Nitrox - Trimix

[Vtach]

Quite a bit of this information has to do with what is termed as "Isobaric Counter Diffusion" or ICD.  From my understanding, on a cellular level, it means that 2 dissimilar inert gases will move in opposing directions at an equal pressure (one going in and the other out).  It seems to be generally explained that mechanism for this movement is across a gradient from high concentration to low concentration (going back to the semi-permeable membrane stuff).  How rapid the movement across the gradient seems to be the unanswered question.  Some folks say it is based on diffusability and others say solubility.  Either way, ICD can work against you or work for you depending on how strategic you are with gas management.  From a diffusability perspective, if you are breathing a "light/rapid" gas at depth (Helium) and switch to a "heavy/slow" gas (nitrogen/air), you will only help to accelerate the removal of the helium.  However if you are breathing a "heavy/slow" gas at depth and switch to a "light/fast" gas you run a significant risk of becoming bent due to the "light" gas quickly saturating the tissue and pushing out the "heavy" and causing bubble formation.  Argon is a "heavy/slow" diffusing inert gas and by using it for suit inflation you help to reduce the risk of a "transdermal" counter diffusion when breathing/circulating a "light/fast" breathing mixture along with the benefit of thermal retention due to gas density and high insulating properties.

That's my synopsis of ICD as I understand it.  If anyone else out there can add or strike me down as a blasphemer, have at it ;D