Isolation Manifolds by John Liddiard (http://www.jlunderwater.co.uk/index.htm)

Isolation Manifolds

Do you dive with twin cylinders and an isolation manifold? How fast can you isolate and shut a cylinder down? Would you bet your life on that?

With independent cylinders, a diver needs to swap regulators to balance the gas used between the cylinders. The objective is that at any point in the dive either cylinder should have enough gas to get back to the surface should the regulator on the other cylinder fail.

On a dive planned by rule of thirds, this amounts to never letting one cylinder get more than 1/3 ahead of the other cylinder. In practice, most divers seem to keep a little extra margin, maybe aiming to not let one cylinder get more than 50 bar ahead of the other cylinder.

With manifolded cylinders, a diver breaths off just one regulator for the entire dive, save for maybe a few breaths of the back-up regulator to check it is working near the start of the dive. There is no need to swap regulators as the two cylinders are interconnected.

Even with an isolation manifold, following a regulator failure with gas escaping, both cylinders will loose gas until the isolator is closed; then the cylinder with the failed regulator will continue to loose gas until the cylinder tap is closed. Once everything is stable, the isolator tap can be re-opened to enable the working regulator to draw gas from both cylinders.

The worst case for an independent twin set should be that at any point in a dive the remaining cylinder will have just enough gas for the diver to get back to the surface. So the target for a manifolded set should be, at any point in the dive, to be able to isolate and shut down before half the remaining gas is lost.

If the isolate and shutdown can be made faster, the diver with a manifolded set has a better safety margin. And if the isolate and shutdown is slower, there may not be enough gas to return to the surface. So a diver with an isolation manifold needs to practice isolate and shutdown drills to make sure that in a real situation they gain rather than loose.

Just how fast does an isolate and shutdown need to be?

I set out to take some measurements. Or rather, I co-opted Steve Chaplin to help me take some measurements. I couldn't do it without help because I have always used independent cylinders. It's a personal thing, I like the modularity of being able to brake gear down and reconfigure it easily. Also I am not very flexible; I am not sufficiently proficient at an isolate and shutdown drill to, as I mentioned, bet my life on it.

On the other hand, Steve has a set of 12 litre cylinders with an isolation manifold. He is also quite flexible and proficient at gas shutdowns. Nicely on the opposite side of the manifold fence to me.

Our experiments began fairly simply in the car park outside our club’s compressor room.

As I mentioned, Steve’s rig is fairly typical. 12.2 litre 232 bar Faber cylinders, Scubapro cylinder valves and isolation manifold equipped with Sherwood rubber knobs for easy shutdowns, and DIN fitting Scubapro mark 20 demand valves. One with a long hose, and one with a pressure gauge. The only modification we made was to add a second pressure gauge so we could measure each cylinder pressure independently.

Steve rigged his twin set with regulators. We doctored it to simulate a range of gas loosing failures, and measured how much gas was lost from each cylinder over 15 and 30 second periods.

Failure scenarios:

• Uncontrolled free flow (by holding the purge on a second stage);
• Low pressure hose failure, on both the long hose and the short hose (by disconnecting a second stage from the hose);
• High pressure hose failure (by disconnecting the pressure gauge);
• High pressure O ring failure (by cutting 25% out of an O ring).

From these experiments we discovered that the worst case scenario was a burst low pressure hose, and that it didn’t matter whether it was a long or short hose. A burst low pressure hose could actually blow a cylinder down faster than just opening the cylinder valve to the air with no regulator attached!

Rounding to the nearest decimal of a litre, the loss rate varied from 32.5 litres per second for cylinders two thirds full to 20.3 litres per second for cylinders at 60 bar. In the middle at 100 bar the loss averaged 24.4 litres per second.

Our other initial finding was that the manifold did not restrict flow between cylinders at all. Even when the isolation valve was only cracked half a turn open, both cylinders lost gas equally until the isolator was closed. Only just cracking the isolator may knock a few seconds off your shutdown time, but it will not significantly restrict the loss of gas across the manifold.

Knowing that the loss rate varied with cylinder pressure, would it vary with depth? We conducted a further experiment to check this, carrying a spare cylinder and regulator on a dive and letting it blow at 30 metres. The rate of gas loss did not change with depth.

Analysis

Armed with a selection of data, I then put it all together into a spreadsheet to calculate target shutdown times for various cylinder sizes and pressures.

The results are displayed as bar charts for cylinder sizes from 7 litres to 15 litres, with TI being the time taken to close the isolation valve and each column showing T2, the additional time allowable for shutting down the relevant cylinder valve.

The first chart is for cylinders at 140 bar at the time of the failure. The “1/3 in” point on rule of thirds, where half the remaining gas is needed to get out safely.

The second chart is for cylinders at 100 bar, selected as typical of the point where an open water dive would be planned to begin ascending if decompression stops were to be made on the main gas, without stage cylinders or separate deco gas. The actual pressure would depend on the specific dive plan, but 100 bar is fairly typical.

