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Brendan, Argon has been used mostly experimentally (and may be still?) as a diluent during decompression. However it is highly soluble and is very narcotic compared to nitrogen and helium. The premise for considering argon was due to its slower diffusion rate into tissue compared to nitrogen and helium. One current decompression model (Buhlmann) suggests the diffusion of an inert gas into tissue is inversely related to the square root of the molecular weight of the gas. For example if you compare this ratio for helium with the ratio for nitrogen you would find the helium ratio is 2.65 times the nitrogen ratio. Similarly if you compare nitrogen to argon, you would find the nitrogen ratio is about 1.19 times the argon ratio. What does this mean? For example, if you look at the 4th compartment (tissue group)of the Buhlmann ZH-L12, 12 compartment decompression model you will find the "half-time" for helium to be 7 minutes and the half-time for nitrogen to be 18.5 minutes. The helium time is 2.65 times faster than the nitrogen time. Amazing! You could extrapolate this information and conclude the half-time for argon for this compartment would be 22 minutes. According to the Buhlmann model the increase in the compartment gas tension (pressure) is a function of the ambient pressure,the time of exposure, and the half-time for the gas being considered. Thus for an equal time of exposure and ambient pressure the gas with the highest half-time would on-gas slower, and off-gas slower. This of course only considers the exponential-exponential hypothesis (exponential on-gas and exponential off-gas). Buhlmann simplified his theory by suggesting the total inert gas pressure, the sum of the inert gas partial pressures in the tissue, determines the tolerated ambient pressure during decompression. This sum of the inert gas pressures is subtracted and multipled respectively by Buhlmann's "a & b" coefficients to derive the tolerated ambient pressure (similar to "M" values in the Haldane based models). The tolerated ambient pressure is the decompression ceiling; if you ascend above the ceiling, decompression sickness may ensue. Stopping at the ceiling (decompression stop) will allow more gas to off-gas and the ceiling (tolerated ambient pressure) will move shallower until you can exit the water. Since argon diffuses into tissue slower, the replacement of nitrogen/helium with argon during decompression would create a situation where the argon is on-gasing slower than the nitrogen and/or helium is off-gasing. This mechanism is referred to as counter-diffusion. This can occur to the degree where the total inert gas pressure in the tissue is less than the surrounding ambient pressure (fraction of inert gas in breathing mixture times the ambient pressure). This process works to shorten the decompression time. The same situation exists during gas switching from high helium content bottom mixes to air. Your decompression obligation can be significantly reduced by switching to air as deep as safely practical because the helium (2.65 times faster than nitrogen) is off-gasing rapidly while nitrogen is on-gasing more slowly. Digressing a bit, its should be noted the reverse process of switching from a heavier inert gas to a lighter inert gas (e.g. nitrogen to helium)can create a "super-saturated" condition, where the tissue inert gas pressure is greater than the ambient inert gas pressure, even with no change in depth from the switch! This is the basis of lengthy and often misguided duscussions on isobaric inert gas counterdiffusion. To cut to the chase, using argon may shorten your decompression somewhat; however the amount of reduction may not be considered significant due to the relatively small difference in diffusion half-times between argon and nitrogen. Other considerations such as its high solubilities in fats and aqueous fluids may imply additional concerns which have not been brought to light, with risks that may not be warranted considering the degree of benefit. Doug Chapman P.S. The above is of course my interpretation and opinion and does not imply anything proven or not. (This is a disclaimer!)
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