Anyone dive a FFM using independent doubles?

[ScottB]

Hello everyone.. Thanks in advance..

If you do, how do you switch gases?

A gas switching block like here http://oxycheq.com/OmniSwivel_Applications.html on the bottom right?

Possibly some sort of manifold with quick disconnects?

If one were to use a manifold without a gas switching block, and were to leave the quick disconnects connected.. would the 1st stage regulator with the highest intermediate pressure deliver the contents of it's tank until the tank pressure fell below the intermediate pressure of the second tank?

And do you run both your bottom gas and your O2 Deco through your FFM's integrated regulator (such as AGA or Poseidon) (I have limited training on Deco, but I have read that once you run 'scuba grade' air through a reg it's supposidly not oxygen clean anymore.. i don't know if that applies to Nitrox as well, since it appears most regulators are only rated to 40% Oxygen..


My intentions at this point are to run the gas line from the second stage regulator to a small manifold having two additional ports which are quick disconnect. This way I could have _only_ the tank I am actively breathing from connected, and for gas switches never loose pressure to my second stage by _temporarily_ having the tank I am removing and the tank I am adding connected at the same time.

Thoughts?

My background:
I'm NSS-CDS Basic Cave certified, have been mentored by my instructor in both Sidemount and Decompression Proceedures, and have _not_ used a FFM yet.

I am interested in moving to a rebreather in the future, and would like to utilize an FFM for purposes of attempting to prevent drowning in the event of oxygen toxicity or hypercapnia. That said I would like to get experience on said FFM before obtaining a rebreather..

I am interested in the AGA and Poseidon FFM's mostly, I have looked at and read about the KM 48 but am not really considering it.

Thanks again,
Scott

 

[DA Aquamaster]

I dive independent doubles using the M&J industries gas block (the one sold on the Oxycheq site) and a Kirby Morgan M-48 FFM.

I run a hose from each first stage to the gas block. They basically route down along the backplate and then forward along the waist belt to the gas block on the left waist. A single hose then routes up across the chest around the neck and to the second stage in the FFM. A 43" hose is ideal for me as it is very streamlined but still gives you a long hose effect if you need to share air.

The M&J valve has three positions, One tank on, both tanks off, the other tank on, which means you can breathe off one tank or the other but not both at the same time. It means you cannot get flow from one tank into the other. It also means you can isolate both tanks from the second stage in the event the second stage freezes up (unlikely, but it can happen). Given that one of the first stages could be isolated from any second stage, an over pressure relief valve attached to a LP port on the first stage is required in the event you have a HP seat leak that could otherwise cause a hose to fail due to over pressure on the unused regulator.

The detachable lower pod on the M-48 allows you to donate your primary and it also allows you to breathe off an alternate second stage or access your deco gas without having to remove the mask. You can purchase additional lower pods to put on deco regs if you want a fully closed FFM during deco as well. The pods incorporate a mouthpiece so that the regs could be used without the FFM.

When diving independent doubles, I use 1/3 from the left tank, then switch to the right tank, use 2/3rds of it and then switch back to the left tank to use the final third. This ensures that I can return to the surface or first deco gas switch on the remaining contents of either tank in the event the contents of the other are lost. It also ensures a 1/3 reserve spilt between both tanks. The gas block makes switching a breeze and it's very easy to ID what tank is selected (mine is configured so that "up" is the left tank, "down" is the right tank)

Your suggestion to use a manifold to feed both tanks to a single hose would potentially work but if both were connected, you would have no real control over which tank you were breathing out of - the first stage with the lowest IP would pass the gas until it went dry, then the other one would feed the system.

You could use quick disconnects to connect each tank in turn to the gas block, but it would be difficult to identify which QD went to which tank. I want mine to be readily identifyable by feel. This would allow your deco gas to feed into the other fitting on the gas block, but it also limits you to only one deco gas.

Doing independent doubles on an FFM is decidedly different at best so I wanted to keep things as normal as possible and also wanted to keep the switchology as simple as possible to avoid any possible confusion.

 

[ScottB]

Thank you for your reply DA Aquamaster

 

[beezwax]

I dive exactly the rig you describe: independent doubles routed to an aga FFM via a gas switching block, with quick disconnects on each line. Last week I did a trimix dive with this system to just over 200ft so I can vouch for it that far anyway. I'd post a picture so you can see my setup but I think the photo gallery is still disabled. Perhaps when it's up again.... Do a search here for "gas switching blocks" and you'll find more info.

 

[beezwax]

The gallery is back! Here's a pic....

​

Click on the thumbnail to go to my gallery, then click on the image to really blow it up. You can see the setup pretty well.

 

[Mr.X]

I don't see a lot of RB folks on Full Face mask. If they do they use one the KM with pod comes to mind. A lot of folks like BOV's instead (Bailout Valves)on their RB's Quick switch to a known gas. My experience with FFM is that if you are doing specialized work needing comms., cold, some protection from bugs they are useful. For just plain scuba diving...??? My feeling they are more of a pain than help with respect to Oxtox and passing out. The buddy system could be of more use than the FFM.

X

 

[MikeFerrara]

Thoughts?

My background:
I'm NSS-CDS Basic Cave certified, have been mentored by my instructor in both Sidemount and Decompression Proceedures, and have _not_ used a FFM yet.

I am interested in moving to a rebreather in the future, and would like to utilize an FFM for purposes of attempting to prevent drowning in the event of oxygen toxicity or hypercapnia. That said I would like to get experience on said FFM before obtaining a rebreather..

I am interested in the AGA and Poseidon FFM's mostly, I have looked at and read about the KM 48 but am not really considering it.

Thanks again,
Scott

I've dived some FFM's but not on technical dives. I was just never happy with the gas switch and gas sharing procedures that I was able to come up with for a FFM. A large percentage of my "technical" dives have been in caves. Throw in a couple of stages and multiple decompression gasses and you can end up with LOTS of gas switches.

I kept the FFM's around for a long time thinking that I would eventually rig them for use at 20 ft on O2 but a buyer showed up befire I ever got around to using them for that.