Fraction of O2 vs PPO2

[jmsdiver]

Curious:

Explain or direct me to resource(s) explaining the concept of Fraction of Oxygen in the Breathing Loop versus PPO2 in the loop.

I know that Semi Closed rebreathers use teh fraction of oxygen theory and Closed rebreathers use PPO2 as the measure to inject O2 or not to inject O2.

What is the different and should it matter to a diver?

Furthermore, how does this affect the survival rate of divers on rebreathers?

Tanks eace:

 

[tekdiver1]

Ok dude here goes,

Fraction of oxygen referes to the amount or oxygen that you are brearthing. For example, breathing air has 21% 02 or .21 02.

PP02 and P02 are the same thing. This stands for partial pressure of oxygen. What this means is:

"what's my Partial Pressure of oxygen at a given depth for the percent of 02 I'm breathing?"

First we convert depth in feet to ATA. That is done by taking:

((Depth)/(33)+1)=ATA (atmospheres absolute)

If you are going to (200FSW) and you are breathing 21% oxygen then your PP02 is about 1.48.

((200)/(33)+1)=7.06 ATA now take your answer

and plug it in to this formula

ATA * O2% = PP02

(7.06)*(.21) = 1.48xxxxxxxxxxxxxx

The general rule is a PP02 of 1.6 is absolute MAX. A safe PPO2 is about 1.2 - 1.3.

To learn more goto "http://www.iantd.com"

They are a tek training group.

 

[padiscubapro]

Divers of SCR rebreathers maintain a constant FO2 which means they must choose their mix for the deepest they plan to dive, Most not all SCR rebreathers have a practical working limit of 130 fsw because of the limited assortment of flow rates available to them and as they ascent the PO2 (FO2 * pressure) drops rapidly especoally near the service and there is a chance of Hypoxia.

CCR rebreathers work on the idea of a constant PO2, you chose a setpoint say .7 at the start of the dive (the best you could hope for would be a 1.00 at the surface (100%) oxygen), then begin your decent, as you are decending your PO2 is climbing but is prevented from going to high by a controlled descent and addition of diluent (air for dives 50m or shallower, He mixes for deeper), upon reaching the bottom, or by 70ish feet you change your setpoint to your bottom setpoint(you do this otherwise you are breathing a mix that would have a higher nitrogen mix than air!) of 1.3-1.5, and regardless of depth you will maintain this point, if you go deeper you will get a reduced FO2 in the loop as you ascend it gets richer. The rebreather always maintains the OPTIMUM mixture in the loop.

To put this in terms of what the usefulness is, say I did a dive to 100 fsw for 30 minutes (this would be the NDL using 36% nitox), as the CCR diver ascends the mix gets richer and you start to off gas, depeding on your setpoint you would have between 46% and 50% O2 at 60 fsw (by 50 fsw your breathing 52%-56% o2)and would be able to stay basically as long as you wanted and still have no deco obligation (your realy limited by NOAA O2 exposure tables around 3 hours) where as a SCR diver (constant FO2) would be near the NDL limits due to the lower FO2. If a connstant PO2 diver maintains a set point of 1.3 by 10 ft they are breatthing 100% O2!! (realistically, without flushing the breathing loop 1.3 at 10 is difficult to achieve most CCR divers lower the setpoint to .7 around 20 fsw and fly it manually to achive the PO2 they want, and flush the loop when necessary to allow high po2 at very shallow depths )