Turning DOWN a valve in a freeflow

[DougK]

I read with some interest the thread about the pros and cons turning off a valve in a freeflow. I did not ready all 7 pages. But I had this thought that no one ever addressed in my class and I did not think about until now - 4 years after certification. Since the first tread got so lengthy I want to start an new post:

In a freeflow would it be of any help to turn the valve about 1/2 DOWN. My thought is it will make the air last longer and potentially the diver could still breath off the reg. Would the valve turned down make it easier to breath off the rush of air which is now reduced?

Seems this would fix the problem, mentioned in the first thread, of not being able to inflate BC when surfacing?

Just a thought, what do you folks think?

 

[Garrobo]

Turning it down half way wouldn't have much if any difference on the poundage going into the second stage. As relayed before in the discussion you are talking about there is a possibility of feathering the first stage to almost off which might slow down the air flow enough to the second stage to surface safely. I wouldn't count on it unless I had a lot of practice doing it. The best thing to do is to have an octo or a pony bottle if you are really worried about it.

 

[rstofer]

You might also try crimping the hose to the regulator and breathing off the octo. However, if the failure is in the first stage (the IP is now too high) then the octo might soon free flow. Still, it's worth a try.

Continuing with the 1st stage failure, there is now a possibility that the higher IP will cause the BC inflator valve to fail and this could result in a runaway ascent.

Trying to use the tank valve as a 1st stage regulator is not likely to be very successful because you can't modulate pressure, only flow. You need high flow at depth and low flow near the surface and, while you are playing open loop regulator, you don't want to overpressure the inflator valve.

Richard

 

[Diver0001]

Turning the valve down most of the way might buy you a few seconds but finding just the right setting would be difficult and it would take time which could be better spent solving the problem at hand. Also, I have my doubts about how many divers would really be in a position to do something like this admist the bubbly chaos of a full-bore free flow.

Personally I would advise securing an alternative first then feathering if it's really necessary. BCD's can be inflated orally, as we all know and practiced during our OW course.

R..

 

[widget]

No, unfortunately turning the valve down by 1/2 a turn will have no effect whatsoever.

Uncontrolled Free flows are generally caused by the first stage stage seat or related parts like springs etc failing, the first stage is designed to reduce tank pressure say 200 bar to a certain intermediate pressure, usually around 9 bar (for the sake of this example) which in turn feeds air at a controlled rate (9 bar) to the 2nd stage.
This pressure reducing is regulated in the first stage by pressure of a knife edge on the seat, if the seat fails, air will be delivered to the second stage at "Tank pressure" (there is no regulation) causing the 2nd stage seat SPRING pressure to be overcome and the air escapes in an uncontrolled manner (free flow).

To have any noticable differance to the free flow on the second stage you would have to reduce the flow of air from the tank to the first stage down to 9 bar or less - allowing the second stage seat to close under spring tension and stop the free flow - that would mean seriously depleting air flow to the second stage and probably make it unbreathable at anything deeper than a few feet, if that.

Second stages usually "leak" rather than free flow, as they are only fed by 9 bar, generally a second stage leak or free flow is not a big issue and the diver can easily still breathe on the regulator and safely return to the surface.

 

[Diver0001]

Well it obviously won't stop the freeflow but at some point it will reduce the speed at which the tank empties.

R..