Question re: Overturning tank valve back & Regulator

[sytech]

I was recently in Cozumel and was made aware of the following.

A diver descended to 40 or 50 feet when his regulator stopped giving him more air other than about 2 gulps.

He then shot to the surface (with no injury) and he told me that he was totally out of breath by that time and his next inhalation would have been salt water. (That's why I carry a Spare Air).

Anyway,when he got back on the boat he realized that he had fully opened the tank valve but apparently "rolled" it back more than a half turn and as he descended his air supply to his regulator was vanishing

Can someone explain the physics or mechanics of what is actually happening inside the tank valve as well as the regulator in such a situation?

Thanks,


Sy

 

[JahJahwarrior]

I can't explain it for you, but I would note that it's very very very important to be able to reach your valve. That's your air, the only thing keeping you alive. If you can't reach that valve to turn it back on in case something turns it off, you are screwed. You HAVE to be able to control that!!

(it's also good to practice this skill...)

 

[GrumpyOldGuy]

A small orifice can pass a reasonable amount of air on the surface at 1 ATM, but restrict air as it becomes compressed and more dense at depth (3ATM). This might happen if the diver or someone else got mixed up and turned his valve off, then on 1/2 turn thinking it was turning it on, then backing off 1/2 turn.

Edit for clarity: This also accounts for the diver reporting the valve was off when he returned to the boat. I am just stating that the valve did not roll off in open water, it was turned off (almost off anyway) by someone.

A check I do is to breath through my primary while watching the pressure gauge right before I decend. If the needle dips, it is indicating something like this has happened. It could have even been a helpful deckhand on the boat. Its not a replacement for knowing which way to turn a valve on, but a backup for me.

 

[Teamcasa]

I doubt a roll of a half a turn would have cause the problem. More likely the tank valve filer or regulator filter plugged up. Those valves require several full revolutions to seat and shut off the air supply. Rental gear/tank?

 

[battles2a5]

It is more likely that his tank was rolled all the off and just a half turn on. Closing the valve a half turn won't shut down the system like that. So the system will remain pressurized but won't deliver the air you need for a full breathe (e.g. the first stage can't catch up w/ the second stage). If I had to guess, I would say one of the following happened:

1) He put his reg on backwards. When he stepped up to the dive platform, the DM rotated the valve the same way he always does to make sure the valve was open and ended up closing it and opening it a half turn
2) He turned his valve on, then mistakenly turned it back off and opened half a turn thinking the valve was closed

This is why you should do the following:

1) Look at your SPG when you are test breathing your system before jumping in. If the needle is sweeping w/ each breathe, your valve is not open all the way (or there is a problem w/ the valve)
2) Don't let anyone touch your valves
3) Always be able to reach your valves and practice shut-downs regularly, regardless of diving singles or doubles.

Glad to made it out alright.

 

[PhilEllis]

It is more likely that his tank was rolled all the off and just a half turn on. Closing the valve a half turn won't shut down the system like that. So the system will remain pressurized but won't deliver the air you need for a full breathe (e.g. the first stage can't catch up w/ the second stage). If I had to guess, I would say one of the following happened:

1) He put his reg on backwards. When he stepped up to the dive platform, the DM rotated the valve the same way he always does to make sure the valve was open and ended up closing it and opening it a half turn
2) He turned his valve on, then mistakenly turned it back off and opened half a turn thinking the valve was closed

It is my experience (and verified through flow-rate measurement through the first stage) that this cannot be correct. If you open a scuba valve 1/2 a turn, it is flowing as much air as it can flow, no matter how far you open it. The details can easily be seen by looking INSIDE of the scuba valve.

I have heard this same story quoted over and over. But you can test it yourself. Open your scuba valve 1/2 turn.....purge your regulator while you watch your pressure gauge....if you see needle movement, you have a problem, but not because the valve is open only 1/2 turn.

Anyway, just my opinion.

Phil Ellis
Discount Scuba Gear at DiveSports.com - Buy Scuba Diving Equipment & Snorkeling Equipment

 

[battles2a5]

It is my experience (and verified through flow-rate measurement through the first stage) that this cannot be correct. If you open a scuba valve 1/2 a turn, it is flowing as much air as it can flow, no matter how far you open it. The details can easily be seen by looking INSIDE of the scuba valve.

I have heard this same story quoted over and over. But you can test it yourself. Open your scuba valve 1/2 turn.....purge your regulator while you watch your pressure gauge....if you see needle movement, you have a problem, but not because the valve is open only 1/2 turn.

Anyway, just my opinion.

Phil Ellis
Discount Scuba Gear at DiveSports.com - Buy Scuba Diving Equipment & Snorkeling Equipment

Ok, here is what I am talking about. Maybe I shouldn't have said "1/2" turn. But if you open the valve the same amount that you were taught to back off a fully opened valve it will not be fully opened and the needle on the spg will sweep when you breathe it. I ran into the garage to put together a little demo:

YouTube - Sweeping SPG from partially opened valve

First take is the partially opened valve, followed by a fully opened valve so you know I am using functioning equipment.

If anyone is interested in picking up some of the stainless steel din plugs in the background, I got them from Phil for a good deal

 

[GrumpyOldGuy]

Phil,

You are right, 1/2 turn does supply a lot of gas. Still I can feather my valve where it supplies a reduced flow of air, but its about 1/16 turn. Still very possible considering the report.

 

[PhilEllis]

That is a great video....and damn fast! I agree that if you just crack it open, you will get this result. A full 1/2 turn will make it fully open, but cracking it (after being confused about the direction) like you are taught in open water would surely cause this.

Great response.

Edit: I actually have a major problem with that video. There is way too much stuff there that you DIDN"T buy from me. And you don't have nearly enough DIN plugs. lol

Phil Ellis
Discount Scuba Gear at DiveSports.com - Buy Scuba Diving Equipment & Snorkeling Equipment

Ok, here is what I am talking about. Maybe I shouldn't have said "1/2" turn. But if you open the valve the same amount that you were taught to back off a fully opened valve it will not be fully opened and the needle on the spg will sweep when you breathe it. I ran into the garage to put together a little demo:

YouTube - Sweeping SPG from partially opened valve

First take is the partially opened valve, followed by a fully opened valve so you know I am using functioning equipment.

If anyone is interested in picking up some of the stainless steel din plugs in the background, I got them from Phil for a good deal

 

[PhilEllis]

Ryan, you win the "fastest video response of the day" $10 Gift Card from DiveSports.com. Watch your mail!

Phil Ellis