Minimum O2 requirements

[TwoBitTxn]

My physiology is more than a little bit rusty so I'm asking this question.
Regular air is approx 21% Ox
By firefighter and OSHA standards anything below 19% requires SCBA
If I remember correctly ( I probably don't ) what we exhale is 12% and can support life for a very short period.

What is the minimum O2 requirement for mixing a breathing gas? I have seen a couple of reports now that appeared the diver was breathing something around 15%. How long can a diver handle percentages that low?

Tom

 

[BILLB]

Gas laws apply! Partial pressure of oxygen will be equivalent to 30% oxygen if the depth you dive doubles the ambient sea level atmospheric pressure. In this means that in seawater the pressure is doubled around 33 feet.

In order to breath normally the air delivered to your lungs (on demand) is the same as the surrounding water pressure. This is basic SCUBA in a nutshell.

 

[TwoBitTxn]

I understand that every 33 ft you increase by one ATM. I havn't taken a Nitrox course yet, but I can imagine this is covered fairly extensively.

So essentially the deeper you go, due to partial pressure, the lower the percentages of oxygen is requred to sustain life?

Tom

 

[BILLB]

First of all, we all need that 21% to be really comfortable. When diving the partial pressures of oxygen increases but our bodies do not use all the excess oxygen. It is simply wasted.

The divers mixing 15% in their breathing mixes do this to avoid oxygen toxicity. At a partial pressure of 1.6, most of us may overdose and convulse. Not good when under water. So, the 15% oxygen mix allows a diver to dive deeper than if a 21% air mix was used. Air becomes toxic around 220 feet.

 

[Uncle Pug]

Tom,
Think of it this way...
What you need is not a certain percent of O2 but a percent of 1 ATM....
Specifically .21 ATM .....

Diving increases your ATA and so...

At 33' you are getting .42 ATM if you are breathing air....
At 66' you are getting .63 ATM... an so on....

So you can decrease the % of O2 in the inspired gas if it is going to be used at depth an still have your .21 ATM....

Lets say you are breathing a mix with only 10.5% O2 at 33fsw...
You would be getting the .21 ATM that your body needs...
But if you were to breath such a mix at a shallower depth you would obviously not be getting enough O2....

I hope this doesn't just add more confusion!!!

BTW expired air contains .17 ATM O2 (17% at the surface) and is still good enough for CPR in a compromised patient... or at least in the small percentage of cases that end in a successful save.

 

[TwoBitTxn]

Thanks Pug,

I understood that. It made sense to me.

Tom

 

[Stone]

Originally posted by BILLB
The divers mixing 15% in their breathing mixes do this to avoid oxygen toxicity. At a partial pressure of 1.6, most of us may overdose and convulse. Not good when under water. So, the 15% oxygen mix allows a diver to dive deeper than if a 21% air mix was used. Air becomes toxic around 220 feet.

I would venture to say none of us would overdose and convulse at PPO2 1.6 under most circumstances. According to the "NOAA PO2 and Exposure Time Limits for Working Divers" at a PO2 of 1.6 a diver can work for 45 minutes during a single exposure, and can work for a total of 150 minutes during a 24 hour period. Since exposure time is limited even at 1 ATA, the time restriction may involve the need to avoid lung damage, not just CNS Toxicity.

The Table looks something like this:

PO2............Single Exposure in Minutes................Total Duration
1.6..............................45..........................................150
1.5............................120..........................................180
1.4............................150..........................................180
1.3............................180..........................................210
1.2............................210..........................................240
1.1............................240..........................................270
1...............................300..........................................300

 

[Rick Murchison]

Originally posted by Tom Vyles
My physiology is more than a little bit rusty so I'm asking this question.
Regular air is approx 21% Ox
By firefighter and OSHA standards anything below 19% requires SCBA
If I remember correctly ( I probably don't ) what we exhale is 12% and can support life for a very short period.

What is the minimum O2 requirement for mixing a breathing gas? I have seen a couple of reports now that appeared the diver was breathing something around 15%. How long can a diver handle percentages that low?

Tom

The question is the minimum... and that, for a healthy human in reasonably good shape who lives pretty close to sea level is about .16 ATM, equivalent to what you get at about 8,000 feet above sea level, the same as the cabin altitude of a commercial airliner. 10,000' is the cockpit altitude above which pilots are required to start breathing supplimental oxygen. Most folks can't do heavy exercise at that level without a little acclimation, but it's "ok."
However, I think most training agencies don't like to use anything less than "normoxic" until at a depth that gives the diver .21 ATM oxygen, equivalent to sea level. Since .16 (16% oxygen at sea level) isn't a big deal, and since you reach .21 at 10' going down, I personally have no problem with starting that gas at the surface going down.
Coming back up is a different discussion entirely.
Rick

 

[Rick Murchison]

At rest, exhaled air usually runs about 16-18% oxygen (the 17% Uncle Pug already mentioned). On a breath hold dive for most of us, we can get that down to about 12% (may be where that number came from - you can use your Nitrox oxygen sensor to check your own). I don't know how low "real" freedivers can get theirs, but I'll bet it's impressive.
Rick