Ascending faster than 60ft/minute

[Thalassamania]

Hi Immersed,

Not feeling picked on at all, what you say is correct all the way down the line, I apologize for oversimplifying and occasionally dropping the uncertainty modifiers.

Thanks

 

[MXGratefulDiver]

Not to be a stickler for exactness, but given that the topic is one that can affect people's lives ...

NO! The speed is irrelevant. AGE is caused by introducing air into the arterial circulation, usually by way of a torn alveolar sac in the lung.

YES! ... well sort of. Removing the pressure so fast that the critical super saturation ratio is exceeded and thus bubbling occurs causes DCS. But the converse, (e.g., ascending very slowly helps avoid DCS) may not always be true.

Two points of observation and a question ...

- You do realize that you are explaining basic physiology to a surgeon ...
- You also realize that you are discussing this topic in the "New Divers" forum ...
- How much do you charge for your "100 hour, 12-dive" Basic Open Water class?

... Bob (Grateful Diver)

 

[MXGratefulDiver]

I'm also wondering why nobody's brought up the effects of Henry's Law on body tissues, and how this affects offgassing? Or other mitigating/risk factors such as hydration, stress, rest, physical conditioning, etc.

Then there's this ... it ain't so much the expansion of microbubbles that'll bend you ... it's the collision of these microbubbles, resulting in larger bubbles, if they are not given an opportunity to release themselves through the process of circulation and respiration. THAT is why a slow ascent rate is so important ... and why there are no hard rules around avoiding DCS. Collisions can occur even if you do everything right ... increasing your offgassing efficiency through slow ascent simply reduces the probability that it will occur.

There are no hard rules for avoiding DCS ... there are only "rules of thumb", which are designed to help you reduce the risk factors to an "acceptable" level ...

... Bob (Grateful Diver)

 

[Fish_Whisperer]

Please refresh my memory: What does Henry's Law state?

 

[Fireman Ken]

Hi Immersed,

Not feeling picked on at all, what you say is correct all the way down the line, I apologize for oversimplifying and occasionally dropping the uncertainty modifiers.

Thanks

This is a great topic and worthy of lengthy discussion. There are multiple theories out there. I'm glad that institutions such as DAN are conducting in-depth research on topics such as ascent rate. Remember, all dives and divers are not created equal. On deeper dives, safety stops are good even if you are within the no-decompression stop time limits of your dive tables/computer.

Intuitively making two safety stops deeper and shallower make sense. Remember every dive you make is a decompression dive-- you always offload nitrogen after a dive! Some dives that are deeper and longer require actual mandatory decompression stops to thwart DCS. In this situation, you are truly in an overhead environment.

In my opinion, slow ascents tend to be helpful. For example, in a dive run time profile where decompression stops are required, you design the profile by measuring the time you LEAVE a mandatory deco stop not when you ARRIVE.

Also if you ascend slowly, your computer recalculates mandatory deco stops based on the gas you've offloaded. My experience is slower ascents cut down on the time & number of mandatory stops called for by the computer. The logic-- slower is generally better.

Of course there are other ways of cutting down time at mandatory deco stops, such as breathing a higher O2 concentrated gas mixture from a stage bottle rather than your back gas. But that is a whole different topic.

In the meantime, my goal is to learn from the experts and be safe out there.

Ken

 

[TheRedHead]

I'm also wondering why nobody's brought up the effects of Henry's Law on body tissues, and how this affects offgassing? Or other mitigating/risk factors such as hydration, stress, rest, physical conditioning, etc.

Bob, I've never thought about Henry's Law in relation to microbubbles. Care to elaborate? A guy on my boat got bent diving basically the same dives as the rest of us. Made me think about DCS risks and why some people get bent and others don't. Everyone talked about it and we all noticed how fast he popped up from 15 feet to the surface.

 

[DiveTub]

This is a really interesting thread
I am currently one of a group acting as Lab Rats in an attempt to determine the likelihood of bubbles after a dive.

Basically they stick us in a chamber and take us to the depth/time limits of the PADI/DECIM Tables then bring us up at the maximum ascent rates recommended by PADI. We then sit round for a couple of hours and they run Doppler on us to see if they can hear any bubbles.

Its pretty cool we took a couple of Mares Nemos in the chamber with us, they didn’t like the ascent rates much…

Cheers
Chris

 

[Thalassamania]

1) Explaining basic physiology to a surgeon should be a waste of time and I sincerely hope that I’ve not offended anyone by doing do. I was under the impression that we’re not dealing with physiology in this case, but rather with basic physics.
2) We'd moved to a bit more detail than perhaps we might, but there's nothing I’ve posted that I don't routinely teach to entry-level divers.
3) Within the university, 4 credits at $205/credit, plus students need to show up with a full set of gear matching a rather specific spec. I don't think that I described it as a "basic open water course," I usally use the term entry-level training program. And ... it's not MY course, its a program developed over the last fifty years by folks far more capable and knowledgeable than I.

If you want the course taught on your schedule, at your location, it depends more on my interest in doing so then on money.

 

[Thalassamania]

Henry’s Law: The pressure of the gas above a solution is proportional to the concentration of the gas in the solution.

 

[MXGratefulDiver]

Please refresh my memory: What does Henry's Law state?

Simplified explanation ... sufficient for the new diver ...

The deeper you go, the more gas your body tissues will absorb due to the increased pressure of the water, and subsequently the density of the gas (partial pressure of N2) you are breathing. As you ascend, your body's ability to retain the absorbed gas decreases proportionally with pressure. Since the rate of pressure change increases (relative to depth changes) as you approach the surface, maintaining a constant ascent rate results in a faster rate of offgassing the closer to the surface you get. As an example … from 99 fsw to 66 fsw, you change the pressure from 4 ATA to 3 ATA … and the density of the gas you breathe changes by 25%. From 66 fsw to 33 fsw, that change is 33%, and from 33 fsw to the surface, that change is 50%.

In effect, holding a constant ascent rate … from, say, 60 feet to your safety stop … will result in more bubble releases, as well as faster bubble expansion, because of the increasing rate of pressure changes with each foot that you ascend.

This is why some computers will allow a certain ascent rate at deeper depths, but will alarm if you hold that same ascent rate at a shallower depth (my Oceanic Versa Pro does this, for example).

Most technical divers will ascend in a manner that they call “shaping the curve” … which, to simplify, means that the closer to the surface they get, the slower they will ascend.

... Bob (Grateful Diver)