So for a diver with twin 12.2 litre cylinders, if the time to isolate (T1) was 15 seconds, the diver would have an additional 35 seconds to close the relevant cylinder valve (T2) before crossing the point at which they would have been better off without the manifold. Or in the worse case, an additional 35 seconds before they condemn themselves to missing stops or drowning.

That doesn’t sound too arduous, after all, you practice gas shutdowns regularly and have no difficult achieving these times. But let’s put the exercise into better context. Incidents don’t happen like practice exercises. They hit you by surprise at the most awkward time.

You are at 35 metres, slightly fuzzed even with nitrox. You are engrossed in whatever you are doing - laying a line, measuring a porthole, teasing a lobster out of its home, taking a photograph, swimming through a wreck, sending up a delayed SMB. Suddenly there are bubbles all over the place.

No matter how experienced, your first reaction will be surprise and shock. It will take a few seconds to gather your wits enough to realise that you need to drop whatever you are doing and shutdown the isolation valve. You then need to fumble around and actually close the isolator. All the time bubbles are gurgling round all over the place. You hit the silt and visibility drops to zero.

Even so, all that practice pays off and you close the isolator in just 5 seconds longer than your usual practice time.

So the isolator is now closed, but bubbles are still streaming out from somewhere. You need to decide which cylinder tap is controlling the offending burst hose and close it down. In the process you may need to swap regulators.

And all the while the stress is building.

Despite all these distractions you get the cylinder tap closed. It is just about in time. Congratulations, you will live through the incident.

But as I have mentioned several times, are you prepared to bet your life on the isolate and shutdown times you achieve during practice sessions? Suppose it takes a little longer? Suppose in all the confusion you can’t work out which cylinder valve to close and have to work by trial and error? Suppose an LP hose failure happens when you have less gas to start with?

I don’t claim that our experiments or analysis are sufficiently complete to be definitive. The numbers may come out slightly different for other makes of manifold or demand valve. Maybe you should allow some additional margin for this, or maybe you should take some measurements of your own, at least enough to confirm that you can trust our results.

Personally I would be aiming for an additional 10 second margin on closing the isolation valve and an additional 15 second margin on closing the cylinder valve. To me that is a reasonable allowance for the confusion of a real situation. The sort of margin I would be confident to bet my life on.

If you have a twin 7 litre set with an isolation manifold, maybe you should re-think your dive strategy to keep the manifold closed. Treat the set as independent singles, swap regulators during the dive, and save the isolator for opening after the cylinder valve for an offending regulator has been shut down.

If you have a twin 10 litre set, you need to be very proficient to make the manifold an advantage rather than a disadvantage. Maybe you should be keeping it closed as I have suggested for twin 7s.

If you have a twin 12 litre set, you may feel happy enough with your shutdown times. I wouldn’t; but you may be more flexible and more practised than I am. It is worth noting that although the various training agencies differ in their practice exercises and targets for isolate and shutdown drills, they all come inside the critical time shown for twin 12s on the charts.

With a twin 15 litre set or larger, the margins don’t look so tight. But don’t get complacent.

One way to speed up shutdown times is with a remote control knob on the isolator valve, so that the manifold can be isolated without having to reach over your head. For a diver who, like me, is not very flexible, this certainly makes the first step of isolating cylinders easier. But you still have to reach back to close down the offending cylinder tap.

You could also try mounting cylinders upside-down, like the police and military divers do. A few minor problems like protecting valves while putting the rig on and routing hoses need to be sorted out, but there are many tried and tested solutions. Some divers claim they can reach the taps more easily this way. Others actually find it more difficult.

Whatever configuration you adopt, practice in controlled conditions before using it in anger. To benefit from a manifold, you need to be able to complete an isolate and shutdown drill fast enough to save sufficient gas to make it back to the surface. Overall, it is your decision what margins you are happy to bet your life on.

What the training agencies say:

• BSAC: Jack Ingle: “There is no hard performance requirement in the BSAC extended range course. Personally I aim to be able to isolate and shutdown in 10 seconds.”
• DSAT: Mark Caney: “The equipment requirement is for a minimum of twin 12 litre cylinders. The performance requirement is for a combined exercise where the diver fully closes and re-opens the isolator valve and both cylinder valves within 45 seconds, switching regulators as necessary.”
• IANTD: Kevin Gurr: “My target is 15-20 seconds for a complete shutdown drill. That is close the centre, close number one, switch to number two, open number one, switch back to number one, close number two, open number two, open centre, check number one and two.”
• TDI: Dave Crockford: “A training skills circuit of closing and re-opening all 3 valves, switching and checking regulators as needed, should be completed within 2 minutes. Simple division gives 20 seconds to close each valve, though I would expect divers to be faster than this.”