victorzamora
Contributor
This is a visual analogy to explain ong-assing and off-gassing procedures. It's not 100% physiologically accurate, but it is close enough for government work. First, the basics:
Take a sink, fill it with water. Poke a hole in the side near the bottom. The more water that is in the sink, the faster it goes through that hole. We will call this the "lung" sink, and it's the only one we can change the level of directly. Now, let's take another sink and butt it right up against the lung sink. We can call this a "tissue" sink. The water represents partial pressure, which, in scuba diving, is influenced by diver depth and composition of our breathing gas. The more nitrogen we have in our tank, the fuller the sink. The deeper we go, the fuller the sink. Water in the tissue sink is nitrogen absorption.
At the surface, there is water in the lung sink that just matches the bottom of the hole. The tissue sink has water in it, too, at that same level. This is "equilibrium" and represents saturation (the maximum amount of nitrogen our "tissues" will absorb while we're at the surface. When we start our dive, the lung sink starts to fill up. As this happens, the tissue sink starts filling faster and faster as we go deeper and deeper. When we start coming back up, the "lung" sink empties out and the tissue sink starts draining water back into the "lung" sink. When the tissue sink starts flowing back into the lung sink, that's off-gassing. The faster we come up, the bigger the difference in water level between the lung sink and the tissue sink and the faster the water flows back into the lung sink.
To take this analogy another step, we need to realize that different tissues absorb nitrogen at different rates. They also don't really interact with each other. The tissue sinks just interact with the lung sink. So now, let's place a whole bunch of different sinks around the lung sink....each with its own hole connecting them to the lung sink. Since they absorb at different rates (fast and slow compartments or tissues, as some people refer to them), the holes have to be different sizes. The fast tissues have big holes while the slow tissues have little holes. This works the same as one tissue sink, but now we have to keep track of a bunch of different sinks.
Well, how does the DeCompression Sickness (DCS, or getting bent) work? What if we just stay at the bottom for a long time until even the slow tissues fill all the way up, and then come up quickly? So now we need to add another step. Let's put a filter in all of the holes and sand in the bottom of all of the tissue sinks but not in the lung sink. Let's pretend sand can't get through the filter at all, so there is never sand in the lung sink. The faster the water flows from the tissue sinks into the lung sink, the more sand is swept into the filter. This actually slows the water down. This sandy filter represents bubble formation in the tissues. The amount of sand in the filter makes a difference. If the filter only gets a little sandy, water will flow through slowly but the sand will eventually settle and fall out of the filter by itself. In our body, these are known as silent bubbles. They are a form of DCS, and are considered getting bent....but theyre often accompanied by minimal side effects. Were beginning to suspect these silent bubbles are more common than previously thought, and checking for silent bubbles (with the help of a medical professional and some equipment) is one way of evaluating the accuracy and validity of certain decompression models. If the filter gets REALLY sandy, then the buildup can cause really bad things to happen. In our bodies, this is known as DCS. For this sand to fall out of the filter, it needs to get blown out. This can be done in a recompression chamber or using in water recompression techniques. I dont want to start a debate on in-water recompression, as its very controversial.....but most can agree that quickly getting to a dry chamber run by a trained team is much safer than in-water recompression.
Well, what if were not diving air? With a nitrox mixture, weve already alluded to the solution. More oxygen means less nitrogen, which means less water in the lung sink at a given depth. This means we can stay deep for longer without as much water flow between tanks. Trimix is another option. Besides narcotic effects, which arent properly discussed with this analogy, helium absorption doesnt really interact with nitrogen absorption. So, we have to think of an identical set of sinks to represent our helium levels. All of the principles and properties transfer over from the nitrogen analogy to the helium analogy, except for one aspect. Helium is much faster at transferring into and out of our tissues. So, the main difference is the viscosity of the two liquids. Compared to helium, nitrogen is a thicker liquid. You can tolerate a greater pressure difference without the filters getting clogged, because the thick nitrogen just wont move as fast as the helium. The helium is like water, it sloshes from one place to another very quickly and efficiently. So, if Helium is like water.....nitrogen can be thought of as very runny honey (perfusion rate of He is about 2.8x that of N2, and the viscosity of honey is about 2-3x that of runny honey). As for deco gas, that makes this analogy harder to stretch.....and should be covered more in a proper deco theory book or class. However, Ill try to add it into this analogy easily. There is a better way of explaining oxygen, but it compromises the simplicity and integrity of the rest of this analogy. The easy way is to say that switching to a deco gas lowers the level in the lung tank but it also makes the sand harder to move.
So, this isnt completely accurate....but its intended as a basic visual for recreational divers to get a better grasp of this. For a better understanding of whats going on, Mark Powells Deco for Divers is typically unquestioned as the go-to source. Also, remember that no amount of reading is a substitute for good training.
---------- Post added November 15th, 2013 at 10:44 AM ----------
The overarching analogy I stole directly from Bob (aka NWGratefulDiver). Adding in Helium was the idea of Kwinters. Adding in the sand/filter was the idea of AlexL. Please, guys, if this neds changing/updating let me know. If you want to modify it or rewrite it, I'll be happy to make it that, instead. If there was something I missed or glossed over too quickly, let me know that as well. I'm trying to make this really worth going to for new divers.
