Bubble Speed

[psinyc]

Does anyone know off-hand how fast bubbles rise to the surface in salt h20, in terms of feet per minute (ft/min)?

Also, wouldn't the bubble "accelerate" during ascent as they slowely get bigger?

 

[Stone]

I recall a myth debunking article in Dive Training or Rodale's that addressed the "never ascend faster than your smallest bubbles" rule-of-thumb.

If I remember the article correctly, the bubbles do accelerate as they rise. The bubbles may be ascending as fast as 100 fpm in the last 20 feet or so (exactly where the rate of pressure change is the greatest, and the diver would want to ascend the slowest).

 

[nickjb]

Hopefully you are still producing new bubbles as you ascend

 

[sheck33]

well if you know a bubble's initial size you can calculate the initial upward force on the bubble. If you know the starting depth you will also be able to calculate the bubble size change as it rizes up to the surface. From there you can come up with some equations that will yield velocity as function of depth as well as bubble size ( of course you already know the end bubble size on the surface, it will be Vinitial x (D/10 + 1) where D is depth in meters)

why do you want to know?

 

[psinyc]

It seems like the old adage of not ascending faster than your bubbles is not a good rule of thumb. I was trained to ascend @ 30fpm, but even if you ascend @ 60fpm, the chances of the bubble ascending faster than even 60fpm is great.

 

[sheck33]

psinyc once bubbled...
It seems like the old adage of not ascending faster than your bubbles is not a good rule of thumb. I was trained to ascend @ 30fpm, but even if you ascend @ 60fpm, the chances of the bubble ascending faster than even 60fpm is great.

you are right, this thing about not ascending faster than your bubbles is not a good way to control your ascent rate. If you could devise maybe a little float that has a constant volume it will ascent at a constant speed, now adjust the volume so you have a 60 ft/min rate (are lower) and you do have something useable. Actually they did do that once, someone filled a ping pong ball with a liquid lighter than water so it ascended at just the right speed, so just dont ascend any faster than your ping pong ball

 

[WreckWriter]

sheck33 once bubbled...
Actually they did do that once, someone filled a ping pong ball with a liquid lighter than water so it ascended at just the right speed, so just dont ascend any faster than your ping pong ball

I remember those things! they came in a little rack/dispenser thing, 3 or 5 of them? what were they called? Dive Bubbles? Dive Balls?

I never tried them but I rember seeing them. This was, of course, back before computers with ascent rate indicators were on every diver's wrist..

Tom

 

[sheck33]

now i feel olddddddddd :goofy:


i have never tried them though

 

[madmole]

Isnt the rule to "never ascend faster than the SMALLEST bubbles" ie the fresh new ones that havn't expanded and accelerated

Not much else to look at on the way up in British water with crap viz anyway

Personally, I nearly always deploy a lazy blob, and I know exactly what hand over hand rate gives me a nice slow ascent. I try and get about 5 m/min from 10m upwards or slower

 

[sheck33]

madmole once bubbled...
Isnt the rule to "never ascend faster than the SMALLEST bubbles" ie the fresh new ones that havn't expanded and accelerated

Not much else to look at on the way up in British water with crap viz anyway

Personally, I nearly always deploy a lazy blob, and I know exactly what hand over hand rate gives me a nice slow ascent. I try and get about 5 m/s from 10m upwards or slower

That was the original rule but how on earth are you going to pick 'the smallest' bubbles in a mess of bubbles of all sizes as you move up and the size of all the bubbles changes continously