75 ft out of air, What do YOU do?

[gfisher4792]

that Spare Air only covers up the symptoms if people buy it for the reason of not paying attention to their gauges. I think that more often than not, it gives a "deficient" diver a false sense of security, esp. in lew of paying attention to everything around them.

 

[The Iceni]

J.P. once bubbled...
I know this is not a safe practice but in an event of running a tank dry or empty a diver should know that on ascent there will be air to breath. I guess now I ask WHY not teach this method? I was able to breathe a total of 20 breaths of air from a tank that was empty at 75 feet. Still able to do a really slow ascent. Does anyone have any comments?
Thanks, J.P.

My two cents,

1) An "empty" 15 litre cylinder at 30 metres will give an additional 15 litres of breathable gas when the external pressure is halved from 4 bar to 2 bar at 10 metres.

OR

2a) An "empty" 15 litre cylinder at 30 metres will give an additional five litres of accessible gas at 20 meters;- ((15 x 4/3) - 15 = 5) since the external pressure is reduced from 4 bar to 3 bar.

2b) When this 15 litre cylinder is emptied at 20 metres it will give an additional 7.5 litres of breathable gas at 10 metres when the external pressure is reduced from 3 bar to 2 bar;- ((15 x 3/2) - 15 = 7.5)

If you do not panic and so assume a tidal volume (TV) of under two litres you appear to have a choice of ascending from 30 metres directly to 10 metres where you can take about seven breaths or you could "stage" the ascent and take about three short breaths at 20 metres followed by about four more at 10 metres. (If you have a miniscule TV like J.P. even more breaths).

I started diving in the 1970's, when - although frowned upon - running out of air was not uncommon (and deco stops in recreational diving unheard of!) I remember not being too concerned about running dry because in the minute or so it took to ascend you could take a few breaths from the dead cylinder provided you did not forcibly exhale during the ascent to "empty" the lungs and simply blew bubbles to prevent barotrauma!

How times have changed!

Please note I am not suggesting running out of gas is a good idea or that the above should be practiced or taught. Far from it!! I am simply indicating that if an emergency swimming ascent is necessary there will indeed be a few breaths in the cylinder.

I am not so sure that voluntarily removing much needed buoyancy from your BC is at all wise or indeed necessary when you have nothing with which to replace it, particularly if this leads to the diver sinking!

As for spare air?

Please note I am no instructor and claim no credentials as such!

 

[MHK]

Dr Paul Thomas once bubbled...
My two cents,

1
I am not so sure that voluntarily removing much needed buoyancy from your BC is at all wise or indeed necessary when you have nothing with which to replace it, particularly if this leads to the diver sinking!

As for spare air?

Please note I am no instructor and claim no credentials as such!

Dr. Thomas,

I'm not sure I agree with your final points so please let me add my thoughts for consideration. I want to start off with the notion of that if you are diving in a unified team AND you run out of air you have pretty well not been paying attention to your dive at all, so my sense then is that it's unlikely that the diver will remain calm. In short for the combination of loosing a unified team AND running out of air that diver has turned his brain off for that dive.

That being said, I'm not sure why if you are ascedning you would need to replace air in your BC.. Moreover, if the choice is between drowning and ditching a weightbelt upon ascent my money is ditching teh weight. Accordingly, absent the weight I can't figure out a reason that a diver would need to replace air in his BC or sink, as you suggest.

I don't want to appear argumentative, I just don't see your point..

Thanks

 

[sheck33]

Dr Paul Thomas once bubbled...
My two cents,

1) An "empty" 15 litre cylinder at 30 metres will give an additional 15 litres of breathable gas when the external pressure is halved from 4 bar to 2 bar at 10 metres.

OR

2a) An "empty" 15 litre cylinder at 30 metres will give an additional five litres of accessible gas at 20 meters;- ((15 x 4/3) - 15 = 5) since the external pressure is reduced from 4 bar to 3 bar.

2b) When this 15 litre cylinder is emptied at 20 metres it will give an additional 7.5 litres of breathable gas at 10 metres when the external pressure is reduced from 3 bar to 2 bar;- ((15 x 3/2) - 15 = 7.5)

If you do not panic and so assume a tidal volume (TV) of under two litres you appear to have a choice of ascending from 30 metres directly to 10 metres where you can take about seven breaths or you could "stage" the ascent and take about three short breaths at 20 metres followed by about four more at 10 metres. (If you have a miniscule TV like J.P. even more breaths).

I started diving in the 1970's, when - although frowned upon - running out of air was not uncommon (and deco stops in recreational diving unheard of!) I remember not being too concerned about running dry because in the minute or so it took to ascend you could take a few breaths from the dead cylinder provided you did not forcibly exhale during the ascent to "empty" the lungs and simply blew bubbles to prevent barotrauma!

