Re: CO meters (again)

[Tami Bond <yark@u.washington.edu>]

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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