This is for my diving friends out there, based on something I recently posted on ScubaBoard. Ill call it The Decompression Sink ... its an analogy I like to use at the OW and AOW level when Im talking to students about what goes on in their body as they dive. Imaging yourself as two sinks ... well call one the lung sink and another the body sink. These two sinks share a common wall, and at the bottom of the wall is a hole. When we breathe in its like putting water in the lung sink. As we descend deeper and the air were breathing compresses, the amount of water were putting into the lung sink increases. The hole in the bottom of the wall lets some water flow into the body sink, and it continues to flow until the two sinks reach the same level. This is a very simplified model of what occurs to us as we breathe pressurized gas during a dive ... and the deeper we go, the more water gets poured into the lung sink and therefore the higher the level in the body sink in order to reach equalization. But the body is made up of many different types of tissues, each with its own ability to absorb gas from the lungs, and those occur at different rates. So lets take that body sink and break it up into a bunch of sinks ... each connected to the lung sink by its own wall, and each with its own hole. The ability of any given tissue to absorb gas at a given rate is represented by the size of the hole connecting it to the lung sink ... the tissues that absorb gas the slowest having the smallest hole and those that absorb gas the fastest having the largest. Now when water gets poured into the lung sink, these tissue sinks will equalize at different rates, depending on the size of the hole. This is essentially what your dive computer is doing when it calculates how much time you have to remain at a given depth ... your so-called no-decompression limit. Now consider what happens when you stop pouring water into the lung sink ... basically the part of the dive where youre not changing depth and youre allowing the sinks to equalize. Some sinks will equalize quite quickly, while others will take much longer. At some point youre going to begin your ascent ... and if youve remained within your no-decompression limit the sinks will all be at different levels. Some may be quite close to being equalized, while others have barely begun to do so. As we ascend, reducing the pressure of the gas were breathing, think of it as though were taking water out of the lung sink. At some point the level of water in the lung sink will get to be lower than some of the tissue sinks ... specifically those that had the biggest holes and therefore filled up the fastest. When this occurs, the water begins to flow back from those tissue sinks into the lung sink. This is what we call offgassing. And for a time a more complex transaction will occur as some of those tissue sinks start to drain while others ... because theyre still at a lower level than the lung sink ... will continue to fill up. This is analogous to what happens during our ascent as our body starts to eliminate pressurized gas from our bodys tissues ... which we call offgassing. Once again you can envision that the closer we get to the surface, the faster well be taking water out of the lung sink, and therefore the greater the differential will become between the level in the lung sink and that in the fastest of the tissue sinks. Now, our bodies are designed to eliminate excessive gas through our lungs ... but only within certain limitations. As long as we remain within those limits well be OK. But when we exceed our bodys ability to remove excessive gas safely we run the risk of decompression sickness. In the context of our analogy, the respective levels in the sinks needs to remain within certain limits of each other ... because the greater the difference in levels between them the faster the water wants to flow through the hole, and when we exceed a certain flow rate we run the risk of bad things happening to our body. We maintain those limits by making a slow ascent ... limiting the amount of water were removing from the lung sink at any given time, and allowing the safe flow of water between the lung sink and the tissue sinks. This is why a slow ascent is important. At some point we make what we all know as a safety stop. The purpose of the safety stop is to allow those tissue sinks that have developed the greatest difference with the lung sink a chance to catch up a bit, slowing down the flow rate through the hole before we continue the ascent and therefore make that rate speed up some more. On certain dives, we might make what we know as a deep stop to allow this to happen. One thing to know about a deep stop is that while the tissue sinks with the largest holes will be offgassing, those with the smallest holes will still be flowing the other way ... ongassing. For this reason, these stops should be relatively short in duration ... a minute or so for recreational dives within no-decompression limits. Its also important to understand that the closer to the surface we get, the greater the differential between the sinks becomes, as the difference in pressure means (in our analogy) that the lung sink is being emptied faster and faster as we get closer to the surface. Therefore the most important time to ascend slowly is the final ascent ... AFTER the safety stop is over. Remember, completing your safety stop does not end the dive. The dive isnt over until you reach the surface. So dont neglect that last, important part of the dive ... your final ascent from safety stop depth should take at least 30 seconds ... longer if youve been diving aggressively and came close to reaching your no-decompression limit. This analogy neglects a number of important transactions that occur in your body while youre diving that are caused by the effects of ambient pressure, and by the pressure of the air youre breathing. It neglects consideration of the type of gas youre breathing, and how your body handles different types of gas once it gets absorbed into your body. But it serves well as a simplified model to help newer divers, in particular, understand the mechanics of pressure on your body, as well as why a slow, regulated ascent with at least a safety stop (if not a deep stop) are important. For those of you out there who teach, feel free to use it as a part of your curriculum. For those of you out there who understand these processes better than I, feel free to chime in with any points of contention or clarification that can help the reader ... especially the newer diver ... comprehend how it works and why it matters. ... Bob
Posted on: Fri, 15 Nov 2013 15:59:11 +0000
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