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The following is close to the equation for semiclosed circuit constant 
volume fraction rebreathers.  For the exact equation see Bennet and Elliot
Second edition.

    PI = PS - 1/rK

Where

   PI Inspired PO2. 
   PS Supply PO2 (equal to supply FO2 times ambient preasure).
   r  Volume fraction expelled with each breath.
   K  Physiological constant relating minute volume to O2 consumption.
      Bennet and Elliot give this constant between 15 and 30.

The above formual would probably approximate the BMD rebreather if it was 
still available.  As far as I know this formula will not work for the 
Odysee because they change r with depth d. ie r = r(d).

The actual formula is slightly different ( but I can't remember it right now).  
The theory is that the inspired PO2 should vary over a predictable range.
For example if r = 1/5 then the error on PI is +- .08 ATA.

Barrie Kovish
Vancouver, Canada

   

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