QKRTHNU once bubbled...
Ok. But all we really care about is the ammount of Nitrogen in the compartment compared to that compartments M-value @ 1ata correct?
Since it's almost always safe to go from 2ata saturation to 1ata were only concerned with greater pressures.
I was trying to say that @ 3ata the timeframe for 2ata M-value stauration would be less than it is @ 2ata. In effect the half times are less because the M-value is different @ different pressures.
Even if the halftimes are constant for saturation @ a given pressure I would have to imagine that @ 4ata you would blow through the 2ata M-value faster than you would @ 3ata correct? Which is why the NDLs are so short at deeper depths.
(edited with corrections to sponge analogy)
Ok, first lets take a step back and then one forward:
STEP BACK
1/2 times never change. A 10 min compartment at the surface is also a 10 min compartment at any depth. The difference is in
how much gas the compartment can absorb in a given amount of time. The comparment tries to equalise with ambient pressure what ever that pressure is. In the first 10 minutes it will absorb 1/2 of it's total capacity. That applies at any given depth. What changes is that the deeper you go the higher the pressure and more total gas can get absorbed into solution (into the compartment).
Let's make it visual. Think about a 1 ata "unit" of nitrogen as a sort of sponge that you use to clean your dishes and think about the tissue compartment as a sort of empty can like the cans that food come in.
At the surface only one sponge fits in your can. 1ata because all that air weighs just 1 ata.
At 10 metres (33ft) your pressure has increased to 2ata so you can start squishing more sponges into your can. Because it's the 10 minute "can" it takes 10 minutes for you to stuff 1/2 sponge in. (it can potentially absorb a whole sponge if you keep at it long enough. ie. "saturated").
AT 20 metres (66ft) your pressure is now 3ata. Because the pressure is higher you can stuff harder. You can now stuff (as compared to the surface) a whole sponge into your can in the first 10 minutes. Eventually you can now get 3 sponges in your can instead of the 2 you can manage at 2ata.
And so on. At 6ata you'll be able to get 6 sponges in the can.
and so on.
Going back up works just the same way in this model. If you go from 66ft to the surface (3ata to 1ata) then it takes 10 minutes for you to get one of the sponges out of the can again. Because it's your 10 minute can.
I didn't talk about "staturation", only the first iteration. The second iteration would look like this.
1st iteration, you decend to 33ft (2ata). You can eventually get 1 extra sponge in your can. In the first 10 min you get 1/2 a sponge in. In another 10 minutes with the same amount of squishing you discover that you can get another 1/4 sponge in. AFter another 10 minutes you find room for another 1/8 sponge and so on..... That's what will happen if you stay longer. Eventually (aftre 6 iterations--ie. an hour--you'll get the whole sponge in)
If you then decended deeper then you'd suddenly be able to get more sponges in again because you can stuff harder (pressure is higher).
To understand the whole model, imagine 14 people standing side by side all stuffing sponges into cans at different speeds. They all go to 3ata and start stuffing at the same time. The 10 minute guy manages to get an extra sponge into his can in the first 10 minutes. However, the 60min guy takes an hour to do the same thing. Obviously the 60min compartment fills slower but because it also empties slower (once the sponge is in it takes him an hour to get the sponge back out again too) it could potentially control your surface interval (residual nitrogen = pressure group)
Did that help?
STEP FORWARD
M-values.
The relationship with M-values has no bearing on how much nitrogen you can absorb (how many sponges in a can). At it's most basic level M-values determine your NDL's at each depth. Nothing more. At least not in deco-101. As long as you stay within your NDL's the M-values of each compartment are not that exciting.
In deco-201 you find out that M-values are important for determing how deep and how long you need to make your deco-stops. But even then it doesn't control how much nitrogen you absorb.
What an M-value *really* tells you isn't actually your NDL per se. It tells you how much supersaturation a compartment can take before it bubbles. In other words, once you get your sponges in your can your M-value tells you how fast you can take your hand off (stop stuffing) without the sponges popping out of the can by mistake. And from that they deduced the NDL's.
That's the short answer. To get the long answer you should read that paper that Mike pointed to. After you do that then we can try explaining it in English
R..
