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At 08:23 AM 3/31/99 -0800, Cam Banks wrote: Hello Cam, >Can someone explain to me how deep-decompression stops work? The purpose of deep-stops is to reduce the fatigue (generally believed to be sub-clinical DCS) that is often associated with deep/deco diving. One popular algorithm (Richard Pyle's) is fairly simple. Here it is in a slightly simplified form. The actual algorithm uses the ambient pressure midpoint rather than the distance midpoint between the deepest point and the first mandatory stop, but for the depths which tech divers frequent, the two are very close and are interchangeable for this discussion. 1. Generate a decompression profile the way you normally do. 2A. If the difference between the deepest point in your dive and your first mandatory deco stop as indicated by the profile you generated in step 1 (or 3) is LESS THAN 30 feet, you already have deep-stops in your profile so no further calculations are necessary. 2B. If the difference between the deepest point in your dive and your first mandatory deco stop as indicated by the profile you generated in step 1 (or 3) is GREATER THAN 30 feet, calculate the midpoint between the deepest point in your dive and your first mandatory deco stop as indicated by the profile you generated in step 1 (or 3) and add two minutes to your original profile at the midpoint depth as if it were bottom time. 3. Generate a new decompression profile with the two minutes from step 2A added as bottom time. 4. Return to step 2A. When you eventually exit the algorithm in step 2A, you'll have a decompression profile with deep-stops. Note that deep-stops have not been demonstrated to decrease the incidence of DCS, however deep-stops will almost certainly NOT INCREASE the incidence of DCS. >It seems >on the face of it that on a deep deco stop (70-100 fsw) you would >absorbing more N2 or He into your body in the absolute sense. Well, yes and no. Decompression theory is a very complex subject, and while it does seem, on the face of it, that your body would be absorbing N2/He during the deep-stop portion of a dive, the truth is much more convoluted. To understand decompression theory, it is necessary to understand the concept of tissue compartment half-time. Most theoretical decompression models use mathematical representations of various body tissue-types called compartments. Each type of tissue, and hence each compartment, has assigned to it a different half-time in minutes. The half-time is the amount of time that must pass before the inert gas dissolved in the compartment reaches 50% equilibrium with it's surroundings. If we look at a single compartment with, say, a 10 minute half-time, we observe that 10 minutes after a pressure increase, the compartment is 50% saturated at the new pressure. After another 10 minutes pass, the compartment once again halves the pressure gradient and becomes 75% saturated. After another 10 minutes pass, the compartment is observed to be 87.5% saturated. Each half-time brings the compartment closer to saturation by half the remaining pressure gradient. After six half-times, the compartment is essentially saturated (~98.5%). The same thing happens if the pressure is reduced instead of increased so long as it's not reduced so much that bubbles form. For simplicity, tissues are sometimes referred to as either fast (short half-times) or slow (long half-times). Nerve tissue is considered 'fast'; fat tissue is considered 'slow'. Now, back to your original statement that it seems like doing a deep stop would cause you to absorb rather than offgas N2/He. In fact, both are occurring. Your fast tissues (nerves) are offgasing because, being fast, they quickly saturated during the deep portion of the dive. Since the deep stops are shallower than the depth at which your fast tissues became saturated, they offgas. Your slow tissues (fat) however, are ongasing because, being slow, they absorbed very little N2/He during the deep portion of the dive and are still very close to the saturation level they were at before you jumped in the water. Since your deep-stops are at considerable depth, your slow tissues ongas. Effectively, deeps-stops slow your ascent to prevent sub-clinical DCS from beginning in your saturated fast tissues yet keep your ascent fast enough to prevent your slow tissues from loading up on N2/He. In practice, deep-stops add only a few (~5) minutes of deco time to the typical tech dive. So, you can see that there's a lot more going on than a casual look reveals. In fact, this explanation is grossly simplified in order to keep this post shorter than a Tolstoy novel. :-) -Mike Rodriguez <mikey@ma*.co*> -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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