FFM in Technical Diving

[Akimbo]

Probably a home-brew. One major problem with a simple manifold of valves is cross-contamination of different mixes, especially without check valves. Let’s say you have a 9% deep mix online and one of the valves on a rich decompression gas leaks into the manifold. Your PPO2 could jump from 1.0 to 4 without knowing it. It is equally dangerous on decompression. You could go hypoxic or get bent.

Better-designed commercial mixed gas manifolds (in the gas shack) have a vent valve between the stop and check valves. This ensures that any leakage vents safely to atmosphere rather than contaminating the online gas with an inappropriate mix. Most operations also have an oxygen monitor on what's going down the hose to guard against operator error.

 

[Jared0425]

I tried doing a tech dive with a ffm. I used the switching manifold that is mounted to the chest. I had to make darn sure that it was in the off position or I would suffer cross contamination. What I learned from this is that an ffm reduces my air efficiency, you can only use one deco gas, and the risk of accidentally having the manifold open would kill you. This was the OTS AGA along with the manifold from psd gear.

 

[kwinter]

I tried doing a tech dive with a ffm. I used the switching manifold that is mounted to the chest. I had to make darn sure that it was in the off position or I would suffer cross contamination. What I learned from this is that an ffm reduces my air efficiency, you can only use one deco gas, and the risk of accidentally having the manifold open would kill you. This was the OTS AGA along with the manifold from psd gear.

I used to dive FFM on tech dives with multiple deco gasses and O2. It has nothing at all to do with the mask, except that the mask becomes a single, common failure point. It is all about the plumbing. You could do the same thing with a single second stage and a normal half mask. A 3-way ball valve with high flow quick disconnects (Omniswivel or Swagelok) connects to a manifold block that feeds whatever your normal first stage does (second stage, wing, etc.). I have never heard of a ball valve leaking for "cross contamination" but I guess it is theoretically possible. With the QDs on the ball valve, you can use multiple gasses. So at the start maybe you have a travel gas and your deep mix connected, with the switch set to your travel gas. On the way down you switch the valve to your bottom gas. At depth maybe you disconnect the travel gas and plug in your first deco gas, switching to that on the way up. After switching, you disconnect the bottom mix and plug in rich deco gas or O2 for the next switch, etc.

It actually made gas switching much simpler. And of course the FFM with comms add a great deal of safety IMHO.

 

[DevonDiver]

I've not seen anyone mentioning gas-sharing/out-of-gas protocols yet.... just saying...

 

[Ben_ca]

What problem are you trying to solve by using a FFM?

 

[Phil_C]

I have just had a flick through Jill Heinerths book on cave diving and I have noticed in quite a few of the photo's she is wearing FFM on cave dives. I'm not sure if this is because they were filming and needed comms to co-ordinate and set-up shots, I will have a read to see if it says anything more.

From my own perspective I am doing the FFM and surface supplied introduction in order to be better able to understand archaeological team diving as this is what I am moving towards, and for this comms are regularly used. The teams I have dived/worked with were (part from me) all using FFM and comms, I'm not sure what the OP's angle was though. - Phil.

 

[DevonDiver]

What problem are you trying to solve by using a FFM?

The ones normally quoted are communication and ox-tox survival.

 

[Akimbo]

… I have never heard of a ball valve leaking for "cross contamination" but I guess it is theoretically possible…

OMG, 3-way ball valves are terrible for cross-port leakage; especially on Helium. They also can’t be used on high oxygen mixes. We use hundreds of ball valves on saturation diving systems and monitor leakage with oxygen monitors and Helium leak detectors. They are OK when the two supply ports are LP air because it doesn’t matter if they leak a little.

 

[Jared0425]

Gas sharing with an AGA absolutely blows, it's like trying to breathe a free flowing regulator and you waste much of your precious gas passing it back and forth. But in real life I would give them my secondary long hose. We practice out of air situations in the quarry in the spring and ripping off that ffm in cold water is a real shocker. If it wasn't for my manifold that supplies emergency air to the AGA, I would be hard pressed in trying to breathe off of a secondary reg in the winter and spring due to the shock. I do like the ffm when I don't feel the water below the thermocline.

 

[kwinter]

I've not seen anyone mentioning gas-sharing/out-of-gas protocols yet.... just saying...

I had a separate second stage on a 7 foot hose coiled up on my right side, fed from the manifold block. Whichever gas was feeding the block was also feeding the FFM, long hose 2nd, wing, etc.

The ones normally quoted are communication and ox-tox survival.

Not to mention MI, stroke, any medical condition or unconsciousness, as well as just being freaked out in a cave when you've lost your buddy, the line or lights, etc.

OMG, 3-way ball valves are terrible for cross-port leakage; especially on Helium. They also can’t be used on high oxygen mixes. We use hundreds of ball valves on saturation diving systems and monitor leakage with oxygen monitors and Helium leak detectors. They are OK when the two supply ports are LP air because it doesn’t matter if they leak a little.

I'm surprised. I never had a single issue. And I have the same setup on my rebreather for bailout without an issue. And I have no idea why you say they can't be used on high O2 mixes. I used my OC setup with 100% for deco all the time.

Gas sharing with an AGA absolutely blows, it's like trying to breathe a free flowing regulator and you waste much of your precious gas passing it back and forth. But in real life I would give them my secondary long hose. We practice out of air situations in the quarry in the spring and ripping off that ffm in cold water is a real shocker. If it wasn't for my manifold that supplies emergency air to the AGA, I would be hard pressed in trying to breathe off of a secondary reg in the winter and spring due to the shock. I do like the ffm when I don't feel the water below the thermocline.

Sharing a FFM back and forth is a skill I have never seen, never practiced, never recommended, and in fact never heard of. No reason to ever do this.