Two questions for diving science nerds only

[shakeybrainsurgeon]

These questions came up on a nitrox thread and I thought they were interesting... I seached recent threads about ascent rates and could find no clear answers:

1) What is the maximum ascent rate of an uncontrolled ascent for a typical diver, from say, 200 feet? Not the maximum SAFE ascent rate, there have been numerous discussions on that, the actual rate. For example, a man in a 7 mil suit plans a wall dive to 120 ft but is distracted, loses buoyancy control and finds himself descending. At 200 ft he dumps his weights but doesn't dump his air and thus winds up in an uncontrolled buoyant ascent. How fast will he be going at 100 ft? 60 ft? At the surface? Just as a skydiver hits a terminal velocity due to air resistance after falling a certain distance, the diver may reach a maximal ascent rate and fairly quickly --- resistance goes up by the cube of velocity in water, only the square of the velocity in air. Will he reach that velocity in 200 ft or will he still be accelerating as he hits the atmosphere? In a thread posting last summer, a "rocket rate" of 180 fpm was quoted, three times the maximum safe ascent rate, but no source was given. Could rates as high as 300 fpm be possible? This would be hard to calculate and would have to be measured using a weighted mannekin, but someone may know the answer.

2) At what ascent rate does pulmonary barotrauma become problematic EVEN WITHOUT BREATH HOLDING? Several facts:1) in the final 30 ft of ascent, it takes only about 4 ft of ascent with breath holding to cause gas embolism --- put another way, the lungs can withstand only about 10-15% increases over atmospheric pressure before barotrauma occurs and 2) the healthy airways, at full forced expiration, can empty 70 % of lung volume in one second at 1 atmosphere, and 90% in about 3 seconds. Lungs afflcited with emphysema or asthma, due to airway obstruction, exhaust air much slowly, taking up to 5 seconds or even longer to exhaust half the lung capacity. Moreover, the denser air at depth takes a bit longer to exhaust.

Thus, even with an open glottis and forced muscular expiration, it takes normal lungs several seconds to equalize and diseased lungs even longer. Thus, if the "rocket" ascent rate equals or exceeds 4 ft/sec (240 fpm), even healthy lungs may suffer barotrauma (and diseased lungs almost certainly will) during an uncontrolled ascent because the rate of equalization, even with expiration, is too slow to match the rate of ascent.

It would be interesting to look at the cases of uncontrolled ascents to examine the risk of AGE as a function of a) pulmonary health (smokers, asthmatics vs normal) and b) the depth at the time of ascent, since that determines the maximum rate of ascent.

 

[do it easy]

Well, I'm not a nerd, but I do know that when I shoot a bag from 50', it seems to take "a while" before the bag hits the surface. Using this as a maximum value, I would guess that the terminal velocity of a diver in an uncontrolled buoyant ascent is less than 200 fpm.

I would compare this velocity with the estimates based on the time it takes me to swim up from a 50' freedive, roughly 15-20 seconds, although noting that I probably have a more streamlined form when free diving. The freediving estimate is 150-200 fpm.

Based on your assumption of a maximum "safe" velocity of 240 fpm, I think that a diver in full scuba gear, with healthy or average lungs is probably safe.

 

[do it easy]

Yet another comparison is downward terminal velocity. It takes me about 3 to 5 minutes to descend 200', although this is while adding air to my wing to prevent hitting the bottom and being negative. The differences here would the fact that gravity may not act with the same amount of acceleration as buoyancy. None the less, the descending estimate is 40 - 67 fpm.

 

[Charlie99]

If you google on things related to submarine escape you may come up with info along the lines you are looking for. One of the methods for departing a crippled submarine is to go into a chamber, pressurize it up to ambient, open the hatch and bolt for the surface. Hopefully, the submarine was close to 1ata, so they will only be pressurized for the short period it takes to fill the lockout chamber and for the ascent.

 

[Charlie99]

, the lungs can withstand only about 10-15% increases over atmospheric pressure before barotrauma occurs and 2) the healthy airways, at full forced expiration, can empty 70 % of lung volume in one second at 1 atmosphere, and 90% in about 3 seconds.

One way to look at it is not that you are trying to empty your lungs down to 90%, but that they are near full and you need to exhale the excess. So the key number would be that 70% of capacity in 1 second figure. That is roughly what you would be doing as you pop from 23' to the surface in 1 second. That far exceeds the maximum ascent rate possible. (20' in one second = 1200fpm = 12 kts.) So my conclusion is that, unless one has abnormal lungs, or you close the airway, you won't be getting lung barotrauma on an ascent. You aren't going to find me trying to experimentally verify this, however. :banana:

As for the actual maximum ascent rate, one way of looking at it is that the maximum buoyant force on your body is dependent upon such things as maximum lift of the BCD, buoyancy of your wetsuit, etc. 30+ pounds of lift isn't an unreasonable assumption. If you go for a scooter ride at 3 or 4mph (280-380fpm) and see how much force is needed to move you through the water at that speed you'll probably find it is in that same range, or less if you are well streamlined. If I try hard to keep streamlined, that's the sort of ascent speeds I might be able to achieve. Obviously, just flaring out horizontally will dramatically reduce the ascent speed.

