start of the dive. This gives you 2184l gas available to be used during the dive, and 600l of gas minimum remaining in the cylinder at the end of the dive. The same applies to the second dive. This means that over your two dives you have 4368l of gas to be used for the dives.
If you were using manifolded 12l doubles starting at the same 232Bar then you start dive 1 with 5568l total gas. Ig you are to finish at the end of dive two with 50Bar minimum, then this will leave 1200l of gas at the end of the second dive, giving you 4368l of gas to be used over the two dives.
If your point is that you can spend longer on the first dive (gas availability wise) and compensate by having a shorter second dive if using doubles then I agree. Otherwise, I don't see how you end up with any extra available gas this way (without cutting into reserve at the end of dive 2).
If I am missing something, please bear with me and explain where I have misunderstood.

 

[mweitz]

That isn't what I'm saying at all. What I'm saying is that the reserve you would have kept on your single tank for the first dive now becomes usable gas on the second dive. If it was a single tank dive, you would lose that gas. On an aggressive dive, that could be up to 1000 PSI (or almost a third of the tank)

Keep in mind that rock bottom on my set of double tanks would be 500 PSI on shallower dives, meaning you can safetly start your ascent at 500 PSI with enough gas to get you and your buddy to the surface. For me, 500 PSI is about 28 CF of gas, quite a bit.

EDIT: Remember, we are talking about recreational dives. So, in the scenario I've suggested the second dive would be in the 30 - 50' (10 to 15 M) range.

Make sense?

 

[The Kraken]

"That isn't what I'm saying at all. What I'm saying is that the reserve you would have kept on your single tank for the first dive now becomes usable gas on the second dive."

If your tanks are connected by a manifold and the isolation valve is open, would not the pressure in the two tanks be reduced equally during the dive ??????

If you were diving independent singles you might have a position here for your argument.

 

[Xman]

That isn't what I'm saying at all. What I'm saying is that the reserve you would have kept on your single tank for the first dive now becomes usable gas on the second dive. If it was a single tank dive, you would lose that gas. On an aggressive dive, that could be up to 1000 PSI (or almost a third of the tank)

Keep in mind that rock bottom on my set of double tanks would be 500 PSI on shallower dives, meaning you can safetly start your ascent at 500 PSI with enough gas to get you and your buddy to the surface. For me, 500 PSI is about 28 CF of gas, quite a bit.

EDIT: Remember, we are talking about recreational dives. So, in the scenario I've suggested the second dive would be in the 30 - 50' (10 to 15 M) range.

Make sense?

Yes, I understand better now. I agree that if your first dive was not infact going to end up with you at your 500PSI (or 50Bar or whatever you decide to use) minimum pressure, then the unused portion of the gas available for the first dive can be used on the second. On that we agree. However, I do not agree with your reasoning as far as the second dive depth since we are talking about recreational dives. It is by no means uncommon that the second dive would be to a greater depth than the 10 to 15M range that you describe. If the second dive is this shallow then why would you need the extra gas available from the first dive's leftovers anyway?

 

[mweitz]

Since I'm basically diving a 200 CF tank, I can plan my first dive to where I surface at 1750 PSI (or 100 CF and 100 CF usable). If you are using a single 100 CF tank for the same dive, you have to plan your dive to arrive at the surface with something like 800 PSI for a 100' dive. Once you hit the surface with a single tank, whatever is left in it is basically lost. On doubles (with an isolator manifold) whatever is left over from the first dive is usable gas on the second dive. Sometimes I'll end the first dive with 2000 PSI (or 116 CF). Sometimes I'll have a great first dive and come up with 1500 PSI (or 87 CF). In any case, I have many more options and get more usable gas, and more redundancy in a set of doubles with an isolator valve then I would with singles. I also have a couple more points of failure (the manifold itself).

BTW XMan, it takes more than a regulator failure to lose half your gas supply. You would need to extrude an O-ring or something along those lines. When you shut down one of your posts with an isolation manifold the other post still has access to all the gas in both tanks. It is just when you shut down the isolator that you lose access to the other half.

Keep in mind that these are just a few scenarios and reasons for using doubles for recreational dives. Where I dive, you can easily do 100+' dives from the shore, so it is very nice to do your first "agressive" dive with 200 CF of gas, and then play around in the shallows for the second dive. More bottom time is simply more bottom time, which is better. Isn't that why we dive

EDIT: Another good reason for doubles is doing a single LONG dive. We recently did a dive to 100' or so, and worked our way back up to 20 - 25 feet. We spend almost an hour in that range and the dive itself was 94 minutes. It was an amazing dive. We generally like to spend some extra time in the 20' range poking around to off gass after a deep dive.

Mark