Independent backmount doubles with DSS backplate

[2airishuman]

What about your ability to reach the valve on the left tank and be able to turn it off and on, can you do it underwater no problem?

I can't reach it easily unless I use a cylinder with a "reverse handed" valve. I have one cylinder set up that way and will be setting up a couple more like that.

Did you have two dissimilar tanks on the BC? Did it cause any issues in any way at all?

For my first dive, I had a matching pair of LP72s, worked great. On my second dive, I had an LP72 and an LP85, as noted upthread. The slight difference in height made made it a little awkward to set on a flat surface for donning and doffing. There was no noticeable roll or imbalance during the dive.

@CuzzA because the doubles are much closer to your centerline than sidemount, the moment arm is much smaller and you can actually dive a pretty wide swing in tank pressures/volumes without significant impact on your roll stability.

Exactly.

72's and 85's should be same diameter....

They aren't quite the same. I've measured my cylinders. My LP72s are 6.8-6.9" in diameter and the Faber LP85 I have is 7.1" in diameter. Coating thickness may explain part of it.

So, since this configuration, like side mount only uses a single second stage per tank do you keep a reserve in either tank for a buddy? What's the air sharing procedure?

I was solo on a freshwater shore dive with a max depth of 25'. I kept a reserve of 500 in each cylinder, enough to reach the surface, and for that matter, swim halfway across the little lake. Under more demanding conditions I'd keep a larger reserve.

I dive a primary donate configuration with a bungeed alternate. The difference in air sharing is that for half the dive I would be breathing on the alternate and have the long hose clipped to a D-ring. I can't donate the bungeed reg so that would have to be something discussed with a buddy before the dive.

 

[rx7diver]

2airishuman,

I, too, have dove independent back-mounted doubles (2 x OMS LP 46's, 2 x HP 80's, 2 x Al 80's, 2 x HP 100's, and 2 x HP 120's). Like others above, I dove them banded--on a DR Al plate and (the HP 80's and Al 80's) on a Scubapro Stab Jacket. I was most comfortable routing the SPG's bilaterally (SPG for left cylinder clipped to the D-ring on my left waist strap, and SPG for my right cylinder clipped to the D-ring on my right waist).

I also experimented with wearing my Poseidon Odin regulators bilaterally, both on standard length IP hoses, a configuration I liked very much. (I didn't wear a canister light with this configuration, though.)

The biggest challenge for me was schlepping the 2 x HP 120's, which are quite heavy (for me) when banded together and, so, are awkward to move around without the aid of either the isolator part or the crossbar.

EDIT: BC LP inflator hose is on left-hand first stage. Drysuit hose is on right-hand first stage, and fills from the right. So, with the bilateral Poseidon configuration, no hoses cross my center line, which makes everything much easier (for me).

FWIW

Safe Diving,

rx7diver

 

[cool_hardware52]

I have personally tested the DSS plate with a pair of E-8 130's The single cambands proved to be sufficient, but one does need to understand how to properly use a camband.

As others have mentioned gas management with independent twins is the same whether the tank valves are in your armpit or behind your head. The 130's I had had right / left modular valves making valve access a little better. I have also used this set up with 2 x al 80's that both had non modular right hand valves. Valve access is a little tougher and something I would encourage divers to verify in a benign setting.

Why did DSS stop making such plates? Very little demand, many questions about what the slots were for, 4 additional slots to cut and deburr, and they make the plate less negative. The vast majority of divers are buying a SS plate in part because they want the ballast, and leaving more Stainless in the plate and less in the bottom of the water jet tank just made better sense.

It works, but today most that want indies are going sidemount.

Tobin

 

[stuartv]

Rock Bottom is supposed to be enough gas to get you and your buddy to the surface or the 1st gas switch, right?

We plan for 2 failures, right?

So, what happens if the first "failure" results in your buddy sharing your air, and, of course, it happens right as you reach RB, then your 2nd failure is that you lose one of your tanks (1st stage HP seat failure or LP hose rupture or HP hose rupture or whatever). Now, you are sharing one 2nd stage which is connected to only one tank.

When diving independent doubles, do you plan RB as keeping that much gas in each cylinder? I.e. RB is double the amount of gas you'd have as RB when using manifolded doubles?

I guess you'd have the same problem with manifolded doubles if the tank valve O-ring blew out or you somehow bashed a hole in your isolator manifold or something. It just seems like there is a higher chance of ending up with only being able to breathe off 1 tank when you use independent doubles. No?

 

[tbone1004]

i plan rock bottom based on one diver having a catastrophic loss of gas which means he had 2 failures. I.e. each bottle holds enough gas for one diver to get up, which is padded accordingly.

You state planning for 2 failures which is true, however if diving singles, then you can't plan for 2 because then everyone is out of gas. If everyone is diving indie doubles or sidemount, one diver has to lose both bottles before you have to share air, so I'm not going to plan for 3 failures because that's impractical.

 

[Ghetto Diver]

I recently started diving ID's with a BP/W I cobbled together.


This all started when I picked up an adapter made by Quickdraw for $40. Holds tanks well, but not worth the msrp though.
So far, only shore dives. Go figure I sold my 3rd Al 80 before I went this route.

 

[2airishuman]

Rock Bottom is supposed to be enough gas to get you and your buddy to the surface or the 1st gas switch, right?

We plan for 2 failures, right?
....

For a recreational buddy dive planned using the "rock bottom" strategy, the assumption is that the diver donating gas has no failures. This is the same whether you have a single cylinder, indepedent dubs, or a manifolded twinset. The main difference with independent dubs is that you have to be sure your gas is split in such a way that there is enough for each diver in an emergency.

For a technical dive, rule of thirds or some sort of detailed gas contingency plan, or both, would apply, and the main difference compared to a twinset is that with independent doubles you can lose access to the gas in one cylinder in the event of a regulator failure.

 

[tbone1004]

kind of hard to do rule of thirds without penetrating so in the ocean rock bottom still applies to technical diving if you aren't going into and out of something

 

[Nemrod]

kind of hard to do rule of thirds without penetrating so in the ocean rock bottom still applies to technical diving if you aren't going into and out of something

Maybe that is a different interpretation of the Rule of Thirds? I use it for all dives but adjust the final amount, sometimes considerably to meet the needs of the dive. The Thirds do not have to be equal, they just need to be planned. In any case all dives end somewhere, that somewhere is an agreed upon point, be it the surface awaiting pick up from a drift dive or a shore dive with the divers back on the beach at the starting point or at some other decided upon point. Whatever and wherever that point is, the final Third (an agreed upon final pressure sufficient to accomplish the purpose of returning a solo diver to a safe exit or a buddy team to a safe end point) should be still in the bank.

For a drift dive, the first two Thirds require no particular change other than awareness, again, the final Third, adjusted as needed to complete the dive safely.

N

 

[tbone1004]

that's an interpretation that I've never heard before. being an engineer, to me thirds are always equal parts, and being a cave diver that means third in, third out, third reserve which is what we follow with any sort of penetration. What you're describing sounds just like rock bottom calculations for the part that you're reserving.