In the 1990s, Professor Albert Bühlmann (1923 – 1994) wanted to reflect the change in blood perfusion to various body organs when the diver is subjected to temperature and/or workload variations, as changes in blood perfusion would alter the inert gas saturation tolerance. In other words, he wanted to develop an algorithm to deal with all the real time variables throughout the dive, not just the depth and time. The result was ZH-L8 ADT, a model using 8 evenly-distributed tissue compartments halftimes ranging from 5 to 640 minutes (for nitrogen). Some of the ‘initial’ halftimes (particularly those of the mid compartments) get altered during the dive to reflect what’s going on with the diver’s body. The idea is that coldness causes vaso-constriction (mainly at the skin and the muscles), thus reduces perfusion. To mathematically simulate this change in perfusion level, the ‘corresponding’ halftimes should be altered.
As you might have guessed, this model is useful for dive computers only. It is implemented in some of Uwatec models. The dive computer has a sensor to monitor the water temperature throughout the dive. The feedback is then sent to the algorithm. One drawback of this approach is that the water temperature is not always a measure of what the diver is feeling. The diver might be wearing a swimsuit and a T-shirt, or a dry suit with a heavy undergarment, so skin temperature would have been a more appropriate indicator. The workload is calculated by monitoring the reduction in tank pressure (gas consumption rate). This is considered by some divers as another drawback, as increased consumption rate is not always an indication of elevated workload. The newer, top line models include an optional ‘integrated heart rate monitor’. This is a ‘belt’ that allows the workload calculation to take into account actual blood circulation. In some models, the workload is an adjustable setting, which is good for unfit divers or, for instance, when the diver knows beforehand that there’d be a demanding situation (swimming against current, long surface swim before the descent, etc…).
Based on his personal communication with Professor Albert Bühlmann and Dr. Max Hahn, my friend Dr. Albrecht Salm provided me with some details on which ZH-L16 halftimes would need getting altered to reflect cold and/or increased workload. This enabled me to introduce ZH-L16D in Ultimate Planner. ZH-L16D is a more conservative model than both ZH-L16B and ZH-L16C, and it would generate more suitable schedules for unfit divers or for anticipated colder and/or more demanding dives. For instance, assuming the last stop depth is at 6 meters (20 feet), the total run time of a 30 minute dive on air to 45 meter (150 foot) depth would be 73 minutes (ZH-L16B), 84 minutes (ZH-L16C) or 87 minutes (ZH-L16D).
P.S.: This article was first published in the fourteenth issue of Tech Diving Mag.