Each breath at depth adds nitrogen to your blood, and your blood carries it to every part of your body. The nitrogen does no harm while you stay at depth. The problem arises if you ascend faster than it can leave.
To work out how fast you can ascend, decompression models treat the body as a set of tissues, each taking up and releasing gas at its own rate. Every model since 1908 describes that rate in the same way: with a half-time.
- Fast tissue
- Slow tissue
How a tissue takes up gas
At 30 m on air, the nitrogen pressure in your lungs is much higher than in your tissues, so nitrogen diffuses into them.
The larger the difference, the faster it moves. A tissue therefore loads quickly at first and more slowly as its pressure approaches the pressure you breathe. In the model it never reaches that pressure exactly.
Half-times
The rate of loading follows a fixed rule. In one half-time a tissue takes up half of the difference between its pressure and the pressure you breathe. In the next half-time it takes up half of what remains, 75% in total. After 3 half-times it has taken up 87.5%.
The compartment shown here has a half-time of about 8 minutes. After about 6 half-times, any tissue is treated as full for practical purposes.
The 16 compartments of ZHL-16C
Body tissues exchange gas at very different rates. Blood and brain take minutes; fat, joints and bone take hours.
Bühlmann’s ZHL-16C, the model used in most dive computers, has 16 compartments with half-times from 5 minutes to 635 minutes. On the same dive, the fast compartments are nearly full while the slow ones have taken up very little gas.
Gas release during the ascent
As you ascend, the pressure you breathe falls below the pressure in the fast tissues. They start to release gas, quickly at first, following the same half-time rule as on the way down.
The slow tissues are still below the new, lower pressure for a while, so some continue to take up gas during the ascent. The compartment that limits the ascent changes as the dive progresses.
Try it
Set the depth, the bottom time and the oxygen percentage of the mix. The bars show all 16 compartments at the start of the ascent. Deeper dives load the fast compartments first, longer dives load the slow ones, and nitrox lowers every bar.
What to remember
- A tissue takes up gas quickly at first, then more slowly: in each half-time it takes up half of the remaining difference.
- After about 6 half-times a tissue is treated as full, or saturated, at that depth.
- Models use many compartments because body tissues exchange gas at different rates. ZHL-16C half-times range from 5 to 635 minutes.
- During the ascent the fast tissues release gas first, while the slow tissues may still be taking it up.
Tissues in the planner
Every DiveLogic plan has a tissue view showing all 16 compartments minute by minute, loading and unloading as in the figures above. Plan the same 30 m dive and move along its timeline.
Sources
- Boycott A. E., Damant G. C. C., Haldane J. S. (1908). The prevention of compressed-air illness. Journal of Hygiene 8(3): 342-443.
- Bühlmann A. A. (1984). Decompression-Decompression Sickness. Springer, Berlin.
- Bühlmann A. A., Völlm E. B., Nussberger P. (2002). Tauchmedizin, 5th edition. Springer, Berlin. (ZHL-16C compartment table.)