Air Siphon Technique

[gcarter]

I came across the below in another thread, and it intrigues me, first time I have seen it mentioned.

Can anyone (yes Thal, you ) expand on this technique? Any more instruction / tips or is it just get in the water and give it a go?

This seems a good place to (once again) mention the "air siphon," something I have discussed before. Each BC is a little different so you will need to experiment and practice if you want this in your bag of tricks. It can result in a highly controllable ascent, kinda the best of both worlds:

Start kicking up. When your BC has expanded enough to keep you moving, relax and let it carry you upward. Hold the oral inflator hose as far down (deep) as you are able and keep the oral inflation button OPEN. Continue to keep the oral inflator button open and move the hose up until air starts to trickle out of the oral inflation mouthpiece, then lower it till it just stops. You now have an air siphon from your BC or BP/W that you can use to control your ascent or descent rate with amazing precision. Just remember to keep the oral inflator OPEN ALL THE TIME. THIS TECHNIQUE REQUIRES PRACTICE - try it on some normal ascents first!

 

[g1138]

Probably works via the Venturi Effect.
This is the effect that allows spray bottles to spray and second stages to deliver constant air after you break the cracking pressure.

I'm not 100% on this but I think once you allow air to flow out of your BC hose, it won't stop flowing until the pressure outside your BC is greater than the pressure inside your BC.
IE. your BC will "auto" dump until it's neutral at the current depth.

 

[tengsr]

wouldn't it be easier on the thumb to just lift up your hose when needed, and dump air?

 

[Thalassamania]

You are keeping the size of the bag, and thus your buoyancy constant by maintaining the position of the open inflator mouthpiece, as you rise the air in the bag expands and flows out, the level of the gas in the bag is determined by the relative position of the open mouthpiece. Oh, yeah ... you need to be vertical in the water column, doesn't work real well in a horizontal configuration.

 

[captain]

You are keeping the size of the bag, and thus your buoyancy constant by maintaining the position of the open inflator mouthpiece, as you rise the air in the bag expands and flows out, the level of the gas in the bag is determined by the relative position of the open mouthpiece. Oh, yeah ... you need to be vertical in the water column, doesn't work real well in a horizontal configuration.

Boy did you screw up, the horizontal accent fanatics will be here shortly telling us all why a vertical accent is bad because it won't let our toes decompress at the same rate as the hair on our head..

 

[nolatom]

wouldn't it be easier on the thumb to just lift up your hose when needed, and dump air?

You're being too practical. We're doing "McGyver" here. ;-)

Next topic will be "How to make a rebreather from a fountain pen if you're low on air"

 

[Thalassamania]

No it would not be eaier, what you are doing is taking another variable out of the system, you are keeping your BC at a constant volume; or if wearing a wet suit, as it expands you are compensating for that by deflating your BC in a highly controlled and precise fashion that is not possible with the "normal" rather gross dump procedure. The critical piece is that as you raise the open valve the bubble shrinks and as you lower it the bubble grows.

 

[gcarter]

So if I am understanding properly, never a given...

So once I have started the flow, button stays depressed, if I keep it held down and slowly finned upward, the BCD would continuously auto-equalize to stay neutral?

 

[Thalassamania]

To review:

Start kicking up or start up by adding a little air to your BC.

As your BC expands a little it'll keep you going up, relax and let it carry you upward a little ways.

As you rise in the water column, hold the oral inflator hose as far down (deep) as you are able and keep the oral inflation button OPEN.

Continue to keep the oral inflator button open and slowly move the hose up (shallower) until air starts to trickle out of the oral inflation mouthpiece.

Then continue to keep it open and lower the mouthpiece till it just stops bubbling.

You now have an air siphon from your BC or BP/W that you can use to control your ascent or descent rate with amazing precision. To speed your ascent, push the mouthpiece lower, to slow it, raise it higher.

Just remember to stay vertical and to keep the oral inflator OPEN ALL THE TIME.

THIS TECHNIQUE REQUIRES PRACTICE

 

[gcarter]

OK, so the position of my inflator hose is my ascent throttle. What kind of a lag / reaction time using this method?