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At 9:29 AM 7/19/95, Dennis Pierce wrote:
>> There's no reason to add a consumable O2 sensor to the rig unless you
>> want to monitor partial pressure.  If all you want to protect against
>
>
>....very necessary, unless you want to go to all the trouble and costs
>of a rebreather and end up diving air or something close to it..

I thought the atlantis was a semi-closed system.  As I understand,
the ppO2 in a semi-closed rebreather is determined by the formula:

  PBO2 = {(Vin * FSO2 - VO2)/(Vin-VO2)} * Pamb

where:

  PBO2    is the partial pressure of O2 in the breathing bag
  Vin  -  is the gas injection rate in slpm
  FSO2 -  is the O2 supply fraction
  VO2  -  is the metabolic consumption rate in slpm
  Pamb -  is the ambient pressure in ATA


It follows then that the PBO2 is controlled by injection rate, depth,
gas fraction and rate of O2 metabolization.  In other semi-closed
designs I've seen, the only fuzzy factor (for lack of a better term)
is VO2, and very conservative high and low values are used to determine
the max depth of a particular gas at a given injection rate.  This
makes the O2 sensor unnecessary (in theory), since you've selected
both an injection rate and a gas mix to match your target depth and
surface breathing requirements.  It doesn't follow that you would
be restricted to breathing normoxic gas just because you're only
monitoring the injection rate.  Did I misunderstand your comment?

Scott.

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