Ascent rate monitoring using air bubbles?

[Subcooled]

I prefer to prepare for the worst case scenario, so that when an incident finally happens, I have a backup plan, and a backup to that. This is really helpfull when things start to fail big time.

Scenario

Night time ice dive, clear silt-free water, wide sea, no reference points, safety line to surface, I'm diving alone as my buddy is at the surface keeping the safety line. My computers fail. The only usable reference is the safety line. It gives me direction but no speed indication, as the rope *floats* as it should.


Question

How do I control my ascent rate?

(yes, with a secondary computer, but let's assume they both failed or got lost)


Some initial thoughts

Air bubbles go up. Stokes formula says that the ascent rate of air bubbles in liquid is relative to the
- density difference between the liquid and air (pretty constant, even with air pressure changing tenfold)
- bubble radius squared
- bubble shape

So bigger bubbles go faster. As bubbles ascend, they also get bigger (due to pressure drop) and accelerate. I suspect that at some point the surface tension is too small and the bubbles break into smaller bubbles. Hence, bubbles should have a maximum speed, and also a minimum speed that depens on the size of the smallest bubbles (once they gain their initial speed).

Now how big are the smallest bubbles, and at what rate do they ascend when they pass my mask? And what is the maximum speed of air ascending freely through water?

 

[Subcooled]

Some comments on bubble shape would be welcome too. Droplets? Spheres? Mushroom cloud?

 

[DevonDiver]

This assumes that there are no factors present in the water which would effect bubble speed, such as current.

It also does not account for expansion of the bubble as it ascends, which would/could increase the velocity of the bubble.

If you have a reference line, you can monitor your speed by using a hand over hand ascent. You could also mark the reference line at regular intervals..

 

[japan-diver]

If you are experienced enough to be doing a night time, ice dive I would think you would be experienced enough to keep a safe ascent rate with a line for reference. Many people started diving without computers a simple depth gauge and watch can give you an ascent rate. The old teaching was no faster than your smallest bubbles- worked for years - if your computer failed it would get you to the surface fine.

 

[Bubbletrubble]

You just described a solo dive in an overhead environment (depending on ice coverage).

I have to agree with japan-diver on this one. Any diver properly trained and with the appropriate experience for that dive would see the solution to the ascent rate problem clearly. Sure, you could use your bubbles to help. But wouldn't it be better to just "walk" hand over hand up the line? It's easy, it would absolutely moderate the ascent, it ensures you will find your exit point (opening in ice), and it will reunite you with your "buddy."

 

[Subcooled]

But wouldn't it be better to just "walk" hand over hand up the line? It's easy, it would absolutely moderate the ascent, it ensures you will find your exit point (opening in ice), and it will reunite you with your "buddy."

Yeah, but that line is tied around my waist, and the line is made of floating polyprophylene. Any slack line is pulled to the surface, too. There is no rope to climb along. It just moves with me. And yes, I have two cutting tools.

Oh, right, I could tell the rope tender to pull the rope in at 60 feet per minute if I pull four times, and 30 ft/min if I pull three times. Might work IF the rope tender can adjust the speed properly.

That "not faster than your smallest bubbles" seems to me as a simple controlled solution. Do we have any idea of the actual speed? Is it closer to 60ft/min or 15ft/min? I haven't really paid attention to such things yet...

 

[DevonDiver]

I just don't understand how this situation would occur.

Overhead enviroment, at night, solo... without proper redundancy and support.

It's like asking; 'What's the solution if I ignore the solution?'

 

[Subcooled]

Some points worth mentioning:

I got training from a qualified instructor on how to do the dives
I got both ice diving experience and night diving experience before combining those
I got a good and experienced surface support (but occasionally no dive buddy in water)
I do a thorough mental rehealsal, part of which this thread is
I am also fully aware of the possible risks, which include at least

- lonely ascent with a free flowing regulator: issues with lung pressure/teeth cooling/breathing out/visibility and the accompanied difficulty at controlling ascent rate (can't see the computer). I have done it once from 30 feet, and it's not really a joy, but with a calm mind and moderate depth, very doable. The risks are real, though. Obviously, an octo helps here, esp. with double tanks.

- emergency ascent without air if there is a catastrophic equipment failure (risk of shallow water blackout, for example). Dangerous. Yes, doubles is the solution.

- the risk of cutting and loosing the line (though it's not a reel). There are standard operating procedures for this event, but it may still be pretty fatal. This is why we mark the entry point and divers with lights. They are not visible from far away, though.

- entanglement of the rope and/or diver. There's a safety diver and a rope tender at the surface, but still...

- having the line pull the regulator while one is turning his head is a potential source of anxiety. It feels weird "Where's my line? What's pulling my reg? How do I free it?". The obvious thought "I'll take the regulator out of my mouth and look at it" is the wrong solution.

- if there are several divers under the ice at the same time, the risk of making a big knot of the safety lines.

- and of course the risk of sending a distress signal by mistake and beeing pulled out at too great a speed.

So... not quite an easy entry level dive.

 

[Subcooled]

I just don't understand how this situation would occur.

Overhead enviroment, at night, solo... without proper redundancy and support.

It's like asking; 'What's the solution if I ignore the solution?'

Actually, I have done solo ice dives during daylight hours only, and on my one (1) night time ice dive I had a buddy. But then I started asking myself the what-ifs.

Besides, the bubbles are an obvious reference, that I will be looking at a lot. I want to learn to know how they behave.

-----
Then there's also the fact that people usually trust the computer until they *notice* it is broken. The bubbles might let me verify the validity of the computer provided data.

 

[String]

"No faster than your smallest bubbles" used to be the ascent rate used everywhere. This however is very rapid. In order of 20-30m/min.