Diving doubles recreationally

[elan]

As I said, it's worse than that of *everyone* I dive with. It's still 25% worse than your low average for cold water. *I* would personally be having a terrible day if I were breathing that high.

In any case, most divers aren't going to have an even higher SCR than that (as suggested by the diver I quoted). That's all.

I agree with your thought on hp 130 being enough for a rec dive

A 100 ft ndl dive on 32 usually leaves my stage "dry" and sometimes I switch to backgas before or on the ascend. I do not remeber ever going over 90 cuf

 

[elan]

If you need a V-weight for AL80s (i.e. you dive in cold water), then they really aren't that light at all. In fact, they'll weigh about DOUBLE what a single HP130 weighs. That's a lot of extra weight for only about 20cf more gas.

Ditto

Al80 have advantages but the dry weight is not one of them when you dive cold and/or salt water. They are actually heavy. My lp72 set is lighter by at least 8lbs having roughly the same amount of gas. And HP100 would be as light as the 72 but would have more gas.

 

[OldNSalty]

OldNSalty - I apologize for the bs/drama that has derailed your thread.

Drive on Airborne.

 

[OldNSalty]

That might work if you just want to consider them as a large single tank, but without some knowledge of and practice with the valves and failures, you'd be giving up the redundancy they offer.

Clearly you can't know what you don't know but I have done some reading on the isolation manifold concept and is another reason why I like this set up. I don't dive with equipment I don't understand and so I would look for someone to at least introduce me to the setup.

 

[Lobzilla]

OldNSalty - I apologize for the bs/drama that has derailed your thread.

James, let's bury the hatchet.

I think we agree that doubles are a great tool even for rec diving if chosen and set-up properly. However, they can also be an aggravating curse if they are the wrong tanks for a certain physique or if they are rigged improperly on your back.

If a diver is head-heavy in a singles rig, we shift the tank towards the tail and the problem is solved. We cannot do that with doubles as we would not be able to reach the manifold anymore. This, plus the fact that doubles are a little head heavier due to the manifold makes the choice of tanks and proper set-up critical.

The most common problem is when people go from vertical to horizontal in doubles they will reach a point where the 'fall on their head' before they reach horizontal trim. Ankle weights are a very poor way to solve this problem that should not exist in the first place. Obviously, many divers choose that quick fix and their instructors do not stop them and suggest better options. Diving in a 20 degree nose up position is also not an effective way to deal with the issue and becomes a serious problem if we operate in close, silty quarters or have to do tie-offs. (My personal goal for a great set of tanks is 10 degrees nose down and stable without finning) Again, too few instructors insist on full trim control.

I started the brew ha ha by suggesting to ask a GUE instructor for help as I KNOW that everyone of these will get it right. Obviously, there are many more instructors out there that understand the physics of the problem and can help OldNSalty and others who are interested in trying doubles.

Here is a recipe that worked for me and many others I dive with. YMMV, take this with a grain of salt as I do not have much practical experience, and double check with your instructor.

Firstly, pick the right tanks. Roughly speaking, tall people need tall tanks and short people short tanks. But there are nuances in the weight distribution of different types and manufacturers. An experienced instructor should be able to look at you and make a good suggestion of what works and what does not. Don't just buy something that is cheap on craigslist.

Secondly, the manifold should be approximately at the same height than the top of your shoulders. If it is lower, you will have grief reaching the valves. A reliable way to do this with multiple tank sets is to band the tanks with about 6" between the center line of the manifold and the top attachment bolt (I use a dollar bill as a gauge). Then, adjust your backplate harness to put the manifold at top of shoulder height. This way, all tanks sets will be positioned the same way.

To get the trim perfect, you can then experiment with a tail weight or with changes in your undergarments. Example: With my HP100s that are a tad head heavy (because I should really use longer 120s) I have to add a fleece vest to get them trimmed. Adding more lead on the tail does not make sense as they are already negative enough. But adding buoyancy to the torso solves the head heaviness nicely and keeps me warm where the 100s get me.

The fine-tuning requires that you can feel when the trim is right. Again, this is were a good instructor is invaluable. If at some point you prefer doubles over singles (except for humping them to the water) you have figured it out.

Now, we only need to understand the failure modes and practice the correct response to take advantage of the redundancy of manifolded doubles. Another job for a good instructor.

 

[mutiny360]

I dive doubles on every dive, except when I fly somewhere. I dive double 100s and the work for 2 boat dives off north carolina. I also like the piece of mind always have redundancy. For me there was no option of doubles and single tanks. I don't have that much gear laying around. I have my double 100s, and a single 119 for my wife. I don't count my aluminum stage bottles because I don't like diving aluminum tanks with a drysuit. Depending on the gear you already have, it may not be that expensive to change to doubles.

