why hate safety devices?

[Crush]

So let's all misinterpret the agency's misinterpretation and not teach that which is accurate and correct?

Premise: it possible to produce a very well-trained diver at the basic OW level. However, given what I have observed in my limited experience, such divers are not the norm.

My response to your post: No. But, given the low level of training associated with a modern basic OW cert through many of the most prolific agencies (as opposed to what you teach and what divers were required to demonstrate in the "good old days"), teaching correct ascent rates so as to obviate the safety stop may be asking too much from those students given their lack of preparation and skill. For them, a safety stop might be the second-best alternative.

The best alternative would be more advanced training.

 

[MXGratefulDiver]

My understanding about the origins of safety stops is that they were suggested after it was shown that divers were very often ascending at rates faster than those used in the model (thus negating the NDL values). I wasn't aware of the diving world at that point in time, but I trust Thal's memory.

They still do. Ever watch a bunch of divers at safety stop on an upline? They'll sit and watch their dive computers religiously until they hit the 3-minute mark ... then they'll be on the surface a few seconds later ... which completely negates any benefit they got from the safety stop.

Many's the time I've asked divers how long they think it should take to ascend to the surface from safety stop depth ... only to get a shrug or blank stare. Most seem to think that once you've done your three minutes, the dive's over ...

... Bob (Grateful Diver)

 

[Kingpatzer]

Safety stops? (I've not read the book yet).

Here is my understanding (correct me if I am wrong):

1) Recreational diving time & depth limits prevent tissue saturation reaching a level where so much nitrogen exists in the tissues that it cannot be eliminated by an ascent within prescribed speed limits. This is calculated using known data about the absorbtion and release of inert gases from varying speed tissues in the body, coupled with the known tolerance for super-saturation before DCI symptoms occur.

An over-simplification in that some people, even fit people, diving within the models do develop DCS. But essentially correct.

2) Ascent rate is based upon a model that accounts for the decreasing ambient pressure super-saturating the loaded nitrogen, but allowing that nitrogen to be off-gassed at a rate that prevents bubble expansion and/or congregation. This is based upon a linear relationship between ambient pressure and tolerated inert gas pressure within tissues of different absorbtion and release speed. If the ascent speed is too slow, then further loading can occur in the slower tissues, even whilst faster tissues off-gas.

Again, essentially correct.

3) The super-saturation of nitrogen on ascent causes the inevitable formation of non-symptomatic 'micro-bubbles' (bubble seeds or nuclei) within body tissues and fluids. Providing these micro-bubbles are not permited to coalesce or grow, they will not result in DCI symptoms. However, the existence of micro-bubbles is increasingly believed to trigger an inflamatory/immune reaction (platelet activation and aggregation) within the body, in addition to micro-infarction of tissues, leading to symptoms of post-dive fatigue, malaise, headache etc. The potential damage caused by sub-clinical micro-bubbles is only now being investigated - but there is speculation that they can lead to long term issues such as cerebral lesions, retinal damage.

There is also some speculation that the presence of micro-bubbles is a precursor to DCI. The exact relationship is not known or understood.

4) It is believed that micro-bubbles can exist in the body for up to 48hrs, post-dive. This has severe implications for multi-day, multi-dive activities. It is also the primary danger of 'saw-tooth' profiles, as more micro-bubbles are formed and able to accumulate with each individual ascent phase within the dive.

Actually, it is not believed, it is known. There are studies proving this. Additional studies show that safety stops can dramatically reduce the number and duration of micro-bubbles forming in the body.

8) The off-gassing occuring during a safety stop is focused on the slowest tissues that have absorbed inert gas during the dive. Faster tissues will have off-gased during ascent. The safety stop provides a margin to deal with the unaccounted saturation into slower tissues, that may not have been sufficiently desaturated on ascent.

The details here depend upon the specific profile, which is why some ascent rates can be too slow.

9) If micro-bubbles have already formed on ascent, then they will be present before the diver reaches his final stop (safety stop). The stop itself will not prevent their occurance. Any off-gassing occuring at that stop is dealing with the gases that remain in solution (micro-bubbles, by definition, are not in solution). Micro-bubbles take longer to eliminate because they have to dissolve back into solution (by gaseous diffusion) into surrounding tissue/fluid or be transported to and
eliminated by the pulmonary filter of the lungs.

Incorrect. Micro-bubbles form post dive. Safety stops have been conclusively shown to significantly reduce the number and duration of micro-bubbles.

10) Safety stops would have very little impact on micro-bubble elimination. Neither would they prevent the formation of micro-bubbles except for very specific dive profiles (v.shallow, long dives).

Incorrect.

