Twin Cylinders? Doubles?

[PoloGreen94]

Hello All,

First, let me state I am not a tech diver. I am an advanced open diver/rescue diver with about 100 open water dives. But one day I would like to pursue tech diving to make it to dive locations which are rarely visited (as opposed to your typical 40 foot reef dives) and to get the chance to see historical sites and/or larger pelagic animals.

I am curious about one thing, and I've always been confused by this. I've seen some folks diving twin tanks in deep dives (beyond 130)? Not your typical side mounts but just two cylinders on the back. Can you perform trimix in such a setup? Is it a more convenient way of tech diving as opposed to side mounts? It certainly looks more convenient. Is there an advantage of one over the other?

Hopefully the question makes sense, I'm having trouble describing it as I don't truly understand all the ins and outs in the tech diving world.

If it helps, this diver (Al Giddings) is using twins in this video while diving the Texas Tower to +160 feet. Starts at around 5:40-5:55


For comparison, there are other divers who are diving the wreck but have different setup than the twins 6:50-6:57. Is there an advantage to one over the other.

Thanks for any feedback you may have to this curious diver!

 

[KevinNM]

Bachmounted doubles were how everyone dove who was diving deep. It’s perfectly viable for trimix. Currently doubles are generally used in deep ocean diving vs side mount. Not universally. Side mount was originally developed as a technique to allow access to areas too low to fit with backmounted doubles. So it it has clear application to technical wreck diving. But some people just like to dive sidemount or have movement restrictions that make them want to dive sidemount in the ocean.

Sidemount is very popular for cave diving, and there are quite a few cave divers on the board. But unless you are diving bedding planes or other tight restrictions it’s because you prefer to dive sidemount.

 

[Olariuas]

Is it possible have a Twinset in Backmount and have Stage and Deco correctly configured as Sidemount ?

 

[RainPilot]

In normal recreational diving, you have two sources of breathing gas when you have a single critical failure. The first best option is to share with your buddy (requires them to reserve enough gas for both of your ascents) and the second, last-ditch option is a CESA so your next source of gas is the surface.

Since you are supposed to be diving inside the decompression limits and with no reason not to go straight to the surface, this gives you one very safe and one (acceptably) risky method of getting to breathing gas.

When you are doing diving where the risk of going straight to the surface is too high (decompression obligation) or impossible (cave / wreck etc) then you need to replace the CESA with something else.

In this case, by having two cylinders with independent regulators on, your first option if a reg fails is to transition to your other reg. In the case of back mounted twin cylinders, this will allow you to have full access to all your remaining gas (if the failure was of a first or second stage, the most common failure). In side mount, you will only have access to the unaffected cylinder unless you change regulators underwater, which is possible but rarely practical. This disadvantage can be ameliorated in the case of a free-flowing second stage (the most common failure) because you can breathe the affected tank by opening and closing the valve as required ("feathering").

If all this doesn't get you enough gas, you still have a buddy for Plan B.

Generally the use of twin cylinders in sidemount or back mount is predicated on gas required (deeper equals more gas use) and redundancy.

Is it possible have a Twinset in Backmount and have Stage and Deco correctly configured as Sidemount ?

I do that. My deco stages are side mounted with a bungee attached to the backplate. It does make it a little more effort to check labels etc for the gas switch but I HATE dangling stages so it is worth it for me.

Edited to add: I am currently only diving with one deco cylinder on OC as per my certification limits, I dive CCR with 2 small bailout tanks (AL40) for streamlining. Once you get to 3-4 additional tanks I imagine that side mounting becomes way more hassle than its worth. Most of the time I see divers like that, they are carrying the additional stages on a leash behind them.

 

[TrimixToo]

There's a set of doubles full of trimix in the car right now, about to head for the water. Differently shaped diver profiles aside, there are pros and cons to both solutions. Gas management is simpler with doubles (just breathe and watch one gauge) unless they are not manifolded or you run them with the isolator closed as independent doubles. Emergency management seems simpler with sidemount (shut down the bad side, breathe from the good side). Access to all your gas after a failure is usually simpler with doubles (shut off bad post, reopen isolator). Managing additional cylinders for deco and staging is easier with doubles IMO, but some sidemounters disagree. People with back and shoulder problems report that sidemount is much easier on them. I've probably left some things out.

 

[Marie13]

Sidemount is also easier with bad knees. SM isn’t optimal for boat diving, but it can be done.

 

[Olariuas]

Sidemount is also easier with bad knees. SM isn’t optimal for boat diving, but it can be done.

What's the relation between backmount and bad knees ?

 

[Marie13]

What's the relation between backmount and bad knees ?

If you have painful, arthritic knees as I do, why in the world would you want to walk with 100 lbs (or whatever the weight is) of tanks on your back? With SM, roll them down to the water line or put them on in the water if boat diving. Take them off in the water and climb ladder without them. Charter op has to be cooperative, though. Some may not want SM’ers.

SM is a no-brainer for many people with physical issues. It’s just common sense. Why put more strain on your body if it can’t handle it?

 

[Olariuas]

How many meters do you do with your twinset on your back ? On the boat you mean ? This is none-sense if you have back or knee pain I understand.

Would this solution be interesting ?
Tank2Go Scuba Diving Twinset Trolley by Venture Diving
Twinset Lifting Bar + Wheels parts
https://www.amazon.com/Magna-Personal-Capacity-Aluminum-Folding/dp/B000HVVSDU/
(can have even cheaper)

Or you can be inspired and build up your own DIY at cheap cost

Found that somewhere - maybe for shore diving it's really practical.

 

[Richard FDC]

I'll toss in my 2 cents here. My garage dive area is a giant tool box and I have the tools I need for the diving I want to do. There's a backplate, harness, and donut wing with a single tank adapter for everyday recreational diving. A set of banded up doubles is used for more advanced dives to go deeper and stay longer. You can put trimix in them and sling stage and deco bottles. Sidemount is great when you need to go through low tight places but a LOT of people just dive sidemount all the time. You don't have to carry a lot of weight on your back but you have more gas and redundancy. All the gear is just another tool in the tool box and the "one size fits all" approach IMHO is not the best. Match your equipment to the dive. Carry what you need to do the dive safely and comfortably and don't bring unnecessary things. The most important consideration is safety. The gear is chosen to provide redundancy when (not if) there is a failure and it is chosen to provide an adequate gas supply to complete the planned dive. Keep asking questions. Keep learning and make informed decisions based on what you want to do.