Isolation valves or full independent Doubles

[Remy B.]

I was reading in to Technical courses and requirements, and came across with some differences, some advocate for full independent doubles others with an isolation manifold.

I only can see that if you have an isolation valve connecting doubles that you have 160cf air source constantly but the down part is that in the unlucky event that one of your regulators decide to take a dump on you, then you lose some air from both in the time you close the isolation valve.

With fully independent doubles you just close the failing regulator tank and have no effect of the volume of the other.

I only see 3 more places that can leak on a isolation valve system, and 3 more places that I have to do maintenance

is there any purpose for this isolation valve, what is the benefit of them ?

different scenarios, different doubles arrangements ?

what arrangement you dive ?

if I'm missing something, please share your light, much appreciated, as well as your opinion and preferences between both systems.

 

[PfcAJ]

Losing a reg is more likely than any other failure. With independents, certain reg failure modes render half your remaining gas inaccessible.

For me, that's an unacceptable trade off.

 

[ajduplessis]

Isolated doubles for me. You have to consider the likelihood of failure types; base your preference on them and live with the consequences.

 

[lowviz]

... is there any purpose for this isolation valve, what is the benefit of them ?

different scenarios, different doubles arrangements ? ...

Manifolded doubles removes the annoyance of having to switch regulators to maintain a sensible balance of gas in the two cylinders. There are two types of manifolds, those with an isolation valve and those without. By far, the greatest number sold have an isolator.

Furthermore, whether to isolate first or shut down an offending primary regulator first is a topic of contention (on SB at the very least). The isolator is also a point of potential failure, and a bad one. You slowly lose everything with no means of stopping the loss. However, this is almost never a big deal as you will *usually* have more than enough time to surface. Without an isolator, an extruded neck O-ring can cause you to lose everything too. So there are issues with both. I, personally, see the isolator as a good thing.

Independent doubles adds the task of breathing each gas supply down in a balanced fashion. You do this by monitoring you gas consumption and switching secondary regs. As for me, if I'm going to accept the minor hassle of breathing each tank down "evenly", I'd just go to sidemount. Backmount is inherently unstable, weight on top and flotation below it. Sidemount has it sensibly reversed, weight on bottom and flotation on top. Thus, there is no "turtling" force on you. No easy answer to this.

As a further complication, it is easier (for the boat and crew) for you to dive backmount doubles vs. sidemount doubles when diving from boats.

 

[kwinter]

As AJ said, it depends on the type of failure. If it is a failure of the tank itself, such as a burst disk or neck o-ring, that would render the gas in that tank useless in either case. Shutting down the isolator would isolate the problem, but you are correct that you would lose some gas during the brief time it takes to close it down. You would still be left with only the gas in one tank. But it would be far more common to have a problem with a regulator first stage, second stage or hose that does not affect the gas in the tank. With independent twin tanks (whether backmounted or sidemounted) you would have to shut off that post and consider the remaining gas in that tank as lost. With manifolded doubles, there would be no reason to shut down the isolator. You would only shut off the post. That leaves the gas in BOTH cylinders still available to you through your other regulator.

Failures that require shutting the isolator are far less common than those requiring only a shut down of the post.

 

[tbone1004]

ok you mention three extra modes of failure with an isolation manifold, but it is actually only one. The barrel O-rings are still there whether the crossbar is installed or blanking plugs, the only mode of failure that would cause catastrophic gas loss is the actual valve failing. That really just doesn't happen during the dive. If something is wrong you'll notice it as soon as you open the valve during predive checks.

I dive both independent and isolated doubles. I have buddies that use an isolation manifold and leave it closed all the time and treat them as ID's and will only open it in a regulator failure. To each his own it is all personal preference. If I am in the ocean, it is manifolded doubles all the time. It is easier to only monitor one gas supply and not have to switch regulators while I am working/spearfishing etc etc. If I am in a cave, I dive sidemount which is basically independent doubles just with your ability to see the regulators as they have issues. It makes valve feathering quite a bit easier so you can still access the gas in that bottle depending on what mode the failure was.

Independent doubles are also somewhat useful if you are a travelling diver. I don't like diving single tank backmount, so if I am travelling, I have a set of independent doubles bands which are just cam bands attached to a central block, and that way I can use my doubles wing and have the extra gas/redundancy, but I can throw two AL80's or any other tank into the cam bands, so it makes a good way to travel. The other solution to having that redundancy is single tank with a stage, but that's a little cumbersome on a small boat.

 

[Remy B.]

Thanks guys I'm more clear now, make all sence to have and isolator, I was looking at it the wrong way.

Please keep posting your opinions, it will bring more light to the thread and others that may have the same question as I did.

Is there a valve in the market where you can do the same isolation for sidemounts, but with a hose of course, don't know if it make sence to have one sounds kind of cucumbersome and maybe dangerus .

 

[tbone1004]

there are many threads on independent doubles going back the whole history of this forum. No new technology or opinions have been formed on them since, so they are all still relevant. Just as an fyi

 

[nimoh]

I only can see that if you have an isolation valve connecting doubles that you have 160cf air source constantly but the down part is that in the unlucky event that one of your regulators decide to take a dump on you, then you lose some air from both in the time you close the isolation valve.

You wouldn't close the isolation valve, you would close the valve that has the bad regulator attached to it (much like with independent doubles).

In the time it takes to close the valve, you will be losing gas (as you mentioned), however you will lose the same amount of gas whether you are using independent or manifolded doubles assuming you close the valve before half your gas has depleted.

The difference is that with independent doubles you will lose a maximum of half your available gas if you don't take action. However, aside from doing things like feathering valves or swapping regs underwater, you are going to lose half your gas anyway when you close the valve.

 

[PfcAJ]

If you can't operate a manifold properly, you've got no business diving manifolded doubles in a technical environment.