 

[DevonDiver]

Exceeding ascent speed (too rapid reduction in ambient pressure) could/would lead to symptomatic bubble formation. However, the damage is done... bubble would be present.

The safety stop is 'extra decompession' to prevent bubble growth, not 'treatment' to resolve them.

A statutory period of 'extra deco' logically serves to compensate for variance in deco obligation. Less logically... it would be a vague 'shot in the dark' remedy against the risk of pre-existing bubbles. There would be better solutions for that....

From what I know about bubble dynamics...a bubble will grow or shrink depending on whether the tension of gases in the surrounding tissue is greater or smaller than the internal pressure of the bubble. To reduce a bubble, the ambient pressure needs to be large, thus keeping the bubble pressure large, whilst the pressure gradient across the skin of the bubble is kept negative... forcing out gas.

If higher ambient pressure is needed to stabilize and/or shrink the bubble caused by an exceeded ascent rate.... why would the remedy be a safety stop (lower ambient pressure)?

What I am getting at is: If they were addressing the bubble risk caused by exceeding an ascent rate... then why did they opt for a shallow stop, rather than an immediate stop?

 

[DevonDiver]

Incorrect. Micro-bubbles form post dive.

I am guessing that this has been measured via doppler/MRI etc? Excitation of inert gas remaining in solution post-dive?

Safety stops have been conclusively shown to significantly reduce the number and duration of micro-bubbles.

I'd be interested to read any articles you have that explain this process. It doesn't fit with what I understand of the process (I am more than willing to accept my own lack of knowledge is the culprit for this).

If it were micro-bubbles in the blood...then it would provide them with a window of time to be filtered by the lungs (micro-bubbles being small enough to pass through that filter). But those micro-bubbles present in the tissue would have to dissolve and return to a soluble state in the tissues before being off-gassed. That process wouldn't be dramatically effected by a further 3 minutes surely?

I understand that this is why safety stops were never noted for reducing 'post-dive fatigue', wheras deep stops were.

 

[Crush]

They still do. Ever watch a bunch of divers at safety stop on an upline? They'll sit and watch their dive computers religiously until they hit the 3-minute mark ... then they'll be on the surface a few seconds later ... which completely negates any benefit they got from the safety stop.

(Emphasis added)

I am not picking a fight with NWGratefulDiver - he is free to interpret this post as he sees fit.

Please correct me if I am wrong, but I fail to see how this statement can be literally correct. If NWGratefulDiver means to say "which partly negates any benefit they got from the safety stop" then I agree. I don't see how a rapid ascent can completely negate the benefits of a safety stop, provided that your dive was deeper than the stop. Or am I missing something?

I was taught by ACUC and NAUI that 30 ft/min [Edit: I originally posted 30ft/s. This was an obvious error. If you made fun of it - please grow up.] was a safe ascent rate above 30 fsw, which would mean that this is an appropriate ascent rate from the safety stop to the surface. Do others disagree with this? Is there a more appropriate value that used to be taught?

 

[Crush]

I'd be interested to read any articles you have that explain this process. It doesn't fit with what I understand of the process (I am more than willing to accept my own lack of knowledge is the culprit for this).

Dr Deco posted a good description of DCI and microbubbles here: http://www.scubaboard.com/forums/5559679-post32.html His post deals with many of your questions.

 

[Dr Wu]

Yep, "Physics in Wu's Universe." That should be a short course.

The following statements are factually incorrect:

How on earth would you have "good buoyancy" control without the BC? Isn't that impossible by definition? Aren't you defying the laws of physics especially if you are wearing a wet/dry suit?

When diving wearing a wetsuit U/W without a BC you are not able to control you buoyancy if you are changing depth.

Just because you "really, really" want them to be true they do not magically become true.

 

[knowone]

All your answers, and the solution to your over reliance on excerpting books and numbers mpetryk are contained here:goingdown:

 

[BoltSnap]

Dr. Wu,

Let's assume that you are diving with a 7mm suit in the Ocean without a BC and you had to use weight to be neutrally buoyant on the surface (or at 15ft) by using somewhere around 20 - 25 lbs. Now you go down to, let's say, 100ft. By definition your wetsuit compresses and loses at least half of its buoyancy at that dept. Now you are at least 10 lbs negative. How would you achieve neutral buoyancy then??

(Note: I am using "back of the envelop numbers for simplicity. I am making some approximation for ease of calculations. The suit also has its own "resistance" to compression in addition to other factors. The suit won't necessarily compress with 50%, it all depends on suit characteristics including material, gas used, age, etc.Even with 25%, the same question still holds, one still needs to compensate for wetsuit compression.).