Ascent Rates-Split from Catalina Diver died today

[wedivebc]

That your instructor did not covey this says volumes about the "quality" of their class.

Jim, I would hate for the measure of my teaching ability to rest on what any one of my students retained some point after my class. "I wasn't taught that" sometimes means "I don't remember being taught that"
There are bad students just as much as there are bad instructors.

 

[Jim Lapenta]

As far as the entire class yes I agree with you. But this is such a basic safety issue that I can't believe the poster would say he does not remember it being talked about if it was. How often do you remind, admonish, warn, etc students to watch their rates of ascent. I know I do it every session. Classroom and pool.

 

[scorpiofish]

I have a question for you more experienced divers about this thread.

At what point does a rapid ascent become a serious safety issue? I don't remember getting anything on this in my OW training on this other than warnings that it is dangerous and to keep it slow.

Is there an approximate depth or rate associated with serious problems?

There are many variables. As noted, bad things can happen from 4ft., e.g. barotrauma. Other variables include length at depth along with general randomness. I've seen a rapid ascent from around 80ft. with the diver suffering no ill effect.

Risks are relative. Ascending at 61ft. per minute vs. 60ft. per minute poses a slightly greater risk. It is not a bright line. No one calculate the exact risk curve.

I've gone from 42' to the surface in about 15 seconds with a flooded camera. With a total dive time of 3 minutes the risks are pretty low.

We don't know the exact injury in this case, nor do we have a confirmed description of events. Once the facts are in, this particular case may provide a sad yet meaningful reference for such risks.

 

[TSandM]

Just to answer the question again, because I'm not sure it's clear from the several answers:

There are two issues with ascent rate. One is nitrogen, and one is gas expansion in the lungs.

How much your ascent rate will impact you from a nitrogen point of view depends on how long you have been down, and how deep you have been. Nitrogen needs to escape from your bloodstream during ascent and surface interval, and it can only do so at a certain rate. If you go up faster than you can offgas, the nitrogen will form bubbles in the blood, and you can develop symptoms of DCS. This is why ascent rate is prescribed in the tables -- because a given depth and time is only a "no deco" dive if you ascent at the prescribed rate. But if you violate that, the likelihood of getting into trouble is related to your nitrogen load. If you have been at three feet for three minutes and pop to the surface in a second, you are very unlikely to get bent. If you have been at 100 feet for twenty minutes and go to the surface in 30 seconds, you're highly likely to be injured. Obviously, nobody studies the maximum safe ascent rate from any product of depth and time, so no one can tell you what the fastest you can go safely will be.

Lung expansion injury is entirely different -- you can get that in a swimming pool. That is related to pressure-related increases in lung volume, and how effective your glottis is at closing off your airway. If you do a panicked ascent from 4 feet and hold your breath, you can get pulmonary barotrauma. Ascent RATE is not the issue, except that during panicked or uncontrolled ascents, it's very easy for a stressed diver to forget to continue to breathe. Arterial gas embolism, one of the most lethal forms of DCI, may be related to barotrauma.

So, there is no answer to the question, "How fast can I go?" We know the ascent rate prescribed in the tables is generally safe. We know that holding your breath at ANY ascent rate is dangerous. And we know that, if you carry enough nitrogen in your blood, too fast an ascent will lead to DCS -- but how fast that "too fast" is may vary from person to person, day to day, and definitely varies with the degree of nitrogen loading.

 

[wedivebc]

This is why ascent rate is prescribed in the tables -- because a given depth and time is only a "no deco" dive if you ascent at the prescribed rate. But if you violate that, the likelihood of getting into trouble is related to your nitrogen load.

I would just like to add to this. When we say no deco dive we really mean the ascent rate has been factored in as the minimum "deco" required. Simply all decompression stops are essentially controlled ascent rates and all dives are essentially deco dives. A more accurate way of expressing that is a no stop required dive.

Just because the PADI table allows 60ft/min ascent does not mean that asent rate is safe when you are at the NDL of a dive computer which is using a different algorithm.

 

[Blackwood]

Just because the PADI table allows 60ft/min ascent does not mean that asent rate is safe when you are at the NDL of a dive computer which is using a different algorithm.

Further, it doesn't mean that an ascent rate >60FPM is unsafe, just that the computed limits no longer apply (they are now shorter).

 

[bleeb]

However, as I understand it, the critical issue is not so much how fast one ascends, but rather how quickly one can exhale to prevent lung over-expansion.

I think it may be a bit of a red herring (i.e. not generally a problem) to think exhalation rate is a limiting factor. IIRC the exact numbers, most people exercising really hard can push roughly a third of their lung volume in or out in about 1 sec in each direction, and peak flow rate is considerably higher. Starting from 33', that's roughly equivalent to an ascent rate of at least 11'/sec, assuming the airway is fully open.

Also, human lungs and diaphragm can generate a lot less than 1 psi pressure differential. So what this all means is that if someone's airway is open, I think the gas is going to go where it wants to as a result of any depth changes, regardless of whether they're busy trying to breathe or not.

 

[DCBC]

I have a question for you more experienced divers about this thread.

