Questions about twinsets.

[BlueTrin]

@Scuba_Nick27 , there is something that is not obvious on manifolded twins: if you look at the simplified schema below

If you close the left or right post, you can still access all of your gas if you leave the isolation opened.

I didn’t know this until someone explained it to me.

So if you can stop a leak by closing the correct post, you can access all the remaining gas by reopening the isolation valve. At this point the SPG would still show the total pressure because opening the isolation would equalise the tanks (the SPG is usually on the left post, the right post is the one the most likely post to fail because you breathe from it).

For example if you have a free flow on the right post, you could do this:

• Close isolation
• Try to close right post and switch regs
• Notice that you are not leaking anymore
• Reopen isolation manifold and stay on secondary reg

PS: I have never experienced a catastrophic equipment failure myself (touching wood) so only repeating what I have been told ...

 

[Tribal]

If you go to the more technical site you have to take some costs in to consideration. Replacing the bcd is quite cheap compared to other costs of gear and training.
It sucks to replace a good working bcd but for doubles a BP/W is way better, definitely when you start adding stages.
Look at the bright side, if it’s only the bcd you need to replace you are lucky. Many of us dit replace a entire setup

 

[Scuba_Nick27]

@Scuba_Nick27 , there is something that is not obvious on manifolded twins: if you look at the simplified schema below

View attachment 599730

If you close the left or right post, you can still access all of your gas if you leave the isolation opened.

I didn’t know this until someone explained it to me.

So if you can stop a leak by closing the correct post, you can access all the remaining gas by reopening the isolation valve. At this point the SPG would still show the total pressure because opening the isolation would equalise the tanks (the SPG is usually on the left post, the right post is the one the most likely post to fail because you breathe from it).

For example if you have a free flow on the right post, you could do this:

• Close isolation
• Try to close right post and switch regs
• Notice that you are not leaking anymore
• Reopen isolation manifold and stay on secondary reg

PS: I have never experienced a catastrophic equipment failure myself (touching wood) so only repeating what I have been told ...

What a useful chart! It all makes sense now, thankyou!

 

[Boston Breakwater]

So I have a question, if you have two tanks connected with a manifold, do the pressures form each tank combine? For example lets say that in each tank there is 3000 psi and 100 cubic feet of atmospheric air, and lets assume that both gasses have identical proportions of other gasses that make up air (nitrogen, noble gasses, etc). Lets say the manifold runs into a pressure gauge halfway between two tanks. Will the pressure displayed on the gauge be 6000 psi or 3000 psi?

That was my first question...

Here is the second question. Since I want to get into technical diving, which includes twinsets, side-mounts and etc. Please provide me with an idea of how things work. Does each side-mount run a separate 1st stage and 2nd stage regulator, or are they connected together? Also, do twinsets run off of one regulator or two separate ones?

Thank you.

Hello. @Scuba_Nick27 You'll quickly find that divers have many views, and it's important to research, and make an informed decision, in regard to gear, and the applications/diving scenarios that the gear will be used. Of course, a lot of this is "Trial, and error." Example; I was certified on 17 Oct 1999 and bought my first tanks U.S. Divers "Double 80's." in December 1999. My first B.C.D. was a U.S. Divers "Seamaster." which at that time, was a top of the line product. Back inflate, Weight Integrated,(32 lbs.) 50 Lbs of lift, heavy duty thick shoulder harness, Etc,
It was designed to accommodate "Twin tanks." using nylon cam bands, and the hard internal back plate. It worked well, for less than a year. I logged approx 90 dives my first winter, and eventually the ABS plastic just broke nearly in half. I'm sure that the temperature differential was a factor, going from 28 degree water to the warmth of a house for 6 months. Ultimately, it was the sheer weight of the "Rig." being consistently used that broke it. (Live, and learn.) My other (2) "Sea Masters." diving single tanks lasted twenty years.
As, stated already in a couple of replies. I suggest not using a recreational B.C.D. for "Doubles."
When, I made the transition to BP/W I decided to buy Apeks gear. I had been eyeballing Apeks for about 8 years, or more. I purchased an "Aluminum Backplate." When, I took it out of the plastic bag, I was a little surprised? It seemed flimsy, and frail.(No offence, Apeks.) I could see the potential for it being easily bent. After looking it over....I just slide it back in the bag, and eventually gave it away.
Again, applications, and diving scenarios...I'm sure for many divers the "Aluminum." is perfect. (Just not for me.) I called and ordered (3) Apeks Stainless Steel Backplates, and when they arrived, I was extremely satisfied.
Last year, I had a little money kicking around, and bought (3) Apeks Black Ice B.C.D.'s You can order a "Twin cylinder kit." for them which is basically what I did 20 years ago. Overall, I don't think it's a good idea. Side note: They have never even seen water, because, when, I go diving I always reach for my BP/W (Probably selling them.)
At this point in time. "Twin Sets." with an isolation manifold, (For Redundancy.) and S.S. bands are widely used, and I would think holds the majority. I dove "Indies" for over 15 years. I won't go over that here, I get in enough "Hot water." on S.B. as it is.
Welcome to the board. There is a boundless fountain of information here,
Cheers.

