decompression

[DRE]

"Side note- who the heck wants to do cold water deco :o."

Dive the Kamloops - 260ft to the sand at the bow. 20 minute bottom time, total dive time 70 minutes, using EAN50 and O2 for deco.


"Anyway, I would think useing EAN 80 would be advantagous because you could start much earlier ie at 30 ft instead of 20 with 100% O2. Furthermore wouldn't your allowable exposure be lower with 80%?"

First of all, at 30ft your PPO2 of EAN80 is 1.53, O2 @ 20 is 1.6 - already there is a benefit for using O2 @ 20. Second, you don't stay @ 30, but most people will go to 20 and then 10, which will lower the PPo2 even more, thus reducing the deco effectiveness even more. Third, EAN80 still has N2 in it, which means that the pressure differential between inspired and arterial gas in't optimal. An analogy: you want to cut bourbon with soda: what's more optimal: to cut it with pure soda or a mixture of soda and bourbon; what will give you the better result?
Fourth, allowable exposure should be thrown out of the window when using correct gasses. Most people advocate the use of fairly high PPO2's for bottom gas (TDI recommends 1.4ATA). This will do much more damage to your CNS clock than using high PPO2 deco gasses, which is where you really need them. Using a backgas with low PPO2, as in the 1.0-1.2 range, will give you the possibility to switch back and forth between your deco gas and backgas during deco, thus constantly resetting the CNS clock (actually you reset your lungs - the CNS clock is a concept that dates back to the 1960's and was so poorly understood that it led to ultraconservatism, kinda like modernday dive computers)