Posted by CalAbDiver on August 22, 2001 at 09:51:54:

In Reply to: Re: you are right on, MuckDiver posted by muckdiver on August 21, 2001 at 20:43:20:

Bill,

a hypoxic mix would be anything with less than the equivalent of 20% O2 at the surface. That equates to a ppO2 of:

ppO2 = %O2 x P ATA = 0.20 x 1 ATA = 0.2 ATA

therefore at whatever depth you are running calculations for, as long as your ppO2 is 0.2 or higher, your mix is not hypoxic at that depth.

thats actually easy to remember: PO2 > 0.2

any gas mix where the O2 is diluted, as in trimix or heli-air, the mix will be hypoxic until you reach a depth where the ambient pressure times the fraction of O2 in the mix equals or exceeds 0.2 ATA.

for a really thin mix like 10/50/40, with a FO2 of 0.1, you would need to be deeper than 33 fsw before you begin to breathe it. normally this would be the depth to switch over to the bottom mix, so that you save your EAN for deco on the return trip / egression.

TDI gives you a standard worksheet with which you calculate your MOD, PO2, PN2, CNS O2 dose, bottom time, travel time, deco time, bottom mix, travel mix, deco mix, gas volumes, tanks needed, tank size needed, and contingencies.

Normally, the leanest deco mix becomes the travel mix. so your travel mix time and travel mix gas volume comes out of your first deco bottle. TDI starts with standard deco mixes of EAN39 and EAN80.

finally you have to fine tune your dive plan to meet all of your objectives, which is usually a given depth and penetration [of a wreck, not a cave], with appropriate travel and deco calculations. photography and/or salvage are other possibilities, although a lot of photographers use modern rebreathers, and most salvage is done with umbilical hoses to surface supplied gas.

before JeffB gets upset with me for sounding so erudite without having dived any of the 8 deep training dives yet, let me point out that all this is just what the manuals say. i havent begun my deep dives for the course.

i have only just learned the TDI rules and run the calculations. the instructor I am working with required me to read all the manuals and pass all the written tests before we go deep. he has a wife and kids and he doesnt want to die.

before MHK-Mike gets upset with me for adopting such "lax standards" for deep diving and wreck penetrations, let me point out that I have never disputed with him the superiority of DIR principles for cave diving. nor do I directly oppose his personal Valhalla of DIR concepts applied to all open water diving.

- Re: hypoxic mixes
**muckdiver***20:43:52 08/22/01*(0)