Anybody know how manifolds work?

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wolves64t

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My question is centerd around the isolation of two tanks. If the tank valves themselves are closed and the isolator is open are the two tanks connected? Or do the tank valves have to be open and the isolator open for the tanks to be connected? In the first scenario you could have a regulator fail shut the iso valve then shut the tank valves individualy until the problem is stabilized. Then having only the tank valve with the working reg open you could open the islolator and have access to the gas in both tanks. If the problem was not with the regulator but the tank neck oring/valve you could leave the isolator closed and surface using the gas in the functioning tank/valve/reg combo.

I know this could be hard to follow but I am hard pressed to describe my mental wanderings in a clear fashion. Thanks in advance for helping to clear this up for me.

E.B.
 
wolves64t:
My question is centerd around the isolation of two tanks. If the tank valves themselves are closed and the isolator is open are the two tanks connected?
Yes. There is no shutoff between the tank valve and the crossbar; the crossbar is open to the tank regardless of the position of the individual tank valves.
Rick
 
wolves64t:
In the first scenario you could have a regulator fail shut the iso valve then shut the tank valves individualy until the problem is stabilized. Then having only the tank valve with the working reg open you could open the islolator and have access to the gas in both tanks. If the problem was not with the regulator but the tank neck oring/valve you could leave the isolator closed and surface using the gas in the functioning tank/valve/reg combo.

This is correct.
 
Back in the dark ages, divers that needed more gas than could be carried in a single tank began strapping two tanks on their back in what is called an "independent doubles" configuration. Each tank has it's own valve and regulator and there is no connection between them - they just sit side-by-side on the backplate. Divers with independent doubles have to alternate between regulators in order to access the gas in the separate tanks, but it mostly works. There are a number of drawbacks to diving with independent doubles, including the futz factor associated with switching between tanks. Many divers found that the changing bouyancy characteristics resulting from one tank becoming lighter as they breathed it down created problems with list (or roll, depending upon your lexicon) and that they had to switch back and forth regularly to maintain good trim.

Thus, the idea of connecting the two tanks together via a manifold was developed. The manifold is simply a tube connected to both tanks that allows gas to freely pass between them. It bypasses both valves on the tanks (what is often referred to now as the left and right post) and is not affected by them - turning off either post will not shut the gas flow off through the manifold. Most divers liked this much better than the independents they had been using but it didn't take long to discover a major potential problem with the manifold: in the event of a valve failure, there was no way to prevent the loss of all of your gas.

Generally, a massive and sudden loss of gas is considered to be a bad thing and a solution wasn't long in coming: add a third valve to the system, in the middle of the manifold, that would allow the gas flow between tanks to be shut down. This valve would allow each tank to be isolated from the other, effectively returning the system to a set of independent doubles. In the event of a valve failure, shutting down the isolation valve will allow you to keep half your gas. These days, almost all manifolds sold have an isolation valve in them. Standard procedure is to leave the isolation valve open except during shut-down drills or in the event of an emergency.

It's all kind of circular but that's the point. In some instances, independent doubles are the best solution, in others you want the tanks to function as one. The isolation manifold allows the diver to choose which is best. It also give doubles divers something new to argue about - what is the proper sequence for a shut-down in the event of a failure, but for more about that argument you should visit the technical forums.
 
Steven,

I think this was by far your most informative post. I'm very suprised by your knowledgeable response. HA

Seriously, this was good stuff.....makes a lot of sense. Thanks Steven! Next time you come down on the south side, let a jepuskar know....1 drink is on me. :wink:
 
Reefraff, the evolutionary order you describe is out of sequence.

If you look at really old diving pictures you see a lot of twin and a few triple tank arrangements all of which use a manifold. The manifolds in question were comparativley simple affairs with one outlet to accommodate a single regulator. This made more economnic sense as another regulator cost a lot more than a doubles manifold. Plus the hose routing for a pair of independently mounted double hose regs would have been really interesting.

The next evolutionary development was manifolds like the Sherwood Selpac where you had a centrally mounted outlet for the primary reg and another outlet on the right post for a back up reg. In that case, both regs could be shut off independently without isolating the tanks from one another so that the contents of both tanks could be accessed with either regulator. This arrangement worked ok but had the potential for total gas loss if you had a failure in a neck o-ring, burst disc, etc.

So..independent doubles were used as a simple and reliable means to acheive complete redundancy. This arrangement does require the diver to switch regs twice during the dive. You essentially breath one tank down 1/3 then switch to the other and breathe it down 2/3rds, then switch back ot the first tank for the 2nd third in that tank. It leaves you ending the dive with 1/3 remaining in each tank as a reserve and it ensures that you could end the dive safely or get to your first deco gas switch with the contents of either tank in the event the contents of one tank were lost.

With this approach, there is also never more than a 1/3 differential between tanks, so even if you use 120 cu ft tanks, the lateral difference in bouyancy of the tanks and resulting roll moment is minimal and has no significant impact on trim.

Where divers get in trouble here is in using 2/3rds of the contents of the first tank before switching to the second tank. This reduces the reg switches to one, but leaves you switching the tanks at the turn point as well as creating an unneccesarily large differential between tanks.

Plus if you use a long hose, starting on the long hose and switching to the short hose after 1/3 leaves 2/3rds of the gas in the long hosed tank where it will do the most good in a gas share situation and it also puts you back on the long hose near the end of the dive where a gas share situation is most likely to occur and where donating the primary would then be ideal.

The whole process really is a lot simpler than non independent doubles divers make it out to be and requires only minimal thought, minimal training and minimal additional task loading. And it carries the advantage that if you have a failure in a restriction, your life does not depend on your being able to promptly close a post and/or an isolator valve. This practice has by no means been relegated to the "dark ages" of diving.

Finally we had the development of dual post manifolds with an isolator valve that combined the ability to access both tanks equally without switching regs with the redundancy that comes with being able to isolate the tanks if neccesary.
 
Ok, as long as we are talking doubles/manifolds....how does the gas monitoring work for these? I would think since their is a possibility you would have to isolate the cylinders you would need an SPG on each tank?
 
jepuskar:
Ok, as long as we are talking doubles/manifolds....how does the gas monitoring work for these? I would think since their is a possibility you would have to isolate the cylinders you would need an SPG on each tank?

Hi Jep,

Most modern hogarthian ("dir") rigged divers have a single pressure gauge on the left post. If it becomes necessary to shut down the left post then you lose your pressure gauge. On the other hand, if it becomes necessary to shut down either post then the dive is over and you're extricating yourself. Does it really matter if you 'know' how much gas you have remaining at this point? It's going to take as much as it takes.. Hopefully you allowed for that in your gas plan :)

Additionally you should train yourself to know what your remaining pressure is based on time at depth - the gauge is there mainly to verify & provide feedback to what you already know based on how long you've been at depth.

I hope this answers more questions than it raises ...

Edit: To more directly address your question - if you isolate the two tanks then you've likely lost all of the gas from one of the tanks. If it was your right tank then you still have your SPG. If it was your left tank then you do not. See the previous points...
 
https://www.shearwater.com/products/teric/

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