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To begin I would like to thank Frank Deutschmann for his excellent posting on 
the development of decompression tables and also ask if he has any suggestions 
for further reading on the subject.  

Secondly I would like to comment on the formation of bends bubbles from the 
point of view of some one who has studied at least a little chemistry.   In 
the course of the very lengthy discussion of the subject I have seen only one 
reference to the problem of nucleation, and this is of primary importance if 
we are ever to gain a real understanding or DCI.  

We learn from physical chemistry that if a bubble of gas is immersed in a 
liquid, because of surface tension the pressure inside the bubble is greater 
than the pressure of the liquid.  Furthermore  the pressure difference is 
inversely proportional to the diameter of the bubble.  Therefore as the size 
of the bubble approaches zero, the pressure difference and thus the pressure 
inside the bubble increases without limit.  For this reason the gas in small 
bubbles will tend to re-dissolve in the liquid, that is the small bubbles are 
thermodynamically unstable unless there is a high degree of supersaturation of 
the gas in the liquid.  This is a fortunate situation; it is why Haldanes 
number is 1.58 and not just 1; without surface tension our "no decompression 
limits" would be drastically reduced.  But now we have another problem.  Why 
is the number 1.58?  Why isnt it infinite?  How is it possible that the 
bubbles ever form?  Clearly we need some special mechanism for the nucleation 
of the bubbles, something that goes beyond the simple surface tension 
explanation.

The problem is not unique to the explanation of DCI; it applies to the 
formation of any bubbles in any liquid; it even applies to boiling water.  
When we boil water, however, it is possible to watch and see what the special 
mechanism is.  As the water begins to get warmer, bubbles of air (actually 
NITROX) appear on the bottom and sides of the pot.  When the boiling point is 
reached bubbles of water vapor form on the bottom of the pot.  Watch closely 
and you will see that they tend to form at specific spots.  These are places 
where some impurity (usually a minute air bubble adhering to some other 
impurity) act to nucleate the bubbles of water vapor.  The nucleation is 
caused by impurities on the inner surfaces of the pot.    

If we are to understand the formation of inert gas bubbles in the body tissues 
it is necessary to understand the special nucleation mechanism, and at the 
present time, to the best of my knowledge, no one has any idea what it is.  
Until we understand nucleation, decompression tables will have to be 
empirical, and it will be impossible to give a definitive answer to questions 
like, "Could the ultrasonic waves used to detect bubbles actually help to 
nucleate them?"  However "bubbles too small to detect" would be in the region 
of extreme thermodynamic instability, and it is extremely doubtful that they 
could exist.  

In closing I shall say that I doubt that DCI is chaotic, just incompletely 
understood.  The weather, on the other hand, is chaotic in the mathematical 
sense of "extreme sensitivity to initial conditions".  


Bruce Gerhard

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