I've read explanations of the various compartments (constructs) and their associated half life times (fast-slow, 5 min-120 min or whatever). I never see the explanation where this is related to depth. The explanation is the 5 minute compartment is half "full" after 5 minutes but this concept is never related to depth. If I dive to 30 fsw and stay 5 minutes the 5 minute compartment isn't half 'full" is it? I know that the deeper you go the controlling compartment is the faster compartment and as you ascend the faster compartment is also the first to clear so as you stay underwater longer the slower compartments would start to control. If I go to 130 fsw the 5 minute compartment would probably be the controlling compartment but if that's the case there has to be some tie in between 5 minute compartment and depth. Without any explanation of course I understand that the partial pressure of nitrogen is increasing with depth and my NDL times are becoming shorter or my deco times are becoming longer. So in effect I know what is happening in reality but I'm trying to relate that to the deco theory terminology.

Great question, I would love to see a good explaination on half-times, too. Working on my DM and this area has me stifled.

See the link below. The half time of a compartment is the time it takes for the compartment to become half saturated, or if we are ascending to become half desaturated. By saturated we mean the stable level of gas loading at that depth - whatever depth that is. In the cases you quoted the answer is yes to both cases. If you go instantaneously to 30ft or to 130ft, after 5mins the 5min compartment will be at 50% loading. There is small simplification here because gas loading is not zero at the surface so you should really do the calculations with deltas but the principle is the same. http://www.dive-tech.co.uk/half times.htm

The time it takes a compartment to reach half of the nitrogen it can hold at that depth is the half-time of the compartment. What amount of nitrogen constitutes "full" varies with depth. If you think about it, the difference in pressure of nitrogen between the air you are breathing and the tissues in your body is greater after a one minute descent to 60 feet than it is after a one minute descent to 30 feet. So the pressure driving nitrogen into the body is greater, and nitrogen moves in faster. But the body can also hold more, so the time to half "full" is still five minutes, but the absolute number of nitrogen molecules that have moved during that five minutes is greater. As you ascend, the fast compartments, which are "full", reach 100% or greater saturation very quickly, but they offgas very quickly as well, so they are almost never the controlling compartment. Deco software can graph this out for you very nicely, or look up Erik Baker's paper, "Understanding M values", which is excellent.

Let me try to give an example of what I understand to be going on: Take the 5 minute compartment. Let's say at 1 ATM it can hold 1 mL of N2. At 2 ATM it will hold 2 mL and and 3 ATM, 3 mL. We start a dive, it is holding 1 mL (since we've been on the surface "forever") Go to 2 ATM (10 meters) for 5 minutes, it is up to 1.5 mL (1 + 1/2 *(2-1)). At 10 minutes it is 1.75 (1.5 + 2-1.5), etc. Every 5 minutes it moves half the distance to saturation. Now, instead, go to 3 ATM (20 m). Now in the first 5 minutes you are at 2 mL (1+1/2*(3-1)), after 10 minutes 2.5, etc. So after 5 minutes at 20m if you *immediately* go up to 10m you will be saturated at 10m at you'll stay saturated. If you come up immediately to the surface, after 5 minutes that compartment will be hold 1.5 mL of nitrogen (2-(2-1)*0.5) and 5 minutes later 1.25 (1.5-(1.5-1)*0.5). Of course, a computer tracks, what 10-12 compartments at a time with different half-lives and the recreational tables generally work on the 60 minute compartment, I think.

Thanks. I get it. At 30 fsw the 5 minute compartment is half "full" but that's not much of an issue as the M value has not been exceeded and won't be at 30 fsw for a long time. At 130 fsw the body can hold more disolved nitrogen and the M value will be reached much quicker but the 5 minute compartment is still half "full" after 5 minutes (even though it can hold more at this depth).

Here is a clip from wikipedia It has been theorized that different compartments (or groups of tissue) take up gas at different rates. The same theory holds that they release gas at the same rate they on-gassed. If the rate is too fast bubbling will occur which can be harmful (the bends). The gradient or difference in pressure between the compartment and the ambient pressure (present depth while ascending) will reach a certain value before bubbling can be expected to happen. This is known as the M-value. So originally it was thought by Haldane the 2ata was the absolute safe maximum or M-value. It was later realized that the gradient was not affected by oxygen, only inert gas (nitrogen usually) so the M-value was revised to be 1.58 ATA partial pressure of nitrogen or 33 ft when diving air. When any one tissue reaches this value it must not be exceeded until it has reduced (decompression) to a safe value. Buhlmann defined 16 compartments ranging from 4 minutes to 635 minute half times. These compartments represent no individual tissure, just theoretical groups of tissures that on gas and off gas at the same rate. Each compartments half time is independant of each other and are what is known as a parallel model.

You will never exceed the M-value of a compartment while you are stable at depth, or descending. M-values are only pertinent when ascending. Wedivebc's post is valuable by pointing out that this is a mathematical construct. It cannot even be called a model, because models are developed out of careful measurements of the behavior of systems under various parameters. This is a theoretical structure based on an assumed structure of "tissues" and gas loading and unloading behavior. There is significant controversy about the models, particularly when you get into the behavior of helium mixes.

Hello gcbryan : Halftimes Halftimes are independent of the depth. Naturally, the deeper you descend, the more dissolved nitrogen will be present in the tissues but the time to reach this is the halftime independent of depth. Actually, halftime also applies to altitude depressurizations where the dissolved nitrogen is being unloaded. Real Halftimes One must also remember that halftimes describe the growth of a gas phase in the body as it relates to dissolved nitrogen. Halftimes as encountered in diving are much longer than would be determined by blood flow. Living tissues could not function with such a small blood supply as to result in a 300-minute halftime, for example. The oxygen supply, nutrient supply, and elimination of waste products would be much too low. Dr Deco :doctor: The next class in Decompression Physiology for 2006 is September 16 17. :1book: http://wrigley.usc.edu/hyperbaric/advdeco.htm[/url