Sounds like you were building a closed loop rebreather!  I agree, just
build the rebreather and monitor the scrubber.

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
> >