RE: CO meters (again)

["Bryan Willson" <Bryan.Willson@colostate.edu>]

I am a bit confused by the response from Paul Borsari of Onset.  Generally,
when a company reports a response time for any type of instrument, they are
referring to the time it takes for the instrument to respond "significantly"
to a step change in the measured value.  Usually, this is the time to
achieve 66% or 90% of the new value.  In this case, we are talking about the
time to respond to a step change in ambient CO concentration.

What Paul appears to refer to is "warm-up time", which is something very
different.  My hypothesis is that the long (11 minute) response time on the
Onset web site is due to the fact that gas only gets to the electrochemical
sensor by diffusion, since there is no sampling pump to "force" the sampled
gas through the instrument.

Having said this, I now have more questions than ever.  When I have a
question, I like to take a measurement so that I know how the test was
conducted and what it tells us.

Does anyone have one of these things that they would be willing to send us
for a few weeks?  I propose putting the thing in a box, feeding it some CO
calibration gas, and plotting it's response vs. time.  You can send me a
message through the Stoves list or offline.

Thanks!

 - Bryan Willson

 Dr. Bryan Willson
 Professor of Mechanical Engineering
 Research Director, Engines & Energy Conversion Laboratory
 Department of Mechanical Engineering
 Colorado State University
 Fort Collins, Colorado  80523-1374
 Phone:  (970)-491-4783
 Mobile: (970)-227-5164
 FAX:    (970)-491-4799
 EECL Web Site: www.engr.colostate.edu\EECL



> -----Original Message-----
> From: Tami Bond [mailto:yark@u.washington.edu]
> Sent: Saturday, September 21, 2002 2:52 AM
> To: Dean Still
> Cc: stoves@crest.org; ethos
> Subject: Re: CO meters (again)
>
>
>
> Stovers and especially Dean, Crispin and Bryan:
>
> I am finally weighing in on the CO issue.
>
> Harmon, thank you for looking that conversation up. I was going to do the
> same but you saved me the trouble! I did remember that we came to the
> conclusion that it wasn't 'really' an 11-minute response time to CO but
> couldn't remember why. Thinking about it-- these sensors are usually used
> for safety rasons. You might want to get out of the space if the CO levels
> skyrocket, and before 11 minutes too!
>
> Following what people have said, these would be my questions about the
> Onset logger.
>
> 1) Does the slow temperature equilibration affect the accuracy at the temp
> fluctuations one might experience in the exhaust? If so, we ought not to
> consider this unit.
>
> 2) Just what size of changes are we looking for? Dean speaks of changes by
> a factor of 10. Bryan suggests that with an inaccurate meter, one might
> not even trust comparative values. True, but I don't think the
> inaccuracies would be off by a factor of 10. And if we are only making a
> 10% difference, our stoves are probably not 'improved' enough.
>
> 3) Dean said that the logger won't handle high heat or humidity. Is this
> because the datalogger piece needs to be kept cool, or is it a problem
> with the sensor itself? If the problem is not with the sensor, then it is
> not too hard to separate them-- put the sensor in the stack and the logger
> outside. It doesn't make for a nice turnkey (=turkey) package, but it's
> not much more expensive, maybe less, and you could pick the sensor and
> even put a baby muffin fan on as Don suggested.
>
>
> About CO sensors. The research I have done suggests that one can get an
> electrochemical sensor with voltage output in the $150-200 range, maybe
> less. Cons-- the sensors are prone to temperature effects, interferences
> from other gases, wear out (chemically) after a couple of years. Pros--
> cheap. The next higher rung is NDIR (non-dispersive infrared). More
> accurate, but also more expensive (>$1000). My sense is that
> cheap-rugged-accessible is going to be limited to the electrochem right
> now. (Please someone prove me wrong and send me a brochure for a $100
> NDIR! ;-))
>
> So we are stuck with the electrochem, with all its faults. It is not as
> good as a fancier system; but it is surely better than what people have
> now, which is nothing. Can it be made to work in service of stoves? It
> needs good response time and reasonable accuracy. We should work on
> defining what 'reasonable' is, because we have many sources of uncertainty
> in comparing one stove versus another. I am guessing that 20% is good
> enough for the field. Anyone?? (Here I am reminded of what my old advisor
> would say: 'Measure it with a micrometer; mark it with a pencil; cut it
> with a hatchet!') Next, we need some data on what the response time is for
> that accuracy. Compare it with the Enerac or the FTIR or the NDIR.
> Anyone who's taking measurements, please log a couple of electrochems in
> parallel, and post the data on Stoves.
>
> Next next, I haven't seen much discussion of what we really need: not the
> CO, but the CO/CO2 ratio. The CO is variable based on dilution. You either
> want to know the concentration in the living space, or the total emission
> from some amount of fuel-- CO concentration alone, at some point in the
> stack, gives you neither. Measure the CO2. Dean's proposed setup will do
> this. I have a misgiving about using an exhaust gas analyzer to measure
> CO2 unless it's right above the flame, but the Enerac people think their
> setup will work.
>
> You might think you could get the CO/CO2 ratio from a hand-held combustion
> analyzer, but in fact, those are all inferred from O2 readings. If the
> exhaust has been diluted a bit, the O2 concentration is high, and the
> calculation of CO2 becomes uncertain. Plus, the inference is made based on
> fuel composition-- which changes as you burn. You have to measure the CO2.
> The other option is to weigh the stove to infer mass loss, and know the
> flow rate through the capture device (hood). I find that measuring CO2 is
> much less uncertain-- but I have not found a way to do it cheaply yet.
>
> Where does this leave Crispin, other than with a pocketful of small bills
> and still nowhere to spend it? I do think the electrochem is a viable
> option for CO, but it's a long way from the answer. My feeling is that you
> could do a lot worse than spending $250 on the meter Dean suggested; know
> that there are a lot of uncertainties, but at least it would tell you
> *something*.
>
> Tami
>
>
>
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