Does the above analysis qualify me for "nerd" status?

 

[JahJahwarrior]

The ascent rate would change depending on how bouyant the diver was. You could come up with a chart though that would give you maximum ascent based on the amount of air in the BC, typical bouyancies of various wetsuits and you have to take in the weight and mass of the diver. Other things will change it too, if the diver spread out, more areas would be affected by friction. Gravity should pull down, the salinity of the water will have to be determined, any current would change the ascent vector--instead of going straight up, the diver would drift, which would make him/her go up less feet per minute, some of the acceleration would be sideways.

It shouldn't be too hard if you knew the bouyancy of the diver. That is the biggest unknown and something that will change based on depth even. The maximum ascent rate will vary widely depending on size of the perrson and how fat vs. muscular they are, how much thermal protecttion they have and how old it is, and how much air is inthe BC, also taking into account how it expands and then will leak out of the BC.

 

[1_T_Submariner]

If you google on things related to submarine escape you may come up with info along the lines you are looking for. One of the methods for departing a crippled submarine is to go into a chamber, pressurize it up to ambient, open the hatch and bolt for the surface. Hopefully, the submarine was close to 1ata, so they will only be pressurized for the short period it takes to fill the lockout chamber and for the ascent.

In Groton Ct. at Submarine school they had a tower to simulate this. It was broken when I went through however maybe one of the other Sub guy's on the board will have had the experience of going through it.

The escape trunk is basically an area between 2 hatches that can be flooded and pressurized you put a hood / K-poc (Life jacket) rocket to the surface. We used to say they where for Wives, Mothers and Congressman on tours of the boat. You could point to it and say that is how we would escape if there was a problem. Ummm yeah in the North Sea, Really deep, Water temp 33 degrees F. How long do you have to live?

 

[chrisch]

I remember being a "dead" diver for a rescue course and getting a fast lift from 6m many years back. The force of the air leaving my lungs is still in my mind all this time later..

Lung expansion injury is all too easy and at all too slow a speed.

Chris

 

[do it easy]

OK, I did some "scientific research" and came up with this thread. It initially discusses the possibility of a diver breaching out of the water and how what velocity would be required. Then, after the usual hijackings, scotch, golf balls, shark skins, sailing ships, and BBQ are discussed. There were some estimates thrown out that seem to put the theoretical terminal velocity in the 200-300 fpm range, but there is one post about a 125m freedive with a 44 second ascent, assisted with a lift bag. I'm not sure if the diver breached completely out of the water, but the ascent has an average speed of 562.5 fpm. The poster did metion that a maximum of 4m/s second was achieved, or 792 fpm. Again, the usual caveat of a diver in scuba gear, etc., etc.

There are a few anecdotes about submarine escape hatches and the training ascents from those. I would guess if it is safe to "blow and go" from a sub at depth, a scuba diver would be in a similar boat.

Most readers will suspect my logic is flawed since I am relying on a previous SB thread for empirical evidence.

 

[shakeybrainsurgeon]

OK, I did some "scientific research" and came up with this thread. It initially discusses the possibility of a diver breaching out of the water and how what velocity would be required. Then, after the usual hijackings, scotch, golf balls, shark skins, sailing ships, and BBQ are discussed. There were some estimates thrown out that seem to put the theoretical terminal velocity in the 200-300 fpm range, but there is one post about a 125m freedive with a 44 second ascent, assisted with a lift bag. I'm not sure if the diver breached completely out of the water, but the ascent has an average speed of 562.5 fpm. The poster did metion that a maximum of 4m/s second was achieved, or 792 fpm. Again, the usual caveat of a diver in scuba gear, etc., etc.

There are a few anecdotes about submarine escape hatches and the training ascents from those. I would guess if it is safe to "blow and go" from a sub at depth, a scuba diver would be in a similar boat.

Most readers will suspect my logic is flawed since I am relying on a previous SB thread for empirical evidence.

I read that thread, a little nasty. The discussion about the velocity needed to breach the surface began to sound like the Monty Python movie The Holy Grail, where two serfs argue over how much airspeed a swallow would have to attain to lift a coconut (of course, it depends on whether it's an African or European swallow )...

looking at DAN data from 2005, rapid ascent was the most common technical mistake and AGE was the second leading cause of death, behind drowning. Most drownings seemed to be secondary to coexistent heart disease, not so much a technical issue but more a medical one. Thus, AGE due to rapid ascent appears to be the dominant source of morbidity that can be traced to technical/procedural error.

The fact that some survive unscathed by a rocket escape and others die of AGE or suffer barotrauma seems mysterious. It may be related to the manner of breathing during ascent. The last 30 feet are the most dangerous, because the pressure gradient is greatest and ascent rate is maximum, and if the ascending diver has full lungs as he enters that zone, it may be disastrous.

Or it may be the orientation/drag of the diver. The terminal velocity in air free falls can vary from about 90 mph for a horizontal skydiver and twice that for one diving head first.

The effect of ascent on free divers who don't breathe pressurized air seems irrelevant.