Twin Cylinders? Doubles?

[Caveeagle]

I dive both BM and SM doubles. It just depends on situation and requirements.

I dive mostly in caves. FWIW...

These are pros/cons as they apply to me..

BM pros: quicker set up time. I can walk from my truck straight to the water and don't typically have to make multiple walks to the site like most in SM. For me, BM makes it easier to deal with stages, for cave dives where you drop and pick up one or more stage tanks during a dive. Also fewer gas switches because BM doubles are (usually) manifolded.
*much easier to transition from single tank diving to BM doubles for most divers.

BM cons: carrying them is not an issue for me. There are several dive sites that are either not accessible in BM, or where they get low and silty enough where it's more challenging to get cleanly through restrictions. Less gear flexibility when you have tanks banded/manifolded, they are not easily switchable for SM or single tank usage. Being able to reach valves for shut down drills can be difficult. Especially in a drysuit.

SM pros: Gives me access to a couple caves that I just don't fit in w BM. Diving river caves from my small Jon boat, SM tanks are much easier to handle and enter/exit the water. Tanks can easily run double duty as singles for OW diving.

SM cons: requires more attention to gas switching and gas management. Gauges can be hard to read. Requires a lot of tweaking to get right. Much less standardization, so lots of trial and error. Stages are a PITA for me In SM. I can do it, but its just more challenging than BM.

 

[PoloGreen94]

Thank You All for the responses, it makes more sense now!

“Also fewer gas switches because BM doubles are (usually) manifolded.
*much easier to transition from single tank diving to BM doubles for most divers.”

Can you please explain this a bit more though? How do you switch with BM doubles? Is it as simple as doing a reg exchange? Also, in a setup with BM Doubles, I’m guessing you have a cylinder filled with one gas (Air, Nitrox, whatever) and a second cylinder filled with another gas (Helium), or whatever is dictated for that dive, correct?

 

[RainPilot]

Both tanks are filled with the same final blend. Imagine if the oxygen tank failed, what would you be breathing?

If you are used to diving with a single tank on your back, the training and adaptation to having two on your back is a lot simpler than moving to sidemount tanks.

The fewer gas switches comment is about changing between tanks on sidemount to keep them balanced, with backmount you would use the same reg for the whole dive.

 

[MaxBottomtime]

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?

I used to have a boat with a flimsy ladder. We got into our doubles with deco bottles on the boat, back rolled into the water and when we surfaced we removed our gear in the water. We never had to walk nor climb the ladder with tanks/weights.

 

[DanielRJones]

Thank You All for the responses, it makes more sense now!

“Also fewer gas switches because BM doubles are (usually) manifolded.
*much easier to transition from single tank diving to BM doubles for most divers.”

Can you please explain this a bit more though? How do you switch with BM doubles? Is it as simple as doing a reg exchange? Also, in a setup with BM Doubles, I’m guessing you have a cylinder filled with one gas (Air, Nitrox, whatever) and a second cylinder filled with another gas (Helium), or whatever is dictated for that dive, correct?

Most back mounted doubles are manifolded which means they are connected together so if using 2x 12l tanks they act as 1x 24l tank so you can use the same regulator for the full dive. With sm you have to swap from side to side to get equal use of each cylinder.

Most people learn with 1 cylinder on their back with a primamy regulator and backup and a single spg. When diving bm doubles you still have 1 primany 1 backup and 1 spg

 

[Akimbo]

If it helps, this diver (Al Giddings) is using twins...

He and the rest of the film team also used doubles on the Andrea Doria in the 1960s... breathing air. Divers who are not familiar with Al Giddings' work might find these links interesting:

• Academy of Underwater Arts & Sciences, Al Giddings
• Scuba Diving Hall of Fame, Al Giddings
• AL GIDDINGS and the first UNDERWATER BLOCKBUSTER MOVIE
• Tech Diving Magazine, .pdf, Interview starts on Page 56

Al was one of my childhood heros. I had the chance to dive with him a number of times in the 1960s and my first chamber dive was at his dive shop in San Francisco, the Bamboo Reef. He is one of the most successful underwater photographers in history.

 

[Caveeagle]

“Also fewer gas switches because BM doubles are (usually) manifolded.
*much easier to transition from single tank diving to BM doubles for most divers.”

Can you please explain this a bit more though? How do you switch with BM doubles? Is it as simple as doing a reg exchange? Also, in a setup with BM Doubles, I’m guessing you have a cylinder filled with one gas (Air, Nitrox, whatever) and a second cylinder filled with another gas (Helium), or whatever is dictated for that dive, correct?

No switching of regs in typical BM doubles, as the manifold gives each reg access to both tanks. Only switching is to test breath the backup reg, and for stages/deco. There are some who use non-manifolded "independent" doubles, but that's not as common.

When I got my kids set up for diving single tanks, other than having single 1st stage, the hose lengths, positions and touch points are nearly the same as a "hogarthian" doubles rig. One of them moved into BM doubles prior to tech training and the transition was very smooth.
FKD - Single Regulator Configuration

If you think you might eventually get into more advanced, or tech diving, you would be well served to start out with a single tank configured as described on the link above.

 

[PoloGreen94]

He and the rest of the film team also used doubles on the Andrea Doria in the 1960s... breathing air. Divers who are not familiar with Al Giddings' work might find these links interesting:

• Academy of Underwater Arts & Sciences, Al Giddings
• Scuba Diving Hall of Fame, Al Giddings
• AL GIDDINGS and the first UNDERWATER BLOCKBUSTER MOVIE
• Tech Diving Magazine, .pdf, Interview starts on Page 56

Al was one of my childhood heros. I had the chance to dive with him a number of times in the 1960s and my first chamber dive was at his dive shop in San Francisco, the Bamboo Reef. He is one of the most successful underwater photographers in history.

I agree. Al is amazing. I still like his work on For Your Eyes Only and the Abyss.

Pretty amazing you got a chance to meet and dive with him. I’ve always heard nothing but great things about him. For a person as successful and as talented as he is, I’ve heard he is extremely humble and down to earth.

 

[stuartv]

I use BM doubles (steel 120s) all the time, unless I have a specific reason to use something else.

E.g. when I'm teaching OW, I use my single tank rig. If I fly somewhere and do recreational dives, I use a single tank rig. If I need some practice time to maintain basic proficiency and comfort with my SM rig, then I dive that. I'm not a cave diver (yet).

Otherwise, whether it's a 20' deep shore dive at the Blue Heron Bridge or a 150'+ plus deco dive on the Oriskany, or anything in between, my go-to is BM doubles. I think they are more convenient than SM, and I prefer the redundancy and extra gas, as compared to diving single tank. My doubles feel like snuggling with my binky (i.e. safe) compared to diving with a single tank rig.

I'm not a huge guy, nor am I a spring chicken, or in particularly great shape. But, I don't find the weight of double 120s to be a problem - even when I'm climbing the ladder in "sporty" ocean conditions off North Carolina.

 

[ajduplessis]

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

Yes, this is how I dive.

Ring mount bungees with a buttplate