Re: [PSUBS-MAILIST] New means having alot to learn.

["Sean T. Stevenson" <sts@telus.net>]

A tank of anything at 1ATM will not deliver gas at 160 feet, since the ambient pressure is 5.85 ATA.  The tank must be pressurized at least that high to have any useful contents whatsoever.

I think what you are getting at is physiological oxygen requirements.  The human body can tolerate oxygen partial pressures from about 0.16 ATA to 1.6 ATA (these figures are specific to a diver in the water.  A person in a gas space will typically 
tolerate a higher maximum PPO2, since the pressure differential across the lungs no longer exists).

To deliver gas to a diver, it must be at a greater pressure than the ambient pressure.  Each 33 feet of seawater is roughly equivalent to one additional atmosphere of pressure.  A person can breathe heliox 18 at the surface (18% oxygen, rest helium).  
This is above the absolute minimum PPO2 of 0.16 ATA.  At a depth of 33 feet, the PPO2 of this gas would be 0.36 ATA, but the gas would need to be delivered to the diver's lungs at twice the density, since the ambient pressure is now 2 ATA.  The 
gas is consumed twice as fast at 33 feet.  You don't get something for nothing.  If you wanted to consume the same amount of oxygen at depth as at the surface, you would need to use heliox 9.

Clear as mud?

-Sean


On Tue, 11 Dec 2001 09:04:33 -0700, Coalbunny wrote:

>So actually, a tank holding 10 cubic feet of O2 at 1ATM lasts longer (in
>theory) at 160 feet?
>Carl
>
>
>Paul B wrote:
>> 
>> Hi, Carl
>> 
>> >Why is it breathable only below those depths?
>> >Carl
>> 
>> Because 3% of oxygen at normal atmosphere pressure is too little for humans
>> to breathe. As you go deeper, the pressure rises, so partial pressure
>> (pressure, created by one single gas in a gas mixture) of oxygen rises, and
>> the mixture becomes breathable.
>> 
>> Let's see if I can explain it by numbers:
>> 
>> For example, air consists of 78% nitrogen, 21% oxygen, and some odd percent
>> argon, helium, etc.
>> 
>> O2 21% @ 1 atm = 0.21 atm partial pressure (that's O2 in regular air,
>> breatheable)
>> 
>> In hydrogen/oxygen mixture:
>> 
>> O2 3% @ 1 atm = 0.03 atm partial pressure (which is not enough to breathe)
>> O2 3% @ 6 atm (approx. pressure at 160 feet) ~ 0.18 atm O2 partial pressure
>> (that's enough to breathe)
>> 
>> See, you have more oxygen in the mixture at this depth, so you can breathe.
>> 
>> Sincerely, Paul.
>> 
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>
>-- 
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