[PSUBS-MAILIST] Life Support Revisited

[Jonathan Wallace <jon@psubs.org>]

I've got a couple of questions regarding life support in a 1 ATM sub that
some research hasn't turned up.  Basically, I'm wondering why an 02 sensor
would be necessary in a 1 ATM vehicle as long as 02 is added as the result
of pressure loss caused by CO2 scrubbing.

My understanding of this is as follows.

1) we have a 1 atm sub
2) we have a CO2 scrubber
3) we have a perfect atmosphere in the sub for human survival at the
   instant the hatch is closed.

So, upon our first breath with the hatch closed we start consuming oxygen
and replacing it with CO2.  The CO2 attaches to the sofnolime in the
scrubber thereby reducing the atmospheric pressure within the sub.  A
barometric device reacts to the decreased pressure and adds O2 to the
sub until 1 ATM is reached again.

I understand the ramifications of a reduced partial pressure of O2, however
an increase of O2 partial pressure should have no life threatening effect
in a 1 ATM sub (assume no fire hazard exists).

Under this scenario, it seems that the amount of O2 added by the barometric
device would have to be sufficient to maintain a .2 ppm because if it
didn't I'd have to add more O2 manually, but then risk raising the
overall pressure within the vessel.

I'm sure there's a good reason why people spend the money on an O2 sensor
within a 1 atm sub, but I'm not seeing it.  What am I missing under this
scenario?

Jon