An altimeter shows the pressure lost as
carbon dioxide is consumed, oxygen is added to bring the cabin pressure
back.
Hi, Peter . . . in what way is carbon dioxide consumed? And why would
pressure be lost? And, is this a closed system we are talking about? A
one-at boat?
CO2 is generated, not consumed, as a waste product of O2 metabolism in the
body. Oxygen is added, not "to bring cabin pressure back", but, rather, to
keep the occupants alive. In a closed system, the cabin pressure has
nowhere to go - there is simply an EXCHANGE of gases from atmospheric air (~
80 % N2, 20% O2). That means a gradual INCREASE in cabin pressure.
Cabin pressure remains the same regardless of whether the inside air is
being consumed by passionate lovemaking, smoking a cigar or burning
electrical insulation.
If pressure relief was all that was required, nitrogen would suffice.
They open an equalizing valve after surfacing before opening the
dome.
That's to equalize the addition of oxygen (that had previously been stored
as a compressed gas in cylinders) to the cockpit. There is a gradual
increase in cabin pressure over time from compressed O2 entering from the
bottles into the cabin.
The occupants generate CO2 that gets reduced by the scrubber fans drawing
the CO2 across racks of Baralyme. The lungs soak up the O2. Interesting
things happen when mammals breathe high concentrations of CO2 and O2. Hence,
the scrubbers.
The body has cells called chemo-receptors that determine when to breathe.
These receptors are sensitive to both O2 depletion and a blood increase of
CO2. These are referred to as blood gases.
The theoretical maximum saturation of oxygen in the blood runs somewhere
around 99%. This can vary according to the concentration of O2 in ambient
air, efficiency of the lungs, whether the O2 is displaced from the red blood
cells by CO, etc. Many factors determine blood gas levels including drugs,
altitude and more.
Rick Lucertini
Vancouver, Canada
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