At 22:25 19/08/99 -0400, you wrote:
>On Thu, 19 Aug 1999, Joe & Gladys Koontz wrote:
>
>> Sounds like you were building a closed loop rebreather!  I agree, just
>> build the rebreather and monitor the scrubber.
>
>BTW, (to whoemever posted that originally) - if you're having fantasies
>about Space, you want Lithium Hydroxide instead of Calcium Hydroxide.
>LiOH has a *vastly* superior weight/volume ratio, as well as a
>moderately superior ratio of CO2 absorbency/volume.
>
>It's rarely used in rebreather applications because it's also
>reactive as hell in water...

Isn't that why they use a dehumidifier before being past into the scrubber?
Else Kaboomb! (Well at least a fizzle and quite a large amount of that user
friendly hydrogen being produced)


Dennis


>
>-Will
>
>
>> good luck,
>> joe
>>
>>
>> "William M. Smithers" wrote:
>> >
>> > On Thu, 19 Aug 1999, Russell McMahon wrote:
>> >
>> > > I tried using a Zinc-Air cell as an O2 sensor REALLY roughly (seemed
like a
>> > > cheap way to make a sensor) and cell voltage seemed to be very
insensitive
>> > > to Oxygen concentration (as you suggest it would be). Stored O2
appears to
>> > > be a major problem.  I'll be trying it again in due course with  a
little
>> > > more finesses (hopefully). We bought a batch of "real" Oxygen
sensors but
>> > > even in 100 volume (I think it was) they cost around $US45 AFAIR.
These DO
>> > > produce a linear pO2 versus Voltage response. Being an
electrochemical cell
>> > > they have a finite lifetime and they drift severely so you need to
>> > > calibrate them before every use. Fortunately there is an easy cheap and
>> > > fairly accurate source of 20.8%-Oxygen calibration gas available.
(Or, if
>> > > you want to argue, 20.95 +/- 0.01% - yep, the ranges don't coincide-
>> > > depends who you listen to :-)).
>> >
>> > Before I address the O2 sensor issue, let me first ask if you
>> > really need one.  If you're looking to do life support
>> > in a fixed volume container, all you have to do is blow
>> > a muffin fan through a can of Calcium hydroxide (Sofnolime
>> > brand is good), and have a demand regulator hooked up
>> > to a source of 100% O2.  The Co2 goes into the scrubber,
>> > which drops the pressure inside the environment, and
>> > causes the demand reg to pump in the appropriate amount
>> > of O2.  This is what they do in mini-subs.
>> >
>> > For the sensors - I did some extensive hyperbaric tests using
>> > zinc-air cells as oxygen sensors.  My interests were a little
>> > different, as I was mostly interested in measuring higher
>> > partial pressures (0-2.0 ATA PPO2).  At any rate, the upshot
>> > is that they have horrible drift and are not particularly linear.
>> > They also have a very short life in hyperoxic environments, as
>> > you might expect.
>> > If you want to do some playing around with them, use a *small*
>> > cell (Duracell DA13, as I recall), and put a few hundred ohm load on
>> > them.  The smaller cells react faster and store less gas internally.
>> >
>> > Galvanics are the way to go for O2 sensing.  If you want small,
>> > Teledyne makes a sensor called the K-1, which is about 3/4 inch
>> > in diameter and 3/4 inch long.  The calibration issue is real - you
>> > do need to recalibrate regularly if you want accuracy.
>> >
>> > There are also fourescent dye based sensors, which apparently
>> > work extremely well, but I don't have any experience with those.
>> >
>> > Finally, there's paramagnetic O2 sensors.  This is really the
>> > ultimate O2 sensor, as they never need to be recalibrated and
>> > never wear out.  Unfortunately, the ones that have no moving
>> > parts are the size of a Coke can and draw major current.  They're
>> > also expensive ($2500 as I recall).  The current draw is because
>> > they've got an internal heater, as apparently the paramagnetic
>> > effect is very temperature sensitive.
>> >
>> > Along these lines, I screwed around with using Hall Effect
>> > and GMR devices to brew up a cheap paramagnetic O2 sensor,
>> > but never got too far with that project.
>> >
>> > -Will
>> >
>> > > decent
>> > > >pO2 sensors.
>> > > >
>> > > >Just looking at their voltage wouldn't work, of course.
>> > > >The voltage would be something like the log of pO2, a weak
function, and
>> > > >subject to stored O2 every time it got a good exposure.
>> > > >
>> > > >But the sensor was run as a current source into a virtual-ground meter
>> > > input,
>> > > >with a small diffusive air leak,
>> > > >the current might be proportional to pO2 with a response time of
under a
>> > > >minute.
>> > > >
>> > > >This might be useful for monitoring inert-gas vs oxygen.
>> > > >I never got around to trying it.
>> > > >Has anybody tried this or heard of it?
>> > > >
>> > > >Jim S
>> > >
>>
>
>