Nitrox on boat with air refill

[NAM001]

You are stuck with the general definition also. so on the first dive of the day it is hard ti visualize how the fastest compartment will not be the controlling one when talking about NDL. The fastest compartment should be the only one to be able to first reach that level.

NDL is the time at which you can no longer directly ascend to the surface. In the dissolved gas models that is when ANY compartment reaches a inert gas partial pressure which would (probably) cause DCS at the surface given an ascent at however m/s the model designer chooses.

I am trying to show that which compartment is first to limit is non trivial and no general intuitive rule covers it.

 

[KenGordon]

On a first dive the fastest will reach equilibrium first, but equilibrium at X metres may still be ok for the fast compartment but not slower compartments when ascending to the surface.
The models are not just about how fast the gas goes in and out but also about how much pressure above ambient is too much for an ascent. So if compartments 2 and 14 had the same ppN2 then compartment 14 would not allow the diver as shallow as compartment 2. The longer the half time, the less over pressure allowed.

On repeat dives the slower compartments may not need much increase in the inert partial pressure to be a problem, but the fast compartments start from scratch. Which compartment limits the dive depends on SI, depth, time, profile. Fortunately we have computers.

Of course all this is just a mathematical model.

 

[NAM001]

Ken

I understand that. I guess I look at it as rec divers should never have to deal with the higher compartments, especially on a 2-3 dive a day outing. I have (for what it is worth) watched the dive nav simulator showing the 16 compartments. and I can follow them and predect what will happen for depth changes and SI periods. Thjis thread makes me want to start digging into the compartment theory. Your definition was in the form of what is not ndl. How would you define what NDL limit is. Other than the point of where you can o longer ascent directly to the surface. The plots I have seen slows a graph 0-100 and a line at about 90 and that is the ndl line. any compartment that touches it puts you in deco and out of ndl. So can you be no longer in NDL and have say 3-4 compartments at the 60 or70 level. IN other words is ndl not just simply hitting the 90 level, and is an interaction of multiple compartments that can put you in deco with out any reaching the 90 level. Do you see my dilemma in understanding. My limited unserstanding on the mater is subject to the most basic definitons provided at the OW level of training. It makes it difficult to discuss because now if use the word saturated as if it was in reference to the 90 level for my understanding the word saturated has a more precise meaning in the technical levels of explanation. Not that it would help my diving any but perhaps I should find a book called ndl for dummies or something. Sometehing written at an ow/aow level.

On a first dive the fastest will reach equilibrium first, but equilibrium at X metres may still be ok for the fast compartment but not slower compartments when ascending to the surface.
The models are not just about how fast the gas goes in and out but also about how much pressure above ambient is too much for an ascent. So if compartments 2 and 14 had the same ppN2 then compartment 14 would not allow the diver as shallow as compartment 2. The longer the half time, the less over pressure allowed.

On repeat dives the slower compartments may not need much increase in the inert partial pressure to be a problem, but the fast compartments start from scratch. Which compartment limits the dive depends on SI, depth, time, profile. Fortunately we have computers.

Of course all this is just a mathematical model.

 

[KenGordon]

Read this book Deco for Divers by Mark Powell

From Dive-Tech: Decompression theory - Robert Workman and Prof A Buhlmann -

"Workman revised Haldane’s model to take into account the fact that each of the various tissue compartments can tolerate a different amount of overpressurisation and that this level changes with depth. He introduced the term "M-value" to describe the amount of overpresurization each compartment could tolerate at any depth."

I do not like the word 'saturated' wrt to deco. It is used vaguely and seems to mean 'in equilibrium with ambient' or 'reached a ppInert such that you cannot ascend' depending on who is talking.

 

[NAM001]

HERE IS A XL PLOT OF HALF LIFE. PROBABLY TOO ELEMENTERY BUT IT IS AS I SEE IT. lEFT TO RIGHT 20 MINUTE DIVE TO 100 FT, 1 HOR SURFACE inTERVAL FOLLOWED BY UN ENDING DIVE TO 100 FT AGAIN. left top blue cpmpartment 1 5 min half life. horizontal is in 5 min increments.

sorry for the caps.

 

[PfcAJ]

I do not like the word 'saturated' wrt to deco. It is used vaguely and seems to mean 'in equilibrium with ambient' or 'reached a ppInert such that you cannot ascend' depending on who is talking.

saturated and supersaturated are the terms, I believe.

 

[Centrals]

Or, did I make an error in one of my calculations?

Safe Diving,

rx7diver

I am withdraw my statement about you making WRONG calculation on CNS% because you had left out the "90mins half-live of CNS%". The 90 mins half-live was built on unproven scientific study(assumption or whatever) and you have every right to ignore it.
However, to make thing more complicate, plenty tec divers are using the idea in planning their deco dives without suffering O2 tox.
Cheers.

---------- Post added November 24th, 2015 at 09:11 AM ----------

You can easily exceed your computer CNS oxygen exposure doing recreational, no stop dives, it really depends on your computer. Oceanic computers use a 24 hour rolling window and the NOAA exposure limits. They do not use a half life of elimination of 90 minutes. This allows you 5 hour in a 24 hour period with a pO2 of 1.0. Doing 4 dives per day to about 60 ft with EAN36, (pO2 just over 1.0) for about 1:15 each, easily gets you over the limit, especially since there a 5 dives in the 24 hour window. I have exceeded my O2 limit many times under these conditions. With a 90 minute elimination half life, I would never even be close. I wish Oceanic would update their O2 exposure algorithm.

Perhaps Oceanis realized that 90 mins half -live was not based on any scientific study. So Oceanic probably won't update their O2 exposure algorithm.
There should be a warning on the practice of 90 mins half-live or make it an option in any dc or soft ware.