At what point does a rapid ascent become a serious safety issue? I don't remember getting anything on this in my OW training on this other than warnings that it is dangerous and to keep it slow.

Is there an approximate depth or rate associated with serious problems?

As we are in the Basic SCUBA discussion area, I wont elaborate much more other than to say that your ascent rate largely depends upon the depth from where you start to ascend and the amount of gas absorption you're dealing with.

At DCIEM, we found that the maximum ascent rate between 33 feet and the surface should be 17 fpm. Unless you are diving outside the recreational no decompression limits however, 30 fpm is what's recommended.

 

[Bubbletrubble]

However, as I understand it, the critical issue is not so much how fast one ascends, but rather how quickly one can exhale to prevent lung over-expansion.

However, even when there is no nitrogen, i.e. a short dive in shallow water, you still need to let air out of your lungs as you surface. The problem, as I see it (which is what I always worry about on my ascents) is that after you exhale, you inhale and if you are inhaling as you ascent, particularly if you are breathing deeply, you increase the risk of lung over-expansion. Slowing your ascent mitigates this. However, it could not hurt to slow even more or even stop on the inhale and then continue your ascent as you exhale.

I think it may be a bit of a red herring (i.e. not generally a problem) to think exhalation rate is a limiting factor. IIRC the exact numbers, most people exercising really hard can push roughly a third of their lung volume in or out in about 1 sec in each direction, and peak flow rate is considerably higher. Starting from 33', that's roughly equivalent to an ascent rate of at least 11'/sec, assuming the airway is fully open.

Also, human lungs and diaphragm can generate a lot less than 1 psi pressure differential. So what this all means is that if someone's airway is open, I think the gas is going to go where it wants to as a result of any depth changes, regardless of whether they're busy trying to breathe or not.

bleeb is correct. With respect to lung overexpansion, it really isn't about how fast one exhales. It's about keeping an open airway. A diver can be inhaling or exhaling upon ascent. So long as an open airway is maintained, the air inside the lungs can expand/compress with changes in ambient pressure.

Lung expansion injury is entirely different -- you can get that in a swimming pool. That is related to pressure-related increases in lung volume, and how effective your glottis is at closing off your airway.

TSandM and others have mentioned the importance of not holding your breath. I just wanted to reiterate that when a diver holds his breath with a closed glottis, the air space inside the lungs becomes a closed system. Taking that closed air-filled system to the surface is not a good idea (even from a shallow depth).

As a side note, I cringe anytime I see a smoker who dives. A smoker can be doing everything correctly (with regard to not holding his breath during ascent) and still be at very high risk of experiencing a lung overexpansion injury. With respect to lung overexpansion, smoking has a double whammy effect:

1. The tar in inhaled smoke paralyzes respiratory cilia which help move mucus up and out of the respiratory tract. This stagnant mucus becomes an obstruction hazard.
2. Inhaled smoke damages the walls of terminal airways, resulting in less tissue flexibility and the development of emphysema.

Even with an open glottis, it's easy to imagine how a mucus plug inside the lungs could trap air in the terminal air spaces. Upon ascent, the expanding air will find a way out -- possibly the weakened walls in terminal bronchioles and alveolar sacs. The result is a pneumothorax. And that's not the only reason to quit smoking. Carbon monoxide in inhaled smoke decreases the oxygen-carrying capacity of the blood. There's also the increased risk of heart disease and several types of cancer.

My point is...divers really should not be smoking. Sorry for going off on a tangent like this, but I think it is related to the topic of discussion.

 

[Ken Kurtis]

At the risk of being tagged a nit-picker (but if the shoe fits) . . .

A rapid ascent from 4 feet without exhaling can cause an embolism.

A SLOW ascent from 4 feet without exhaling can cause an embolsim.

Embolism or lack thereof is dependent on exhalation, not ascent rate. If you don't exhale while you ascend, you risk the danger of embolism. It's not a simple statemenment or equaition because you need to factor in depth, fullness of lungs, volume of exhale, etc., etc.

For instance, I'm pretty sure with a half-empty lung, I could hold my breath on ascent from 100 feet to 96 without a problem. But with a full lung, I wouldn't dare hold my breath from 4 feet to the surface.

Rapid ascent factors into embolism when the rate of your ascent and the associated change in volume in your lungs doesn't keep pace with the volume of air you are expelling on the exhale.

This is why we stress a "normal" ascent (30fpm by current agency standards) so that a normal rate of breathing will take care of lung expansion issues.

Where rapid ascent can also be problematic is with the bends because it now alters the rate of the outgassing that is taking place on ascent. Don't lose sight of the fact that, in the original tables, a prescribed rate of ascent was a form of decompression and the numbers were based on an assumption that you were doing that rate. Altering the rate altered the numbers (maybe).

Put simply: A rapid ascent could cause a bends hit on a dive that otherwise would not have produced any problems.

But I'm also 100% in areement with Jim's final concern that your instructor didn't cover these things with you in your class. Knowledge is power and more to the point, a knowledgable diver is a safe diver. What you don't know (in diving) CAN hurt you.

- Ken