Thanks Tom. So that o ring failure would lead to both tanks emptying through the manifold valve stem eventually? Obviously it’s an easy fix by closing the valve, but it is possible?
Redundancy on twin sets.
- Thread starter John McGuinness
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
No.
The cylinder valve will isolate the cylinder from the DIN or A clamp outlet. The point of an isolation manifold is this enters BELOW the cylinder valve.
So, with the cylinder valve off, gas can still be removed from the cylinder via the manifold.
File:Scuba Manifolds Schematic.png - Wikimedia Commons
(See the above link for a diagram)
So in the event of a regulator failure/freeflow. The regulator can be shut down but the gas can be accessed via the manifold.
The isolator valve should only ever be open a quarter turn. In the event of a problem the isolator valve is closed FIRST. the cylinder is then shut down SECOND. Then the isolator is reopened the quarter turn.
This ensures you loose the minimal amount of gas, but can access any remaining gas once you have managed the situation.
The potential catastrophic failure is a failure of the manifold, which could potentially dump both cylinders. Although, if you shut the manifold you MAY potentially restrict to the one cylinder.
This why people get very pissed off if you lift their twinset via the manifold, you are adding a force to the manifold that it is not designed for. Potentially increasing the risk of damage to or failure of the manifold.
The question is can you shut down the isolator? Some divers spend a lot of time practicing this. An alternate solution is a 'slob knob' which connects to the isolator, and extends it 2 feet or so on a flexible connection. These have there own issues, normally corrosion related.
A number of people use the 'Navy' method, of inverting the twinset, making it easier to reach the isolator. I know a number of divers that have shoulder issues that mean a conventional (behind the head) shutdown is impossible, they use the navy method.
My late partner always dived with her manifold closed. (Like independents.) If she shut the free flowing post down. She then had the possibility of either opening the manifold herself, or getting her buddy to do it.
Personally, I would always use a twinset with an Isolation manifold, even if I have it closed, rather than independent twins.
Last edited:
Because of the way iso manifolds work, depending on what the failure mode is (assuming you haven't completely destroyed the whole thing), at MOST you could lose half your gas through a leak. The procedure would be to breathe down that tank so that you're not wasting that gas, then switch when appropriate.
Manifold failures are exceedingly rare.
Manifold failures are exceedingly rare.
Thanks Gareth. I meant if the valve that fails is the isolation valve, can the gas then leak from that valve stem? If so then while it’s open the gas from both tanks will leave from the centre post.
- Messages
- 20,562
- Reaction score
- 14,954
- # of dives
- I'm a Fish!
if the manifold one is the one that fails, then yes it will drain everything. It is a fairly loud leak though so you're going to notice it as soon as it happens. Unless the stem also shears so you can't turn it, it can still be shut down with no issues. The valve failures that led to the use of isolation manifolds occurred from that type of failure. Valve knob sheared off with part of the stem when it struck part of a wall. The valve was already open, and you have no way to close it down. The manifold valve should be cocked forward so that can't happen
@Gareth J over here we leave it all the way open. It doesn't take any longer but the risk with having it partially open is that you turn it the wrong direction first
Oh we know you have no problem diving a manifold z-system boy!!!
You'd need a pretty catastrophic failure to cause both the seat and the o-rings to fail. Probably one that would also leave you unconscious on the cave or wreck floor. You'd have to destroy all the o-rings as well as the seat in order to prevent being able to isolate. Or at least torch the valve enough that you couldn't actually manipulate the knob, in which case an o-ring failure would leak both.
Hi @tbone1004
Having watched people taking time to shut down a valve (isolation, or cylinder), I always think a quarter turn is sufficient, quick to close, if you do a full turn, your going the wrong way.
A quarter turn is sufficient to allow cylinders to equalise, but quick to close. These days I'm not sure my shoulders would allow my to dive a conventional twinset with a high degree of confidence I could close the manifold quickly in an emergency. But then I dive CCR mostly these days so its academic, note the O2 cylinder is only ever open half to a turn at the most.
Gareth
- Messages
- 20,562
- Reaction score
- 14,954
- # of dives
- I'm a Fish!
That was a categorical no, no, on my course.
An open solenoid failure with a fully open O2 valve would result in an O2 hit. We had to be able to shut down the O2 in a fraction of a second as we did a loop flush.
Gareth
Sorry to the OP for the divergence
Similar threads
