On Thu, 19 Aug 1999, Craig Lee wrote:

> Smithers!!
>
> Have you been skulking in Monty's bathtub again!!
>
> (I am a very sick P.IC)
>

Busted!!  Damn, it's a small world...

-Will
P.S. Dudes, in case you're wondering, there's a list for rebreather nerds.




> > -----Original Message-----
> > From: pic microcontroller discussion list
> > [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of William M. Smithers
> > Sent: August 19, 1999 8:25 PM
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: Re: carbon dioxide removal for life support
> >
> >
> > 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...
> >
> > -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
> > > > >
> > >
>