ppO2 varying during OC dive

[Jef Mangelschots]

Just started using the Petrel 2. Very nice computer.
When looking at the logs, I noticed the ppO2 (green line) was varying between 0.2 and 0.4 atm.
This was an OC Tec dive with a single tank of 21/00 air at sea level.

From other posts, I understand you use the ambient air pressure times the F02 (21% in this case). But how does it adjust for surface ambient air pressure during a dive.

I was expecting the ppO2 and ppN2 to remain constant on an OC dive.

 

[tcoen]

Hi Jef,

In an OC dive the FiO2 (Fraction Inspired Oxygen) and FiN2 remain constant.

PPO2 (Partial Pressure of Oxygen) and PPN2 vary directly with the total ambient pressure (the water pressure). That is to say, PPO2 is a function of depth. Wikipedia describes it well:
Partial pressure - Wikipedia, the free encyclopedia

Surface pressure is determined while on the surface before the dive and then remains constant for the rest of the dive. Surface pressure is used for some calculations (like depth and decompression modelling), but isn't actually needed to determine the PPO2 since dive computers contain pressure sensors that measure absolute pressure.

Best regards,
Tyler Coen
Shearwater Research

 

[hroark2112]

PO2 = ATM X FO2

Breathing air at 21% O2, your dive must have varied between the surface and 33'.

 

[tbone1004]

what was said about. On OC your FO2 is fixed, and PO2 is variable vs a CCR where your FO2 is variable, and PO2 is fixed.

 

[Jef Mangelschots]

Thanks, I got my Fractions and Partials mixed up. Thanks for setting me straight.