Thoughts? Comments? Questions?
Take a sink, fill it with water. Poke a hole in the side near the bottom. The more water that is in the sink, the faster it goes through that hole. We will call this the "lung" sink, and it's the only one we can change the level of directly. Now, let's take another sink and butt it right up against the lung sink. We can call this a "tissue" sink. The water represents partial pressure, which, in scuba diving, is influenced by diver depth and composition of our breathing gas. The more nitrogen we have in our tank, the fuller the sink. The deeper we go, the fuller the sink. Water in the tissue sink is nitrogen absorption.
At the surface, there is water in the lung sink that just matches the bottom of the hole. The tissue sink has water in it, too, at that same level. This is "equilibrium" and represents saturation (the maximum amount of nitrogen our "tissues" will absorb while we're at the surface. When we start our dive, the lung sink starts to fill up. As this happens, the tissue sink starts filling faster and faster as we go deeper and deeper. When we start coming back up, the "lung" sink empties out and the tissue sink starts draining water back into the "lung" sink. When the tissue sink starts flowing back into the lung sink, that's off-gassing. The faster we come up, the bigger the difference in water level between the lung sink and the tissue sink and the faster the water flows back into the lung sink.
To take this analogy another step, we need to realize that different tissues absorb nitrogen at different rates. They also don't really interact with each other. The tissue sinks just interact with the lung sink. So now, let's place a whole bunch of different sinks around the lung sink....each with its own hole connecting them to the lung sink. Since they absorb at different rates (fast and slow compartments or tissues, as some people refer to them), the holes have to be different sizes. The fast tissues have big holes while the slow tissues have little holes. This works the same as one tissue sink, but now we have to keep track of a bunch of different sinks.
Well, how does the DeCompression Sickness (DCS, or getting bent) work? What if we just stay at the bottom for a long time until even the slow tissues fill all the way up, and then come up quickly? So now we need to add another step. Let's put a filter in all of the holes and sand in the bottom of all of the tissue sinks but not in the lung sink. Let's pretend sand can't get through the filter at all, so there is never sand in the lung sink. The faster the water flows from the tissue sinks into the lung sink, the more sand is swept into the filter. This actually slows the water down. This sandy filter represents bubble formation in the tissues. The amount of sand in the filter makes a difference. If the filter only gets a little sandy, water will flow through slowly but the sand will eventually settle and fall out of the filter by itself. In our body, these are known as silent bubbles. They are a form of DCS, and are considered getting bent....but theyre often accompanied by minimal side effects. Were beginning to suspect these silent bubbles are more common than previously thought, and checking for silent bubbles (with the help of a medical professional and some equipment) is one way of evaluating the accuracy and validity of certain decompression models. If the filter gets REALLY sandy, then the buildup can cause really bad things to happen. In our bodies, this is known as DCS. For this sand to fall out of the filter, it needs to get blown out. This can be done in a recompression chamber or using in water recompression techniques. I dont want to start a debate on in-water recompression, as its very controversial.....but most can agree that quickly getting to a dry chamber run by a trained team is much safer than in-water recompression.
Well, what if were not diving air? With a nitrox mixture, weve already alluded to the solution. More oxygen means less nitrogen, which means less water in the lung sink at a given depth. This means we can stay deep for longer without as much water flow between tanks. Trimix is another option. Besides narcotic effects, which arent properly discussed with this analogy, helium absorption doesnt really interact with nitrogen absorption. So, we have to think of an identical set of sinks to represent our helium levels. All of the principles and properties transfer over from the nitrogen analogy to the helium analogy, except for one aspect. Helium is much faster at transferring into and out of our tissues. So, the main difference is the viscosity of the two liquids. Compared to helium, nitrogen is a thicker liquid. You can tolerate a greater pressure difference without the filters getting clogged, because the thick nitrogen just wont move as fast as the helium. The helium is like water, it sloshes from one place to another very quickly and efficiently. So, if Helium is like water.....nitrogen can be thought of as very runny honey (perfusion rate of He is about 2.8x that of N2, and the viscosity of honey is about 2-3x that of runny honey). As for deco gas, that makes this analogy harder to stretch.....and should be covered more in a proper deco theory book or class. However, Ill try to add it into this analogy easily. There is a better way of explaining oxygen, but it compromises the simplicity and integrity of the rest of this analogy. The easy way is to say that switching to a deco gas lowers the level in the lung tank but it also makes the sand harder to move.
So, this isnt completely accurate....but its intended as a basic visual for recreational divers to get a better grasp of this. For a better understanding of whats going on, Mark Powells Deco for Divers is typically unquestioned as the go-to source. Also, remember that no amount of reading is a substitute for good training.
---------- Post added November 15th, 2013 at 10:44 AM ----------
The overarching analogy I stole directly from Bob (aka NWGratefulDiver). Adding in Helium was the idea of Kwinters. Adding in the sand/filter was the idea of AlexL. Please, guys, if this neds changing/updating let me know. If you want to modify it or rewrite it, I'll be happy to make it that, instead. If there was something I missed or glossed over too quickly, let me know that as well. I'm trying to make this really worth going to for new divers.
Thoughts? Comments? Questions?
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