12
3 Calculation Model ZH-L8 ADT
I Uwatec
®
Aladin
®
Air Z
3.1 Description
The Model ZH-L8 ADT (8 compartments with a nominal half-time period from 5 to 640 minutes) differs
considerably from other models by the consideration of additional physiological processes:
1.Blood perfusion of the organs is not constant. Skin and muscles in particular are subject to considerable
changes of blood perfusion depending on temperature and workload. A change in blood perfusion also
causes a change of the saturation tolerance. The model takes these effects into account. Therefore, skin
and muscle compartments show variable half-time periods and saturation tolerances.
The necessary decompression times are calculated according to the diver’s workload and the decreasing
skin temperature. The decrease of skin temperature is estimated based on the water temperature and
the dive time. At the surface, the diver’s physical performance is lower than during the dive. By consi-
dering the differences in physical performance the no fly time becomes considerably longer.
2.The model considers inert gas not only in the dissolved state, but also in the gaseous phase (microbub-
bles). Formation of microbubbles is calculated with allowance for the different influences in arterial and
venous blood. With normal ascents gas bubbles mainly form in venous blood, with fast ascents also in
arterial blood and on ignoring decompression stops in the tissues. As the model calculates microbubbles,
those physiological processes are reconstructed which actually occur.
The bubbles on the venous side of the circulation are carried into the lungs where they change the arte-
rial nitrogen pressure. This influences mainly repetitive dives, dives with very long decompression times
and the no fly time.
• Gross disregard of the ascent rate, exceeding the decompression depth and repeated ascents to the sur-
face (yoyo dives), can all cause microbubbles to form on the arterial side and in tissues. Where the bub-
bles partially impair circulation, the rate of gas diffusion and the saturation tolerance are changed.
Decompression time, and if necessary decompression depth, are adjusted so that existing bubbles stop
growing. In addition, altered decompression assures that the tissue desaturates sufficiently if microbub-
bles impair circulation locally.
• The calculation of the gas bubbles results in a new ascent instruction. The ascent rate near the surface
is reduced down to 7m/min. This prevents with high reliability the formation of microbubbles in the arte-
rial circulation and minimizes the formation of microbubbles in the venous circulation after the dive.
