How Far to Open Valves on Rebreather Tanks

[reefrat]

A valve opened 1.5 turns is going to take less contact to roll-off in an enclosed/overhead environment, I would want to make sure that my O2 valve was not too exposed in such a situation. Of course a roll-off like that should be easily identified and corrected by a diver paying any attention to HUD/handset- but there have been cases!
Also, on my valves 1.5 turns seems to allow a significant flow of gas- so I guess the intent of the 1.5 is to allow a faster closing of the valve in a boom scenario rather than restricting the rate of gas escaping, an issue when the O2 cylinder is only a 19 or even 13 cu/ft.

 

[macado]

Others have already mentioned it and explained it really well.

For open circuit I teach all the way on and all the way off. That way there is no confusion here it's either on or off. I teach all divers to take 5-6 deep breathes from their regulator while watching their pressure gauge before splashing. Opening open circuit tanks 1.5 turns or some variant of that can be very dangerous; It can severely impede gas flow at depth. Some valves will not deliver full breathes of air at depth when valve is not fully open / partially open.

The open minus a quarter turn back is basically a holdover from old days. While it's not wrong, per se, I personally think it causes more problems and confusion than it's worth. Valves are made of brass which is a relatively soft metal. If you forcibly jam open a valve the rationale is the valve could stick open or you could damage the valve. It's unlikely to happen with newer valve designs (note: not impossible, just unlikely).

Rebreather is a different animal all together. You don't need the same kind of gas flow since you're breathing off counterlungs, not directly off a regulator. Having the valve only partially open won't significantly impede your ability to add O2 or diluent to your counterlungs.

The rationale here is because rebreather thanks are so small (~2L-13cu/ft ~3L-23cu/ft) you want to minimize the amount of catastrophic gas loss in an emergency as quickly as possible. During rebreather training you practice something called a boom drill. A boom drill is basically simulating you have a some sort of leak on either your oxygen or diluent side.

To complicate things further, this could be an internal or external boom. Internal boom means gas is leaking into your counterlungs/loop while an external boom means gas is leaking outside counterlungs/loop. Your job is to shut off your valves as quickly as possible to diagnose where the leak (the boom) is coming from. Unlike on open circuit you can still breath from a rebreather even when the valves are completely closed provided you have enough counterlung volume and the ppO2 is safe.

This is the primary reason why a lot of rebreather divers only open their tanks partially (say 1.5 turns). It allows them to rapidly shut down the valves in an emergency.

 

[lv2dive]

I open both onboard tanks about 1.5-2 turns. After a couple of roll offs of my dil, thinking I was out of dil, and calmly going to offboard, I realized the first thing to do is check for roll offs and voila, no more issues. My bov is plumbed into offboard so that is a non-issue, and I like the ability to shut off my onboard tanks almost immediately.

 

[wedivebc]

You should do what you are taught in your RB training. You should never rely on internet training, or worse, internet speculation.

What if the guy in the video was my rb instructor?

 

[Andrew Donn]

For starters not all valves are created equal. Some of the valve seats have fine threads and other have very coarse threads meaning that 1.5 turns on one valve will result in a different degree of open than on another valve. Try marking the knob with a marker or tape and counting the number of revolutions from fully closed to fully open. You will get different values for different valves.

On open circuit you always want your valve to be all the way open. Whether you turn back 1/4 turn is a topic that has been debated ad nauseam and is the topic for another thread. Any potential benefit of being able to shut down quickly is out weighed by the real danger that the valve will not be able to deliver sufficient air flow at depth. With a partially opened valve the regulator will test perfectly normally at the surface but fail to deliver adequate gas at depth. It will give the sensation that you have an empty tank. In some cases you can get a partial breath and then nothing. The pressure gauge will drop from full to empty and then slowly rebound to full when you stop inhaling has the hoses slowly refill through the restricted valve. In most cases you won't be able to detect this at the surface even with a deep inhalation. Throw in any breathing stress due to current or workload and it is a recipe for disaster.

On a rebreather it is not quite as cut and dry. It is true that you don't usually need the gas throughput that you do on OC but the same principles apply. Is your 1.5 turns a half open valve or is it only open 1/4 of the way? If you have bailout, either in the form of a BOV or an independent second stage, plumbed into the diluent bottle the same rules as in OC would apply. Even if you don't I would argue that the benefit from partially opening a valve would be outweighed by the risk of not getting enough gas. Keep in mind the deeper you go the more gas the valve has to supply and there is no way of judging at the surface how much is enough. Usually the last place you want to find out is on the bottom when a stressful situation arises. My opinion is the diluent valve should be all the way open.

The only exception to this in my judgment is the O2 side. The flow requirements on the O2 side are much lower than for the diluent and risk of too much oxygen in the event that the leak is through the solenoid and into the loop are real enough that you want to be able to shut down quickly. I usually open my O2 valve about half way by counting the number of turns to full open and then closing it by a little less than half that number.

 

[Joneill]

The open minus a quarter turn back is basically a holdover from old days. While it's not wrong, per se, I personally think it causes more problems and confusion than it's worth. Valves are made of brass which is a relatively soft metal. If you forcibly jam open a valve the rationale is the valve could stick open or you could damage the valve. It's unlikely to happen with newer valve designs (note: not impossible, just unlikely).

The attached video provides good historical perspective on where the "open fully and turn back 1/4 turn" advice for opening scuba tank valves came from and why it is not necessary today (starting at ~ 5:06)

 

[Andrew Donn]

I love that he goes into the fun trivia question on why they are called "J" valves and "K" valves in the video. You have to watch all the way to 19:30

 

[lermontov]

I love that he goes into the fun trivia question on why they are called "J" valves and "K" valves in the video. You have to watch all the way to 19:30

thats really funny

 

[JohnnyC]

O2 open a couple turns. Dil full open. For various reasons.

 

[steady1570]

O2 open a couple turns. Dil full open. For various reasons.

ditto

-Mark