At 100 fsw we are nitrogen loading. At 15 fsw we are off gassing nitrogen. My question - what is the depth of equilibium where we are neither absorbing or off gassing, and are there variable factors that have relevance. Can this depth be incorporated for planning in extended diving i.e. liveaboards where doing 5 tanks in a day isn't unheard of.-Thanks
When we first descend we are ongassing from the time we leave the surface until the time we reach our maximum depth. Once we begin our ascent, some of our tissues will start offgassing while others will continue ongassing. The variable factors will be the tissue type (although mathematical models will divide these into "compartments" there are actually a more complex set of relationships than the compartments will accommodate), depth, amount of time spent at that depth, and the makeup of the gas you are breathing.
For my classes I use the following analogy to explain the ongassing/offgassing process ...
Think of decompression as a series of sinks ... all of which are connected to a common trough. Each sink represents a type of tissue in your body. The trough represents your lungs. Each sink has a hole in it that allows water to drain between it and the trough. Since different tissues ongas and offgas at different rates, each sink has a different size hole that represents the rate at which that particular tissue ongasses and offgasses ... slower tissues have smaller holes, while larger ones have bigger holes.
The trough is filled with water. That represents taking a breath. Each sink will receive water from the trough at the rate that it's able to pass through the hole. When you are on the surface you are at equilibrium ... all of the sinks are at the same level as the trough.
When you begin a dive, you effectively put more water in the trough. Since the water level is higher in the trough than in the sinks, each sink will begin to fill with more water through the hole ... each at a rate proportional to the size of the hole. As you go deeper, more water goes into the trough. The greater the difference between the water level in the sink and trough, the more "pressure" is applied to the water flowing through the hole.
At any given point, the water level in all the sinks will vary because some will be filling up faster than others. The sinks with the bigger holes ... which represent faster tissues ... will fill up faster than those with smaller holes.
At some point you will begin to come up from the deepest part of your dive. When this happens, the water level in the trough goes down ... which represents what happens in your lungs when the pressure of the gas you are breathing goes down. When it reaches a point where the level in the trough is less than the level in a sink, water will begin to flow from that sink into trough. That represents "offgassing". But because the sinks are all at different levels, some will start to "offgas" sooner than others. By the same token, during an ascent your faster tissues (mostly your circulatory and nervous systems) will begin to offgas while your slower tissues (muscular and skeletal systems) will continue ongassing ... because the level of their "sinks" are still lower than that of the trough. Each sink will only start to flow back into the trough when its level exceeds the level of the trough.
So as you can see, since there are literally dozens, if not hundreds of "sinks" in your body, the point at which each will begin to offgas depends on the rate at which it ongassed during the descent part of the dive, as well as the amount of time you spent at a given depth.
There are additional factors which impact this process ... which vary from person to person and even from day to day. These include such things as hydration, blood pressure, overall health, personal physiology (i.e. fat to muscle ratios), stress, fatigue, effectiveness of your exposure equipment, and host of other variables.
Which is why decompression is more an art than a science ... there are simply too many variables to be able to predict its effect with anything resembling accuracy. So we use mathematical models based on "risk factors" and an idealized body, add some safety factors, and come up with a decompression algorithm that works most of the time.
So there really is no hard answer to your question ... my best advice for a 5-dive a day liveaboard is to use nitrox (if it's an option), limit the time you spend deep, spend a longer than normal time in the shallows on each dive, drink lots of water or other hydrating fluids, limit your alcohol and caffeine intake, and get a good night's sleep between those 5-dive days.
... Bob (Grateful Diver)