Saturation and desaturation: Basics
During each dive, the body absorbs inert gases, primarily nitrogen. Inert gases are those gases that are not metabolized in the body, but are simply there. They contrast, for example, with oxygen, which is used by the body and converted into carbon dioxide (CO2).
This chapter is about the basics of how inert gases get in and out of the body, where the limits are (M-values), and how to make a model from this that can be used to calculate decompression.
Gases and liquids
How do gases get into liquids?
To understand what is going on in the body during diving, the uptake and release of nitrogen and other gases is fundamental. Gases and liquids that are adjacent to each other are visibly clearly separated from each other – but there is always an exchange between them. Gases dissolve in the liquid, or escape again into the gaseous phase. This happens even continuously, but as long as the pressures of the individual gases in the liquid and in the gas phase are in equilibrium, the same number of atoms or molecules pass over in both directions per time. In net terms, therefore, there is no change in the concentration. However, the situation is different if there is a pressure difference. Then there will always be an exchange between gas and liquid in the direction of the lower pressure, until the pressure of each gas is the same on both sides. This pressure equalization can be described by two physical principles:
“Principle of Le Chatelier”: It is in nature to avoid constraints. When the pressure of a gas on one side increases, the gas will move to the side of lower pressure.
At the end of this process, a state is reached which we know in diving as “Henry’s law” : The concentration of a gas dissolved in a liquid is proportional to the partial pressure of the gas above the liquid – so this is the state after pressure equalization has taken place.
If several gases are involved, Dalton joins the game: Each individual gas in the mixture exerts a part of the total pressure, the partial pressure. A high proportion of a gas naturally means that more of it will dissolve.
Tissue saturation
Saturation curves
The greater the pressure difference, the faster gases pass from the air into the bloodstream and also from a tissue with higher to one with lower pressure.
The pressure equalization thus takes place quite quickly at the beginning, then more and more slowly. This results in a slowly flattening curve.
What is a half-life?
The saturation curve shows on the vertical axis (y) the saturation in %, on the longitudinal axis (x) the elapsed time. Each tissue reaches 50% saturation after a certain time: This time is a half-life. The term may be more familiar to some in the context of radioactivity than diving. There, a half-life means the period of time in which a radioactive metal has lost half of its activity. Such radioactive half-lives are of quite different lengths: from fractions of a second to hours, days (in the case of iodine-131, notorious from tragic reactor accidents) to billions of years, as in the case of naturally occurring uranium-238 and the waste from nuclear energy.
In diving, on the other hand, we fortunately don’t have to deal with such extreme times. For us, periods of minutes to hours are decisive. So back to our diagram: if you follow the curve further, you can see that after another half-life, 75% of complete saturation has been reached. Sure – the tissue halved the remaining pressure difference just as quickly as the first time.
After 6 half-lives we arrive at over 98%, for our purposes we can then assume complete saturation.
Desaturation
Simply explained: Diagram
Desaturation takes place accordingly. When the pressure on the gas side decreases, gases escape from the liquid again. This also happens very quickly at the beginning, then more slowly – but this process has a few special pitfalls, as we will see further on.
Since it is more than complicated to have all model tissues (compartments) in view at once, we will first take a look at the saturation and desaturation of a single model tissue. Helpful for this is this basic graph, where the ambient pressure is plotted on the X-axis, and the inert gas pressure in the model fabric is plotted on the Y-axis. If the ambient pressure is higher than the gas pressure in the model tissue, the model tissue saturates up – if the ambient pressure is lower, it desaturates. The dashed middle line shows the moment when the pressure equalization has completely taken place.
We spend a dive at a certain depth, saturate a model tissue to a certain point – and then, of course, want to end the dive. In doing so, we will inevitably have tissue saturation above ambient pressure at some point during the ascent. Of course, only then can you desaturate.
The relevant question, in fact the only really important question, is now: how far can you reduce the ambient pressure without the nitrogen (or any other inert gas in mixed gas diving) causing problems in the body? This is exactly what the important topic of M-values is all about.
M values
The limits of oversaturation
Unfortunately, gases do not always diffuse out of the body unimpeded, but can form bubbles if the pressure change is too abrupt, and if some unfavorable factors combine. A certain amount of these bubbles is usually tolerated by humans. However, many and large bubbles in the body can also cause a variety of problems, precisely that decompression sickness (DCS).
It is impossible to predict exactly when this will happen. But one can determine values at which the statistical risk of falling ill is low – very roughly summarized, this is exactly the way many decompression models have come about. Statistically determined risks are used to set limits up to which the risk is considered to be “acceptable.” Acceptable is a level of risk accepted by the diver:s following the model, and where exactly that lies is a matter of agreement – risk is never zero, and the outcome for a specific person on a specific dive is unpredictable.
THIS risk is what is meant by the M-value: from here on, your risk of getting DCS with clear symptoms is higher than accepted by the modelers – and the risk grows more and more from here on.
Where exactly do these limits lie? How are they even known? And why is the search for limits like poking in the fog?
Thinking of the process of surfacing in our diagram, we have in the left triangle the area that we have to dive through as we ascend: As the ambient pressure drops, at some point the pressure in the tissue is above the ambient pressure
We are faced with the question of exactly how far we want to go with our current tissue saturation every time we ascent. The fact that overpressure inevitably occurs in various tissues cannot be avoided. The higher this overpressure, the greater the risk of DCS – at the same time, it is necessary if you want to surface.
