NC Fatality?

[Bob DBF]

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

The SCUBA piston first stage is a specially modified high pressure control valve. It was modified by a Sherwood engineers in 1958, and has been the basis for piston regulators since. You would need a specific chart for that valves operation. In addition the valve opening is controlled by the IP, closed at IP set point and fully open somewhere below the set point. Its operation is not controlled by flow, other than indirectly by the flow through the second dropping the IP.

The first stage regulator is a normally open valve, and closes fully only when the IP is met, and I would believe it is very aggressive opening as the flow rate needed to work at say 200' would be considerable. This may be the modification made by Sherwood for scuba use, as normally a HP control valve would have more stable parameters to meet, being used only at 1 atm for a start.

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.

The limiting factor is the throttled tank valve, if it were fully open there would be no problem. This flow restriction by the tank valve at depth does not provide the pressure to close the first stage or provide enough pressure fo the second stage to provide air properly.



Bob

 

[Bob DBF]

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?

Because of the way it was cut in, I can't tell if he opened it fully to check its position prior to the cut, then put it back where it was in the scene we saw.


Bob

 

[rjack321]

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

View attachment 522983

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.

My impression of scuba valve is that (on this graph) they would be defined as quick opening. The orifice to seat interface opens rapidly at first and later turns do little to allow for increased flow. In "valve knob turn" units this is going to be somewhat contingent on whether you have the fine or coarse threaded HP seats though. At least for the more common coarse threaded seats, the valve is fully open within one rotation of the knob and the HP seat orifice becomes the flow limiting restriction.

 

[Dan]

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?

To me it looks like he made a quick check whether the valve is all the way open. If he feels a dead stop after 1/4 turn forward then she’s good to go. Otherwise he’ll ask her about it when she pops back up on the surface giving him OK sign.

 

[infieldg]

It's great that nobody questioned the habit of opening the tank all the way and back a bit, a lot of divers love to run around telling people who do that NOT to because it was based on old tech but I did it simply because leaving it in a position where it moves immediately no matter which way you turn it prevents the guy checking my valve from absolutely swinging on it before trying the other direction. God I hate that. Some people are pretty ham fisted

 

[JohnnyC]

To me it looks like he made a quick check whether the valve is all the way open. If he feels a dead stop after 1/4 turn forward then she’s good to go. Otherwise he’ll ask her about it when she pops back up on the surface giving him OK sign.

If he turns it 1/4 FORWARD and feels a hard stop, he's just turned OFF her gas. Righty tighty.

 

[Dan]

If he turns it 1/4 FORWARD and feels a hard stop, he's just turned off her gas. Righty tighty.

My bad. I should say counter clockwise, not forward. On first glance at the video his hand seems to grip and twist counter clockwise then quickly release the grip and return to the relax position by rotating clockwise. It happens very quickly it’s hard to tell.

 

[dmaziuk]

My impression of scuba valve is that (on this graph) they would be defined as quick opening.

On that graph it would mean that turning it back 1/3rd of the way will only cut off 10% of the flow. Obviously, it's not to scale and so on, but you have to wonder how far back you have to turn it for it to become the limiting factor. All the way FORWARD and then 1/4 turn back?

 

[johndiver999]

The threads on the valve seat in a scuba valve are uniform. Which means that the longitudinal movement of the valve seat opening, is directly proportional to the degrees of rotation of the handle and this relationship is fixed. I'm not sure if gas flow is directly proportional to the opening area, but the whole discussion of how a valve opens on a scuba tank doesn't seem that meaningful for the practical utilization of the device,

I prefer to have the vale opened and then backed off a little bit, when you crank the valve open hard, you squish the hell out of a little o-ring on the stem. That seems like something we can avoid by gently opening to the stop point and then closing a little.

 

[Bob DBF]

Obviously, it's not to scale and so on, but you have to wonder how far back you have to turn it for it to become the limiting factor. All the way FORWARD and then 1/4 turn back?

I have tried a couple of my tank valves, and at 1/4 turn open they will pass any but the most aggressive tank check. My training was to make the tank check to insure you had air, not to check if the tank was open, which can only be done by checking the valve itself. If you have a problem getting air as you decend, surface and check the valve first.


Bob