Re: [PSUBS-MAILIST] L.S.S.-Life Support System

[William Alford <walford@dbtech.net>]

>> > Hi, Peter . . . in what way is carbon dioxide consumed?  And why would
>> > pressure be lost?  

repost from the expert: Dr. Nuytten--

In an ideal situation, you capture a
volume of air in a rigid container (your sub) when you close the hatch.
With no increase in pressure, there is no uptake of inert gas (nitrogen) so
it acts a filler or diluent gas - ( be careful of the term 'inert' -
nitrogen is only inert in the context of metabolic oxidation, not in it's
ability to affect neurons under elevated pressures - as all you deep air
divers are well aware!) As the O2 is metabolized and - for all practical
purposes - converted entirely into Co2  . . .and the Co2 is chemically
bound to the scrubber alkalai - the cabin pressure falls and the barometric
device ( 'bellows-add', or such) is triggered and O2 inflows until the
cabin pressure reaches the set point of the barometric device and the O2
flow ceases . .and so on.  What actually happens with a sensitive enough
barometric device/add valve is that the add valve achieves a 'just cracked'
position, commensurate with O2 uptake- and will only shut off entirely if
breathing ceases ( R.I.P.) or open 'more' if respiratory rate increases (
Jeez! - it's a Giant Squid!). Don't forget the constant-flow safety device
( 'steady-rate by-pass', 'fixed-orifice', or similar) This is fixed at a
rate that will support life but not high-normal respiratory rate.
         So . . after much divergence . . .why have an O2 sensor?? Well . .
.what if the O2 stops? ( runs out, swerf or debris jams orifice, hul- stop
accidentally turned off, etc., etc.,) Air flows into lungs, O2 is converted
to Co2, Scrubber snatches Co2, next breath of 'air' (cabin gas) is
deficient in O2 . . . as you can tell. Yeah? how do you tell? The increase
in Co2 which would normally warn you of decreased O2, is not present. Since
our primitive design specifications never  envisioned a situation where the
oxygen could diminish without an increase in carbon-dioxide, there is
virtually NO warning until the bod suddenly realizes that death is next on
the dance card,shuts you down, and shunts what little O2 is left to the
brain.  Would have been nice to have an O2 meter read-out when you  started
to feel a bit peculiar or noticed gas draining from your eustachion tubes
and sinuses!
        Probably the best argument for an O2 sensor is that they're 'cheap
like borscht' and bring a peace of mind that is far more than their cost. A
good unit should run about $300 -  more with hi/lo audio/visual alarms. Be
careful of the cheapies that 'get you by the sensor-cells' - much like the
printers that offer high quality at a low entry price and then gore you
unmercifully on 'proprietary' ink cartridges for the life of the device! If
the vendor wont direct you to the sensor cell's EOM, go somewhere else. We
(Nuytco) make a unit called the 'Newtrox' ( what else!) which was designed
for sub use - runs over a week steady on a regular little 9 volt battery. A
shameless commercial - BUT - it is just more rugged than usual - it is
functionally almost identical to a bunch of other, cheaper, models. So, go
with the best deal - but make sure you can buy the cells direct from the
cell manufacturer! 
        I did a paper on barometric activation on sub life-support, years
ago. I think there is a copy on/in P-subs archives. Probably worth a look
if you're going to use this type of system.
Phil Nuytten



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