Assembling Double Cylinders | Dive Gear Express®
HOW TO ASSEMBLE A SET OF DOUBLES - SDI | TDI | ERDI | PFI

 

[Brett Hatch]

Late to the party on this thread, just going through it. Lots of interesting points.

When i run doubles, I usually setup my first stages where one has my SPG and a console computer, the other first stage has the wireless sender for my wrist computer. I run redundant computers, and also prefer being able to see the tank pressures of both tanks. Helps in the event the isolation valve is closed, in the event of isolating and sharing air I can see how much they have left vs me. There's a multitude of reasons I do this. However I do know individuals who have a hose with an SPG on both second stages tucked between the hip and each tank, then also have a sender for their computer and those SPGs are there for redundancy.

This post struck me. Can you or someone else elaborate on the scenario where you are closing the isolation valve during an air share? Seems like you'd usually be best off leaving the isolator open -- I can't think of an air-sharing scenario where closing it would improve the situation.

I'm not trained on diving doubles, it's something I'm interested in for the future, so please don't take this as criticism. Just interested for my own education.

 

[BlueTrin]

Late to the party on this thread, just going through it. Lots of interesting points.



This post struck me. Can you or someone else elaborate on the scenario where you are closing the isolation valve during an air share? Seems like you'd usually be best off leaving the isolator open -- I can't think of an air-sharing scenario where closing it would improve the situation.

I'm not trained on diving doubles, it's something I'm interested in for the future, so please don't take this as criticism. Just interested for my own education.

One reason it might be useful to have two SPG/AI is when you deal with a leak: once you close the manifold you cannot see the valves in your back, you still need to know if the left post or right post is leaking. If you have a SPG on one and an AI on the other you can see which one is leaking.

However, in my experience, most people diving doubles will tell you to only have a SPG on the left post and plan gas for a loss of half of your back gas. You thumb the dive on such a failure so it should not make a difference.

Some will argue that the AI transmitter is an additional potential failure point, I don’t know if this is a valid point if you put the AI transmitted on a short HP hose.

Some with argue that the AI on a HP hose is a small entanglement hazard.

Some will tell you that having a single SPG reduces the chance of having the manifold closed accidentally and not noticing because you look only one of the SPG/AI transmitter

 

[KenGordon]

Some will tell you that having a single SPG reduces the chance of having the manifold closed accidentally and not noticing because you look only one of the SPG/AI transmitter

In the usual manifolded twinset config you are breathing off the right post while the SPG is off the left post. If the isolator is closed then the SPG will not drop at all. This should be enough of an indication, no need to compare two gauges.

These are the subtle details that go unappreciated and why completely making it up as you go along rather than using one of the various common and conventional setups is not a great plan.

 

[BlueTrin]

In the usual manifolded twinset config you are breathing off the right post while the SPG is off the left post. If the isolator is closed then the SPG will not drop at all. This should be enough of an indication, no need to compare two gauges.

These are the subtle details that go unappreciated and why completely making it up as you go along rather than using one of the various common and conventional setups is not a great plan.

I should have added: that this is not something I agree with personally (for the reason you mentioned).

Disclaimer: I put an AI transmitter on a HP hose so I can have my air consumption in Subsurface, but I didn’t put the AI pressure remaining on the display of my computer: i.e. I use my left post SPG to check my pressure.

 

[BlueTrin]

In the usual manifolded twinset config you are breathing off the right post while the SPG is off the left post. If the isolator is closed then the SPG will not drop at all. This should be enough of an indication, no need to compare two gauges.

These are the subtle details that go unappreciated and why completely making it up as you go along rather than using one of the various common and conventional setups is not a great plan.

Agree, I should have added: that this is not something I agree with personally (for the reason you mentioned).

Disclaimer: I put an AI transmitter on a short HP hose so I can have my air consumption in Subsurface, but I didn’t put the AI pressure remaining on the display of my computer: i.e. I use my left post SPG to check my pressure. This does not address the issue of having an additional failure point or an additional potential entanglement hazard.

 

[RX8Bob]

Late to the party on this thread, just going through it. Lots of interesting points.



This post struck me. Can you or someone else elaborate on the scenario where you are closing the isolation valve during an air share? Seems like you'd usually be best off leaving the isolator open -- I can't think of an air-sharing scenario where closing it would improve the situation.

I'm not trained on diving doubles, it's something I'm interested in for the future, so please don't take this as criticism. Just interested for my own education.

Personal preference. The theory that a diver whom is probably also on doubles needs to take air from you, the assumption is that their breathing rate is elevated. Since if their on doubles and need to share air from me, something had gone horribly horribly wrong. So the thought is they are going to highly stressed. So my preference is close the isolator so they can only breath through half of my remaining air. This is assuming the need to make stops or at least a safety stop.

My thought is in an emergency where I have to share air, I plan on ensuring I make the surface while hoping that sharing air will get us both to the surface. Someone potentially hyperventilating may empty a cylinder in no time.