NC Fatality?

[Dan]

First stage generally stays open until closed by pressure.

So, what do you think happened to this fellow's 1st stage?

"an Edgewater, Florida, diver perished October 9 with more than 140 bar/2,000 psi remaining in his tank. His valve, however, was only open one-quarter turn."

The Quarter Turn That Kills - Cave Diver Harry

When I say at depth, I mean at ambient pressure higher than at the surface. At 100' depth the ambient pressure is closed to 44 psig. If the pressure drop across the one-quarter turn valve is 1950 psia, then the 1st stage would only receive 50 (2000 - 1950) psig supply, not enough to open the 1st stage against 44 psig ambient pressure. Do I get that wrong?

 

[Vicko]

That is not how valves work.
The pressure between the valve and 1st stage would keep increasing until the 1st stage opens or the pressure equalizes. When this happens it feels like you have to suck air out of the tank, depending on how deep you are it would be harder and harder to breath (would take longer to draw in a full breath). No matter how little you crack the tank open the pressure inside of the tank and outside will always equalize.

When I'm in a hurry on the dive boat I often jump in with a closed or part way closed tank (yes, it's idiotic of me I know and I'm in no way trying to say it's ok to do) and I can say that the feeling of the reg delivering air slower and slower the deeper you go can be terrifying to someone who does not know what's happening.

 

[Steve_C]

That makes sense. Pressures will equalize if connected. But if air flow is reduced that will happen more slowly. Too slow is bad news.

 

[Dan]

Once you breathe the air out of the regulator, it will take a while for a barely open valve to build up enough pressure to open the 1st stage. That's what I mean by large pressure drop. The slower the pressure built up, the harder we inhale from the regulator.

A fully open valve would have small pressure drop and build up enough pressure to open the 1st stage quickly. The faster the pressure built up, the easier we inhale air from the regulator.

Sounds like this is not the case for this incident.

 

[BRT]

So, what do you think happened to this fellow's 1st stage?

"an Edgewater, Florida, diver perished October 9 with more than 140 bar/2,000 psi remaining in his tank. His valve, however, was only open one-quarter turn."

The Quarter Turn That Kills - Cave Diver Harry

When I say at depth, I mean at ambient pressure higher than at the surface. At 100' depth the ambient pressure is closed to 44 psig. If the pressure drop across the one-quarter turn valve is 1950 psia, then the 1st stage would only receive 50 (2000 - 1950) psig supply, not enough to open the 1st stage against 44 psig ambient pressure. Do I get that wrong?

Seems fairly obvious, his valve wasn't open far enough.

 

[Bob DBF]

Once you breathe the air out of the regulator, it will take a while for a barely open valve to build up enough pressure to open the 1st stage.

The first stage opens when the IP is below its setpoint, so if it drops so low that one cannot breathe from the second stage the first stage will be open.

If one has an unbalanced second, at a certain point the diver cannot overcome the spring pressure that holds the valve shut against IP to take a breath, a balanced second will do the same only at a lower IP.

The limiting factor to breathing is first the tank valve which is metering the flow rate, and second the regulator second stage which takes a certain IP to operate properly. Without more flow than the diver is using, the first stage regulator cannot maintain the internal pressures that are needed to operate the second stage properly. Ultimately the diver will not get enough air once he goes deep enough.


Bob

 

[johndiver999]

If they didn't report a problem with the gear, then I would suspect a medical issue.

As for the tank valve opening question - it surprises me that some divers don't have the curiosity to see EXACTLY what will happen when the valve is only slightly cracked open.

Rather than listen to people on the internet, why not take 3 minutes and a 100 psi of air and try it yourself after a dive. Just leave the reg attached after a dive, shut the valve down, suck the air out and then open the valve 5 degrees and see how the regulator performs. If you get nothing open it a tiny bit more. It seems obvious to me that a diver should gain the experience of exactly what it feels like to have the valve partially open (while sitting safely on a bench on a boat).

Similarly, it amazes me that people don't experiment and see how the regulator performs (with the valve fully open) and as the pressure drops to 100 psi or even "zero" on the gauge. So many people seem to think the air will just stop, but it doesn't -at least on the regulators I have used - it just gives you an extremely reduced flow rate for probably longer than most people suspect.

 

[Bob DBF]

Rather than listen to people on the internet,

Are you telling me someone on the Internet is wrong? I'm shocked!

As for the tank valve opening question - it surprises me that some divers don't have the curiosity to see EXACTLY what will happen when the valve is only slightly cracked open.

And if you want it realistic, sit with your back to the tank and have your buddy slowly throttle the valve closed at a random point in time.



Bob

 

[Dan]

May be the following valve curve would help explain the air flow vs valve opening relationship.

In general there are 3 type of control valves, a quick opening, linear or equal percentage. Not sure which type of valve is on the scuba tank. Let say it is linear type, which means if you open the valve half way, or 50 percent of rated travel, you will get half of the maximum flow or half the speed needed for the regulator to reach the intermediate pressure (IP). In other words, it’ll take twice longer to supply air in the IP chamber.

Typically IP is set to about 150 psig (165 psia). At 100’ depth, the ambient pressure is about 45 psig (60 psia). So, you’ll suck the IP down quicker at depth than the regulator can replenish it when the valve is barely open, say 10% open or 10 times longer for the regulator to fully supply air to IP chamber. That’s the feeling of getting harder to breathe the air out of the regulator.

 

[TDiddy]

I found the article: The Quarter Turn That Kills - Cave Diver Harry

Just last night we were watching a promo video for Anthony's Key Resort. Go to roughly 2:50 and it sure appears like the crew member turns the lady's valve 1/4 turn forward as she starts her giant stride. I saw that last night and thought WTF is he doing, and at that moment of all times?