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At 05:12 AM 4/20/01 -0500, you wrote: >Is there a real hyperbaric specialist who can back me up ? >I'm bored to read always the same replies for wkpp >guys who are full of themselves and shout louder. I can't back you up but I think I can provide a better understanding of the subject with an explanation: Counter-diffusion problems are very rare unless you're in a steady state situation -- breathing one inert gas while you're being surrounded by another inert gas for a prolonged time. As gas shifts are concerned it's safe to switch to a "slower" gas at any time but not to a "faster" one without increasing the ambient pressure. Thus, breathing air / nitrox after a helium exposure is fine while changing to helium from air at a given ambient pressure may create problems. The key phrase is "at a given ambient pressure". If you're treated on a heliox table the chamber is "moving down" the moment you start breathing helium and all transient supersaturation problems are being cancelled by the higher gas tension allowed at a higher ambient pressure. You can always breathe nitrox on the surface after a helium exposure. If this was a problem you couldn't breathe air on the surface either. The potential for problems arise if you're saturated with a "slow" gas (nitrogen) and start breathing a "fast" gas without increasing the ambient pressure. Why? For some time your body will pick up more "fast" gas than it's eliminating the "slow" gas and the total gas tension will increase. If you're on the verge of bubbling already that surge of "fast" gas moving in may tip the scales. Don't switch to a "faster" gas while you're ascending Always descend while switching / immediately after a switch to a "faster" gas Fast to slow = safe Slow to fast = unsafe unless you're descending Oxygen (or, a higher FO2) is safe and beneficial no matter what you have been breathing -- inert gases only contribute to DCS. Whatever gas you've been breathing you can ALWAYS breathe oxygen and you can ALWAYS be treated in a recompression chamber. From a practical point of view the only problem scenario would be to breathe heliox on the surface just after a "heavy" air dive. That would be a strange use of helium indeed. Hans -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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