 

[TSandM]

Getting back to the OP's last post . . . 72s are reasonable doubles. I don't like them, because for whatever reason, I cannot trim them out. (Spent a whole morning working with Bob Sherwood, who has a reputation of being the guru of trim, and he finally said, "You're right. I guess you CAN'T balance those tanks!" It is also almost certain that, unless your gas consumption is lower than mine (which would be unlikely) you won't get two dives off a set of 72s, unless the dives are very shallow or fairly short, which again negates some of the value of doubles. You still get the redundancy, and the stability in the water (I think doubles dive MUCH better than single tanks, and I would dive them exclusively if someone else would get them into the water for me ) We have two sets, keep them full of 32%, and use them when we just want to dive doubles for some reason, or for a single dive where redundancy is wanted.

If you have the tanks and the regulators only need some hose changes, then look at the Deco Stop for a used doubles wing. People are always either getting out of tech diving, or switching to CCR and selling their OC stuff. The bands and manifold for 72s, you are likely to have to buy new, though; rarely are the appropriately sized things available for sale without the tanks that go with them

 

[ianr33]

If a diver is head-heavy in a singles rig, we shift the tank towards the tail and the problem is solved. We cannot do that with doubles as we would not be able to reach the manifold anymore. This, plus the fact that doubles are a little head heavier due to the manifold makes the choice of tanks and proper set-up critical.

Ankle weights are a very poor way to solve this problem that should not exist in the first place. Obviously, many divers choose that quick fix and their instructors do not stop them and suggest better options.

I don't understand your point about manifolds at all. On both my double Al 80's and double 95's the bands are as high as they can reasonably go on the tanks. (Just at the point where the tanks start curving over) Can reach the valves no problem.

I don't use ankle weights as I prefer Jetfins.

 

[TSandM]

ian33, it depends a lot on body type and tank size and characteristics.

A lot of people have trouble reaching their valves, which causes them to move the tanks up on their backs. This can result in the diver being off-balance and head-heavy.

On the other hand, people who are head-heavy try to move the tanks down. Once you have the bands at the break of the tank's shoulder, you can't move the tanks down in the bands any more, so your only option is to move the whole doubles setup down, either by selecting a different hole to put the bolts through, or by loosening the shoulder straps. Past a certain point, you may no longer be able to reach the valves, and then you have lost the benefit of double tanks over a big single one.

The better you get at reaching valves, the lower you can put the tanks, and the better they'll balance for most folks. If you dive where you require additional weight, you can also use this creatively for balance. I have a 14 lb v-weight I sawed in half, so I can hang 7 lbs off the bottom bolt -- that will ALMOST balance the LP72s I have, but does a spiffy job of balancing the Al80's in Mexico (as well as, conveniently, being all the weight I need to carry to be properly weighted with those tanks).

 

[DevonDiver]

I dive doubles for recreational diving all the time. I don't have any of the problems stated on this thread by other divers. The reason is; for recreational diving, I use independent (cam-band mounted) doubles.

I typically do 3-dive trips, diving wrecks. I get on the boat, set up my kit...and slip two rental AL80s into the cambands. It's nice and secure, very stable and offers me the safety and flexibility I need to conduct the dives I do.

The way I use them depends on the dive plan. It's just a matter of gas management. For easy penetrations and lite-deco, I 'balance' the tanks (swapping between both tanks during the dive) and sometimes (for truly 'recreational' dives) I just drain one tank, keeping the other as reserve. Either way, I ensure I have redundant 'rock bottom' gas.

A lot of wings won't take doubles without steel bands or some form of an adaptor and some modification. But, if you shop around and do some research, you can find both wings and jacket BCDs that will take indie doubles without the need for steel bands and bolts.

I use a Custom Divers TDB. The backplate is cut and shaped to accept either a single tank or doubles with cam band. The wing itself has matching cam-band slots.

Doubles are a great solution for many problems. They aren't a great solution if you cannot use them because of the logistics/cost of hiring tanks when away from home... but that isn't necessarily the case. People just seem so fixated on the idea of steel banding/manifolding them.

Another options is side-mount. I've seen that mentioned once on this thread, but nobody seemed to notice. Side-mounting is getting much more popular nowadays and offers a great solution for carrying two cylinders without the expense and logistics needed with back-mounted (steel banded) doubles. Again, you can step onto any charter boat and slide two rental AL80s into the kit Courses to use side-mount are becoming easily available - PADI have even brought out a speciality course.