How Far to Open Valves on Rebreather Tanks

How far do you open your valves on rebreather tanks?


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    18

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Vavylon

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Messages
11
Reaction score
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Location
SoCal
# of dives
50 - 99
Hi everyone,

I have been doing some researching on rebreather systems and came across a piece of knowledge that I would like some input on. The video in question is a Youtube video published by InnerSpace Systems for the assembly of their Meg15 CCR:

This question was also cross-posted in the comments by myself.

In the video (checklist step 26 at around the 4:40 mark) the manufacturer suggests only opening the valves to both tanks 1.5 turns. He claims to do this based on the idea that if they are only open that much you could save gas loss during a catastrophic "boom" scenario.

The meat of my question is whether or not this is true, and if so can it be applied to Open Circuit diving as well?

To an uninformed layman (myself) it used to seem that valves felt more or less to be an on-off switch. There was that 5 degree knob turn between no air and "OMG ALL THE AIR!" then there is the 5 more turns of "why did they add this many turns on the knob" to open the valve to the stopping point (full open) then back it off a quarter of a turn. Once I began diving deeper I was informed that the valves were not simply on/off but indeed a gradient and that not opening them all the way could cause WOB to increase at depth due to the lag time created by air passing through a marginally open valve. In addition, I was told that full open is better and reduces the risk of the knob being bumped or rubbed accidentally into the off position.

To extrapolate on my first question, is the "1.5 turn open" standard practice across all CCR units? And is it widely believed to be an effective stoploss method as stated in the video? I understand that on CCR the amount of gas being provided does not need to match the flow rate of OC and therefore the 1.5 turn method would be acceptable. To me the logic seems to hold up.

To revisit the second question about it being applied to Open Circuit, why wouldn't this method be taught the same to new divers in entry level certification courses? The benefit during a catastrophic free flow seems like it would translate to any type of diving. Sure there is the potential WOB issue, but I wouldn't think an entry level diver is going deep enough for it to be a problem. It may well also be that it has been deemed safer to have new divers error on the side of caution with a more or less fully open valve versus one that is closer to being closed.

My final question is if CCR divers practice the 1.5 turn open and then bails out (assuming the BOV or emergency second stage was connected to the CCRs diluant tank first stage) does he/she then open the diluant tank valve to the "standard" full open position to breathe as would be the standard position for OC?

Thanks in advance. I really hope I didn't make an incorrect assumption or miss something obvious.
 
On OC open the valve all the way and leave it there. Backing off a fraction means that you loose information about which way open and close are. Was is open a bit or closed a bit? Not everyone had a misspent youth turning valves or spanners in awkward places to know which way is which without thinking.

The backing off thing I think dates to when valves were made from chocolate.

OC I have encountered a valve which was giving insufficient gas at depth due to not being properly opened.

CC I open the valves a little, as you describe, to make closing them in the case of a stuck solonoid or ADV faster.
 
Open CC a little (# of turns depends on the valve but 1.5 would be about right)

OC open all the way
 
OC I have encountered a valve which was giving insufficient gas at depth due to not being properly opened.

CC I open the valves a little, as you describe, to make closing them in the case of a stuck solonoid or ADV faster.

These two points are important. On OC, if the valve is only partially opened, you will get a feeling that you are out of air at depth or lower tank pressure (roughly <1800 psi). On CC this isn't an issue because you're typically not trying to get a full breath of air. The other thing with CC is that you have a substantially smaller volume of gas than your typical open-circuit diver, so expediency in shutting your valves down is also important.
 
You should do what you are taught in your RB training. You should never rely on internet training, or worse, internet speculation.

Internet training? Speculation? OP asked a well thought out and valid question intending to benefit from the breadth of experience of the forum users.

Diligence on the evolving safe practices of rebreather diving is part of being a good CCR diver. If everyone only did what they were taught in training the world would be full of crap divers and half of us would still be stirring our scrubber between dives...
 
You should do what you are taught in your RB training. You should never rely on internet training, or worse, internet speculation.
The OP is not a RB diver. He is asking whether some RB technique might be applicable in OC.
 
Thank you all for your replies so far, I really appreciate the input.

Yes, to clear things up, I am not a rebreather diver- currently doing some research into various eCCR units both in anticipation of possibly making the switch as well as to further learn about diving in general.

I am relatively new to diving and don't recall a time when valves were made out of chocolate, but I am sure I would have been in deep trouble on a long and hungry deco stop back in that day :) I was taught (in 2008?) to still do the open minus a quarter turn so I suppose there is still some misinformation out there on the topic. Always liked the idea full closed or full open. Either way the consensus for valves on RB tanks seems to be mixed as of now with a majority selecting "other" in the vote, I suppose I omitted more specific options like "full open", but the comments seem to favor the 1.5 open and it appears that it is not specific to which standard rebreather unit is being used.
 
https://www.shearwater.com/products/swift/

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