How does the Nitrogen/DCS model work?
A brief résumé follows. It assumes some basic knowledge of diving theory.
In common with most other models, Diver-Sim is based on the work of J.S.Haldane conducted at the beginning of the century. It uses a series of mathematical 'compartments' to represent actual tissue structures within the body to bypass the unmanageable complexity of directly modelling the interaction of nitrogen within the structures themselves.
As a diver descends, the atmospheric pressure surrounding him grows proportionally, and correspondingly, so does the partial pressure of nitrogen (partial pressure of nitrogen at 1atmosphere = 0.79bar). This pressure forces nitrogen into the blood and tissues at a rate proportional to the pressure until the pressures are equalised. Haldane assumed this rate to be exponential in nature and related to the type of tissue. Therefore, each tissue 'compartment' has its own 'half time' (analogous to radioactive half-life).
The tissue half-times used by Diver-Sim are:
|
|
|
1 |
5 (fastest) | kidney |
2 |
10 | stomach, bowels, liver, central nervous system |
3 |
20 | central nervous system, liver, stomach, bowels |
4 |
40 | skin |
5 |
75 | skin, muscles, heart |
6 |
160 | muscles |
7 |
320 | muscles, joints, bones, fat |
8 |
640 (slowest) | fat, joints, bones, rest |
As the dive progresses, tissues will 'load up' with different amounts of nitrogen. During ascent, these tissue begin to off-load this excess nitrogen in exactly the same way.
The current pressure differential for each compartment is important in determining the current ratio of ambient pressure to tissue pressure. This ratio is important because it is proposed that this value can be used to determine whether dangerous numbers of bubbles will be formed as the diver ascends. The picture (screen-shot) below shows a diver whose slower tissues have absorbed less than the faster ones (as you'd expect), and are still soaking up nitrogen. In the meantime, the faster tissues in red are busy off-gassing excess nitrogen according to their pressure gradient.
There are a number of other factors to throw into the pot, and Diver-Sim aims to roughly model a few of the more important, such as heart rate, breathing rate and some degree of cross-connection between tissue compartments during off-loading.