Example to illustrate de/saturation processes

deutsche Version

Through the following dive profile, calculated by the aid of the software Tausim , a short explanation of saturation and desaturation processes will be given.

At the beginning all the tissues are saturated according to an ambient pressure of 1 bar at sea level. The violet bars demonstrate this, showing a value of 100% for all compartments. The red bars indicate the saturation concerning the maximum tolerated excess pressure. One easily sees, that the faster compartments have a greater tolerance limit than the slower ones (with longer half-time). The slowest tissue is the leading compartment at the beginning of the dive, indicated by the red 16.

Now we descend with constant rate within 5 minutes to a depth of 20m (66 ft). The tissues (even the fastest) cannot saturate with inert gas as qiuck as the ambient pressure rises.

When the diver stays on constant depth saturation of the compartments can take place. The bars show that this process is running quicker with the faster tissues than with the slower ones, which show practically no difference compared to the above picture.

After a time of 20 minutes at a depth of 20 m (66 ft) the fastest tissue No. 1 is nearly saturated concerning the ambient pressure (violet bar shows a value a little bit less than 100%). The compartments with greater half-time show only slight differences. Tissue No. 16 is the leading compartment, still.

Ascending now within 2 minutes to a depth of 10 m (33 ft), the four fastest tissues show excess saturation. The violet bars exceed the 100%, which Tausim indicates with yellow color. This indication is no longer in correct scale. These four compartments are desaturating back, while the other ones are still saturating. Leading tissue is now No. 3.

End of dive after 30 minutes and an ascent time of 5 minutes from a depth of 20 m (66 ft). All tissues are now supersaturated concerning the surface pressure (the extent of the supersaturation cannot be seen from the graphics !). Tissues 3 and 4 reach maximum values for the tolerated inert gas excess pressure. At surfacing compartment No. 3 is the leading tissue (no longer indicated).

For this dive a desaturation time of 30 h 45 min is calculated. "98.5" means, that the desaturation time is calculated up to an extent of 98.5 percent, as all dive computers do (due to mathematical limits of the model it's not possible to determine an extent of 100% - this would last endless). The lack of the remaining 1.5% to 100% are practically meaningless and conform to differences in air pressure at the change of weather conditions from high to low pressure conditions.

The "do not fly" time is 24 h 50 min, calculated for a reference pressure according to a height of 13000 ft. Conclusive is tissue No. 12, compartments with greater half-times (No. 13 through No. 16) cannot be taken into account, because the argument of the logarithm becomes negative, which isn't allowed.

This is a recapitulation of the saturation data of this dive :

compartment $t$1/2 N2 [min] do not fly 15000 ft do not fly 13000 ft do not fly 6000 ft satur. in % concerning to 0 ft satur. in % of the max. tol. value
1 4,0 0:00 0:00 0:00 260 60
2 8,0 0:06 0:05 0:01 264 78
3 12,5 0:14 0:12 0:05 246 82
4 18,5 0:24 0:21 0:09 222 79
5 27,0 0:43 0:36 0:14 197 77
6 38,3 1:20 1:06 0:22 176 74
7 54,3 2:14 1:44 0:25 157 70
8 77,0 3:39 2:37 0:15 143 66
9 109,0 5:54 3:46 0:00 131 64
10 146,0 11:10 5:54 0:00 124 63
11 187,0 --- 9:48 0:00 119 62
12 239,0 --- 24:50 0:00 115 62
13 305,0 --- --- 0:00 112 62
14 390,0 --- --- 0:00 109 62
15 498,0 --- --- 0:00 107 62
16 635,0 --- --- 0:00 106 62
Calculated "do not fly" times concerning three heights and desaturation data in percent concerning surface pressure and maximum tolerated value. Maximum values in columns are printed in bold. "---" : value couldn't be determined (negative argument of the logarithm).

Remarkable is that the "do not fly" time calculated for a pressure concerning a height of 13000 ft is much greater than that determined for a pressure concerning a height of 15000 ft - which should be longer than the first, for the safety tolerance is much higher in this case (comparing the values of a certain tissue one sees, that the values for 15000 ft are greater). Conclusive is that in the case of 15000 ft only the first 10 compartments are to contribute to the calculation, but in the case of 12000 ft these are two more tissues. If all the compartments could help to determine the "do not fly" time (or another algorithm would be used :-) ) it would be even much greater.

It's interesting furthermore, that the "do not fly" time for 6000 ft is considerably shorter - though all tissues contribute ! - than the two other "do not fly" times. Here not the slowest but compartment No. 7, a mid-range tissue, is decisive. In this case the faster tissues have desaturated back, and the slower ones haven't saturated so much so far.

Let's have a look at the last but one column (saturation of the tissues concerning surface pressure). One easily sees, that when reaching the surface the fast compartments are exposed to a pressure more than twice the surface pressure, whereas in the slower compartments the pressure is only slightly higher. Although the second fastest tissue is exposed to the highest excess pressure with 264%, compartment No. 3 acts as the leading tissue reaching 82% of the maximum tolerated excess pressure.

Although in this example during ascent neither the allowed velocity was exceeded nor a deco stop was demanded it's good behaviour to perform always a safety-stop of some minutes at 10 ft and only then proceed to the surface.

What about using nitrox with an oxygen fraction of 40% by volume instead of air ? The following picture illustrates the tissue saturation for the same profile as above using Nitrox 60/40 as the breathing gas. Now compartment No. 6 is the leading tissue and the red bars are significantly less high than with air.

compartment $t$1/2 N2 [min] do not fly 15000 ft do not fly 13000 ft do not fly 6000 ft satur. in % concerning to 0 ft satur. in % of the max. tol. value
1 4,0 0:00 0:00 0:00 260 45
2 8,0 0:01 0:00 0:00 264 59
3 12,5 0:05 0:03 0:00 246 62
4 18,5 0:08 0:06 0:00 222 62
5 27,0 0:17 0:13 0:00 197 60
6 38,3 0:34 0:26 0:00 176 59
7 54,3 0:52 0:36 0:00 157 56
8 77,0 1:08 0:41 0:00 143 53
9 109,0 1:15 0:28 0:00 131 50
10 146,0 1:37 0:19 0:00 124 49
11 187,0 2:20 0:14 0:00 119 48
12 239,0 4:30 0:46 0:00 115 47
13 305,0 10:48 2:45 0:00 112 47
14 390,0 60:12 9:07 0:00 109 47
15 498,0 --- 21:12 0:00 107 47
16 635,0 --- --- 0:00 106 47
Calculated "do not fly" times concerning three heights and desaturation data in percent concerning surface pressure and maximum tolerated value using Nitrox 60/40 as breathing gas. Maximum values in columns are printed in bold. "---" : value couldn't be determined (negative argument of the logarithm).

Besides the fact that the saturation data now lie much below the maximum tolerated values, it's striking that the "do not fly" time for 6000 ft is nearly the same as for air as breathing gas, but for 15000 ft it rose sharply to over 60 hours. This is due to the fact that now all but the last two compartments can contribute to the calculation.

Dive computers avoid these shortcommings of the model in using only the mid-range tissues for the computation of the "do not fly" time. This guarantees that the output of the "do not fly" time is always consistent. Because dive computers don't show the leading tissue (potentially later on the PC), one could be irritated otherwise and think of a malfunction of the dive computer.

Kai Schröder, 14.12.2000