On Fri, 20 Aug 1999, Dennis Plunkett wrote:

> >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)
>

No, actually, Hydrogen has nothing to do with it - the issues are purely
thermal.  But this is a list about PIC's, so I figure this thread
has probably extended as far as it properly should, without going
to private email...

-Will






>
> 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
> >> > >
> >>
> >
> >
>