How times have changed!

Please note I am not suggesting running out of gas is a good idea or that the above should be practiced or taught. Far from it!! I am simply indicating that if an emergency swimming ascent is necessary there will indeed be a few breaths in the cylinder.

I am not so sure that voluntarily removing much needed buoyancy from your BC is at all wise or indeed necessary when you have nothing with which to replace it, particularly if this leads to the diver sinking!

As for spare air?

Please note I am no instructor and claim no credentials as such!

wait a second here. First of all a scuba tank is a rigid metal container. The outside pressure has no influence on the air inside the tank. You cant apply Boyle's law to a rigid container this way.

what do you consider an 'empty' cylinder?
Usually a cylinder at atmospheric pressure, i open the valve and nothing comes out, is considered empty correct?
Then, if i take an empty cylinder as defined above to ANY depth i will not be able to breathe because the pressure in the tank is not high enough.

suppose i am diving and run out of air at 30m. The reason i can take some breaths on the way up is because the cylinder is NOT empty as defined earlier, rather the cylinder pressure at that depth is too low to supply air to the regulator. Depending on regulator design the tank pressure has to be above a critical pressure, depending on the surrounding pressure the reg is in, for the regulator to work. When i ascend the tank pressure will at a certain shallower depth be above that critical pressure again allowing you to take a breath.

To find out exactly how much air you would get on the way up depends on regulator design.

Your calculation is incorrect.
Think about it.
You say you will get 5 L out of this tank at 20 m (coming from 30m), 5L at 20m (3 bar)corresponds to 15L of 1 bar (which would then per definition have to be the tank pressure, 15L tank holds 15L of gas at 1 bar)
Then at 10m you get an additional 7.5 L (2 bar) which again corresponds to 15L of 1 bar (tank pressure).
Doing this calculation from 20m to the surface would imply getting (15 x 3/1)-15 = 30 L which then implies a tank pressure of 2 bars!
You are getting many liters of gas out of the tank and the tankpressure INCREASES , this is simply impossible. As said, you cant apply Boyle's law this way.

 

[Grajan]

I am sure the doc will explain it better but 'empty' is not empty. It is a term casually used in pressure vessels for having the same internal pressure as the ambient pressure.

If a cylinder has 50psi absolute in it and the ambient pressure is 50psi it is 'empty' i.e. no flow when you open the valve.

Now reduce the ambient pressure to 15psi and the differential pressure becomes 35 psi - lo and behold flow resumes.

magic

 

[sheck33]

Grajan once bubbled...
I am sure the doc will explain it better but 'empty' is not empty. It is a term casually used in pressure vessels for having the same internal pressure as the ambient pressure.

If a cylinder has 50psi absolute in it and the ambient pressure is 50psi it is 'empty' i.e. no flow when you open the valve.

Now reduce the ambient pressure to 15psi and the differential pressure becomes 35 psi - lo and behold flow resumes.

magic

of course 'empty' is not empty or you wouldnt get air
We can argue about what an 'empty' cylinder is but Netdocs application of Boyle's law, IMHO, is still incorrect :wacko:
A truly 'empty' cylinder is a cylinder pumped to an absolute and perfect vacuum , how about that for empty

 

[Grajan]

It is pretty obvious, given the context of OOA (at least to me) that the expression "an "empty" 15 litre cylinder at 30 metres" means no flow or differential pressure at that depth - or ambient pressure.

 

[Lloyd]

I made a free ascent from 90' with no problem. I had a first stage failure at 90' right after a "professional rebuild". My brother was right by me, But I wanted to see if I could do it on one breath.
Lloyd

 

[leadweight]

Back at the start of this thread Rick mentioned that if there was a catastrophic equipment failure a bouyant ascent rather than a swimming ascent was called for. This would seem to imply a need to get to the surface faster.

Walter (I think) posted that an O-ring failure allowed some time to get back to the boat.

On a recent dive in Roatan the DM had a medium pressure hose rupture. He took off his bc and crimped the hose, continuing the dive holding his gear in his arms. (Let's save the debate on what he should have done for another time.)

I thought that it is possible to breathe off a free flowing regulator, and I have done this with momentary free flows that happen on entry to the water, or when replacing a regulator that free flowed because it was not pointing down when it was removed.

So, it seems like many of these failures would allow a swimming ascent.

Which also makes me wonder why anyone really needs a pony bottle on a dive without overheads or deco, especially if the depth is limited to 60 feet. (Yeah, we could go on forever about pony bottles, where is your buddy and solo diving which is what you are doing if your buddy is not nearby.)

 

[Braunbehrens]

DEEP SEA once bubbled...
Hey, anyone ever think about diving with a Spare Air? If your in the habit of not paying attention

Then you should forget about diving and pick up golf. The spare air won't save you.