PO2 or FO2???

[dc4bs]

Hey, can anyone settle an arguement for me?

Oxygen sensors...

Do they all read PO2 or FO2 or are the different varieties that do one or the other.

I'm prety sure they all return PO2 but I have found printed in some manuals that they return FO2 (most do say PO2). My argument is that if they return FO2 then flowrate would not matter as FO2 does not change with pressure.

I know a guy who insists that the ones used for nitrox tank checking return FO2 and that a different type is used to return PO2 for rebreater aplications.

I even found one book that stated PO2 very strongly in one chapter and FO2 just as strongly in another... Wheee!!!

I know the distinction isn't all that impotant at 1 ATA but just for general knowledge, I'm curious.

 

[VTWarrenG]

All oxygen sensors report PPO2, because they depend upon a chemical reaction, which is sensitive to PPO2.

Most sensors (such as those from Teledyne) can measure PPO2's from something like 0.10 to 2.0 ATA.

As you stated, FO2 = PO2 at an ambient 1 ATA pressure. Oxygen analyzers for mixes are for use at 1 ATA pressure. You first calibrate them to a reading of 0.209 ATA in air by turning a potentiometer. This calibration effectively cancels weather- or altitude-induced pressure differences. The number on the display is then taken to be FO2, but it's really still just the PPO2.

The sensors used in rebreathers are usually the same as those used in oxygen analyzers -- the only difference is in the interpretation of their output voltages. All oxygen sensors report PPO2, though, no matter how you interpret them.

- Warren

 

[omar]

In the case of the Galvanic oxygen sensor, the voltage depends on the partial pressures of oxygen. The cell voltage is also dependent upon temperature and humidity. The higher the temperature the greater the voltage for the same ratio of oxygen.

Fiber Optic Oxygen Sensors are spectrometer-coupled chemical sensors for full spectral analysis of oxygen pressure. A fluorescence method is used to measure the partial pressure of oxygen.

Laser diode absorption spectroscopy gas analysis measures the fraction of oxygen. As the oxygen concentration increases, the light intensity is attenuated, and the photodetector response varies linearly with the oxygen concentration.

Paramagnetic oxygen analyzers are based on the magnetic properties of oxygen. The instrument output will be directly proportional to the concentration of oxygen in the gas mixture.

omar

 

[VTWarrenG]

omar,

Nice summary... but the only sensors I've ever seen used in diving equipment are galvanic (PPO2).

- Warren

 

[omar]

some of the more gung-ho rebreather guys have paramagnetic ones for bench testing etc.

omar

 

[VTWarrenG]

Nice to know, omar -- thanks.

- Warren

 

[dc4bs]

Thanks guys.

It's exactly what I thought.