Manual Injection Valves

[doctormike]

I've had an ADV stick open necessitating shutting it off by a gas shut-off. Adding DIL came down to either opening the ADV back on/off or using a DIL mav.

Yeah, the CE version of the JJ includes an ADV inline shutoff. I keep hearing about ADVs sticking open, but that hasn't happened to me yet. I would just shut the dil off if that happened at depth, since I would only need it for a dil flush or dewatering at that point. I just don't really see the need for the dil MAV. But then again, I'm a newbie.

 

[kensuf]

I also run my wing from the onboard DIL, so shutting that off at the valve means loss of inflation gas.

My DIL had 9% oxygen because it was for a 100m dive. PO2 drops like a stone when you have a low fO2 and you're shallow. Another option is to also use a BOV; you can easily add DIL through that too. IMHO, while an inline shutoff is pretty easy to engage/disengage, I like having two ways I can add DIL (MAV + ADV, MAV + BOV, ADV + BOV).

 

[doctormike]

I also run my wing from the onboard DIL, so shutting that off at the valve means loss of inflation gas.

Right, but just like dil, wing inflation is something that you generally only need on descent. And shutting it off doesn't mean that you don't have it, it just means that in the unusual situation that you needed to do a dil flush or add gas to your wing (like in a cave type sawtooth ascent), you would have to feather it on for a bit. We're fine doing that with O2 in case of stuck open solenoid or MAV, and in that case you would need to feather it for your entire ascent.

I know that you are a lot more experienced than me, and I'm not trying to be deliberately argumentative. I just really like thinking these things through...

My DIL had 9% oxygen because it was for a 100m dive. PO2 drops like a stone when you have a low fO2 and you're shallow. Another option is to also use a BOV; you can easily add DIL through that too. IMHO, while an inline shutoff is pretty easy to engage/disengage, I like having two ways I can add DIL (MAV + ADV, MAV + BOV, ADV + BOV).

I see, so that is a reason for a dil MAV combined with an ADV shutoff - that a free flowing ADV with hypoxic dil would be dangerous in shallow water. Maybe that's why the CE version has those two things.

So it would be for a situation where you were above the minimum operating depth for your dil (with FiO2 9%, that would be 10 m = 2.0 ATA = PO2 0.18), but below the depth at which you could just forget about dil and run your unit as an oxygen rebreather (6m = 1.6 ATA = PO2 1.6).

Again, just trying to think all of this through, I always appreciate your explanations!

 

[JohnnyC]

An FO2 of 9% will still support consciousness at 8m (.16), and in theory up to 3m (.12) so realistically it wouldn't be an issue unless you've got a long deco obligation, you're out of O2, and have no bailout. That's a LOT of failures to put yourself in that situation. And if you're in that situation, running the unit as an SCR, which would really be your only choice to maintain a life-supporting PO2, you're gonna torch through your dil way before you clear your deco and can surface, and you'd probably black out on the way up anyway.

At that point I'd probably write my will in my wetnotes, and try to hot lip my inflation bottle on the way up, hoping there was a chamber on the boat.

It's all about having options, which is one of the things I like about the SF2 is that both MAVs have QC4's on them. You can plug anything into either side. In theory, you could run open loop with your bailout through the O2 MAV offboard plug. It's an option I'll probably never use, but it's nice to know that I could if I had to. Hell, you could carry a hose and an allen key in a ziploc bag in your pocket, put a hose on any random safety bottle in the cave, plug into your MAV, and monkey dive that tank out in front of you like those fancy sidemount pictures. Options are good. Options keep you alive.

 

[doctormike]

It's all about having options, which is one of the things I like about the SF2 is that both MAVs have QC4's on them. You can plug anything into either side. In theory, you could run open loop with your bailout through the O2 MAV offboard plug. It's an option I'll probably never use, but it's nice to know that I could if I had to. Hell, you could carry a hose and an allen key in a ziploc bag in your pocket, put a hose on any random safety bottle in the cave, plug into your MAV, and monkey dive that tank out in front of you like those fancy sidemount pictures. Options are good. Options keep you alive.

Right, I'm just trying to figure out which options are worth the tradeoff of cost, extra hoses, and potential failure points . Like for example, what are the situations where you need to plug in offboard gas but the O2 MAV isn't an acceptable option?

Again, not being argumentative, just trying to learn.

 

[JohnnyC]

It's all what you're comfortable with. I don't find the additional potential "failure points" to be a deal at all. It's honestly a fairly specious argument in the first place. No static hose is going to be any more prone to failure than any other, and as long as you have some semblance of situational awareness when you build your unit, bubble checks, etc., no hose failure should be a surprise. Catastrophic hose failures are so rare that removing 2 hoses in an effort to remove that "failure point" isn't worth losing the MAV in my mind. Minimizing risk and minimizing risk at the expense of utility are not the same thing.

I could literally run my Pelagian with one hose coming from the dil side to the ADV, one from the O2 side to the needle block/mav, and the hose that goes from the needle block into the head. Of course I have to find wing and suit inflation from somewhere else, BOV feed from somewhere else, I won't know how much gas I have, but sure as hell I've minimized "failure points" on the rebreather side of the equation. One hose coming from each first stage is about as failure proof as it gets. Of course I still need wing gas, I still need suit gas, I still need BOV feed, and I still need to know how much gas I have. So it's not like I can really remove them, I'm just relocating them. So, if you've already got n+1 points of failure, adding a couple "extra" like you'd get with a MAV isn't really increasing the risk substantially, but the utility of having it more that outweighs the risk in my mind.

Again, it's all a choice you have to make. If you're comfortable diving without a MAV, or comfortable diving without an ADV, and you can adequately control your unit without one or the other, drive on. For many many dives you may not need the additional utility of it, in which case you're not really losing out by missing one.

 

[doctormike]

No static hose is going to be any more prone to failure than any other, and as long as you have some semblance of situational awareness when you build your unit, bubble checks, etc., no hose failure should be a surprise.

I wasn't thinking of the hose, I was thinking more of the valve itself - getting stuck open.

Again, it's all a choice you have to make. If you're comfortable diving without a MAV, or comfortable diving without an ADV, and you can adequately control your unit without one or the other, drive on. For many many dives you may not need the additional utility of it, in which case you're not really losing out by missing one.

I guess my question really boils down to this. Is my ADV a sort of a combination ADV and dil MAV, because it has an injection button? Are there ADVs that you can't trigger manually? Or does the concept of a dil MAV imply a redundant dil source?

 

[kensuf]

I guess my question really boils down to this. Is my ADV a sort of a combination ADV and dil MAV, because it has an injection button? Are there ADVs that you can't trigger manually? Or does the concept of a dil MAV imply a redundant dil source?

Yes, there are ADV's you can't trigger manually. The SF2 ADV is a second stage buried inside the counterlung, no way to reach it. My KISS ADV did have a little button, but it was nearly impossible to get to with cylinders mounted. I'm sure there are others.

 

[doctormike]

Yes, there are ADV's you can't trigger manually. The SF2 ADV is a second stage buried inside the counterlung, no way to reach it. My KISS ADV did have a little button, but it was nearly impossible to get to with cylinders mounted. I'm sure there are others.

I guess this is all semantics, but then would you say that I already have a dil MAV? Or does the button just give me one dil MAV feature without the redundancy to deal with a failed-closed ADV, and the second offboard gas access point?

 

[kensuf]

Beats me. I've never dove a JJ, and I'm happy to admit I'm ignorant of some of the finer details on the unit.