Decompression terminology and mechanics

[gcbryan]

Just thinking aloud here and trying to make some of the pieces fit (in my mind). I'll also mention that I did just order Mark Powell's book "Deco for Divers".

I understand that saturation is when the tissues will hold no more nitrogen (at that depth). Saturation diving is when that point is reached and divers simply stay down for days to do work without incurring additional deco obligations (returning to a bell at night) before eventually returning to a recompression chamber on the surface.

Supersaturation is when your tissues are saturated and you start to ascend...they can't eliminate the nitrogen fast enough so you get bubbles. Is this correct. So, you may not be saturated but if you ascend your tissues are supersaturated. Is this the correct usage of this terminology?

How does hitting the NDL relate to this. This is a separate concept right? Your 5 minute compartment could be saturated (can hold no more) but if you are at 20 feet this still isn't a problem as your body can deal with this without developing the DCS symptoms. Right?

So, when you are at 100 fsw and cross the NDL and now you have a decompression obligation this is still not considered saturated since your tissues can still hold more and you can still incur an even longer deco obligation. Saturation is only when your deco obligation will not get any longer with more time spent at that depth.

So the concept of tissue saturation is not the same as incurring a deco obligation. When you first began to incur a deco obligation you don't refer to this as being saturated...right?

I'm trying to clarify the proper use of the terms saturation and supersaturation. When I see them used or defined the above points aren't made clear (to me).

I also understand (I think) the concept of M-values.

[Thalassamania]

Originally Posted by gcbryan

Just thinking aloud here and trying to make some of the pieces fit (in my mind). I'll also mention that I did just order Mark Powell's book "Deco for Divers".

I understand that saturation is when the tissues will hold no more nitrogen (at that depth). Saturation diving is when that point is reached and divers simply stay down for days to do work without incurring additional deco obligations (returning to a bell at night) before eventually returning to a recompression chamber on the surface.

For any ambient pressure there is a finite amount of gas that will go into solution in a liquid. The greater the pressure the greater the amount. The advantage to saturation diving (at least for scientists) is that if you are in a habitat at 2.6 ata (as deep as you can go on air for long term) you can make excursions to 95 ft or less for three to six hours in the morning or early afternoon, followed by a "surface" interval in the habitat (4 hours are needed for maximum bottom time on the second dive), with a second excursion of up to three hours.

Commercial divers use an on deck system where they live under pressure and and are then Transferred Under Pressure (TUP) in a bell, which is basically a pressurized elevator.

Originally Posted by gcbryan

Supersaturation is when your tissues are saturated and you start to ascend...they can't eliminate the nitrogen fast enough so you get bubbles. Is this correct. So, you may not be saturated but if you ascend your tissues are supersaturated. Is this the correct usage of this terminology?

I know it seems counter intuitive but you can be supersaturated without bubbling. Supersaturated just means that you have more gas in you than you would have dissolved in your tissues if you were saturated at that depth. There is a calculable amount of supersaturation that is a function of the tissue half-time (how fast gas dissolves in or is give off). The faster the tissue the greater the supersaturation it can withstand without bubbling.

Originally Posted by gcbryan

How does hitting the NDL relate to this. This is a separate concept right? Your 5 minute compartment could be saturated (can hold no more) but if you are at 20 feet this still isn't a problem as your body can deal with this without developing the DCS symptoms. Right?

Sort of. At 20 feet you can be fully saturated in all your tissues and safely go to the surface without staged decompressing. So let's drop down to 60 feet. The fastest tissues start to fill up, but they are also the most "forgiving" tissues. Eventually you get to the point that a tissue that a slow enough tissue has absorbed enough gas that you can not go up to the surface without bubbling. Now you are at the NDL and will have to make a staged decompression stop to get to the surface without the bends.

Originally Posted by gcbryan

So, when you are at 100 fsw and cross the NDL and now you have a decompression obligation this is still not considered saturated since your tissues can still hold more and you can still incur an even longer deco obligation. Saturation is only when your deco obligation will not get any longer with more time spent at that depth.

Correct, when you hit the NDL at 100 feet it is a faster tissue that in keeping you from sufacing than it was at 60 feet in the previous example.

Originally Posted by gcbryan

So the concept of tissue saturation is not the same as incurring a deco obligation. When you first began to incur a deco obligation you don't refer to this as being saturated...right?

You are not saturated until you have absorbed all the gas that you can at that depth, more time will not result in more gas.

Originally Posted by gcbryan

I'm trying to clarify the proper use of the terms saturation and supersaturation. When I see them used or defined the above points aren't made clear (to me).

I also understand (I think) the concept of M-values.

All supersaturation means is that a tissue has more as than would disolve in it at that depth if it were there for a long time (saturation). Being "saturated" usually reffers to having absorbed all the gas that you are going to in all of your tissues.

[gcbryan]

Which brings up the question how do you get more in a tissue than would dissolve in it if you were there for a long time (saturation)? Isn't this only possible when you are ascending?

[Thalassamania]

Originally Posted by gcbryan

Which brings up the question how do you get more in a tissue than would dissolve in it if you were there for a long time (saturation)? Isn't this only possible when you are ascending?

Correct

[Jrock1062]

Deco for Divers is a great book I just finished it and it really does put things into perspective first off i am a new at this so please correct me if am wrong but to my understanding super saturation occurs when you are ascending and because your tissues have a high saturation then the ambient pressure am i not understanding that right

[Thalassamania]

You are understanding that right

[Jrock1062]

thanks It is really a great book and I thought I read a name that was similar to yours at first i thought it might be you the name was Thalmann

[wedivebc]

Originally Posted by Thalassamania

So let's drop down to 60 feet. The fastest tissues start to fill up, but they are also the most "forgiving" tissues.

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

[Thalassamania]

Faster tissues have higher surfacing ratios.

[Tassie_Rohan]

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.