Decompression terminology and mechanics

[gcbryan]

I would like to hear your explanation for this comment since I am inclined to disagree with you unless I am misunderstanding your meaning.

Let me see if I can guess your concern with this statement (just for my learning purposes)... he uses an example where we are going deeper and his comments could be read to mean that with depth the overpressure (surfacing) ratio is increasing which is not the case. However, I believe he is only referring to the ratio being higher in the faster compartments (than in the slower ones) which is the case even though the ratio for all tissues decreases with depth.

Was that the concern?

 

[Jrock1062]

All ascending divers will have bubbles.

Even non divers on the surface can have bubbles caused by cavitations as joints move around.

The key is how your bubbles are managed.

Bubbles form when the pressure of nitrogen in solution exceeds, by a certain amount, the ambient pressure. The extra amount of pressure above ambient required to form a bubble (leading to a state of 'supersaturation') is because the growing bubble needs to overcome both ambient pressure and the surface tension of the bubble’s 'skin'.

Small bubbles have higher surface tensions than larger ones, so small bubbles need a higher internal pressure to form and grow compared to larger bubbles.

If you looked at your blood under a microscope as you ascend you would see millions of microscopic bubbles form, and just as quickly redissolve. You would also see a much smaller number of larger bubbles hanging around that are stable, due to their lower surface tension, and lower internal pressure.

The key is to maintain a pressure so that all those millions of small bubbles keep redissolving and you keep only a few larger bubbles. This allows you to transport the gas to your lungs, where the 'supersaturated' gas can escape.

If you ascend too quick for the tissue and blood to transport the dissolved gas to your lungs then all those small bubbles have a chance to grow to a size (a 'critical radius') where they become stable - and can then go on to grow to the point that they can do damage: leading to DCI and/or foam for blood.

Cheers,
Rohan.

As i said before I am new so please correct me if I am wrong I understand and agree with everything you said except my understanding is that Supersaturation is only when the Partial Pressure in the tissue compartments exceeds the ambient pressure which occurs when you are ascending

 

[Tassie_Rohan]

Yep – that’s correct.

In my first post I meant to say that if you looked at your saturated tissues as you ascend you would see lots of microbubbles forming.

When you return to undersaturated conditions you would think bubbles dissolve completely – but you can often have relic 'bubble seed' nuclei held together by the surfactants that were on the bubbles surface. These can seed later bubble growth when you return to supersaturated conditions.

Cheers,

Rohan.

 

[Thalassamania]

TR is correct - I was trying to keep it a little simpler and cleaner.

 

[Jrock1062]

thanks all for the help and clarification it is great the way you can bounce information around in SB

 

[Dr Deco]

Thanks...the reponders gave some nice answers.

 

[wedivebc]

Was that the concern?

Actually my only concern was with the term "forgiving". Type II DCI usually involves fast tissue compartments and even though they tolerate a greater surfacing pressure the consequences of type II DCI can be anything but "forgiving"

 

[wedivebc]

Was that the concern?

Actually my only concern was with the term "forgiving". Type II DCI usually involves fast tissue compartments and even though they tolerate a greater surfacing pressure the consequences of type II DCI can be anything but "forgiving"

 

[Thalassamania]

Actually my only concern was with the term "forgiving". Type II DCI usually involves fast tissue compartments and even though they tolerate a greater surfacing pressure the consequences of type II DCI can be anything but "forgiving"

This is true and "forgiving" was a bad choice of word, I should have put it as you did, "tolerate a greater surfacing pressure."

 

[gcbryan]

The more I read on this subject the more it seems that since Haldane it's all just tweaking the numbers to match a result that we already know.

We know how many people per 100k or whatever get bent at certain time/depth combinations so when a new theory is tested the numbers are tweaked to verify that indeed the new theory is working.

Buhlmann's ZHL16 b and c versions (for tables and computers) had the a coefficents tweaked to make them more conservative. Not that it had anything to do with how the body actually works.

I could come up with a theory regarding the moon's effect on decompression and as long as I tweaked the formula's so that a 20 min dive at 100 was OK then we have another valid decompression theory.

I see that some are using what is called the statistical model (based on statistics and not even pretending to model the body). It seems to me they are all (after Haldanes original work) based on the statistics of whether they are working or not as measured by DCS incidence.

It's an interesting subject that I'm still in the process of reading and trying to learn more about but it does seem like trying to be more precise in a subject that is more akin to voodoo.

In a not entirely related questiion...is it true or rather it seems to me to be the case that both traditional and bubble theory models assume ongassing and offgassing at the same exponential rates. Bubble theory slows things down on ascent and has deeper first stops but I think (I may be wrong) that they both assume on and off gassing at the same rate.

I'm reading that bubbles are forming on ascent (in any model.....even bubble theory..just to a lesser extent) and that bubbles slow the rate of offgassing. Therefore none (very few) of the models are accurate in this basic regard.

Am I correct in my reading in this regard?