Testing with a CO + CO2 Meter
Crispin Pemberton-Pigott crispin at NEWDAWN.SZ
Wed Jun 30 16:36:30 EDT 2004
Dear Stovers and Testers
I have been keeping my nose to the grindstone here in Swaziland and
practising collecting measurements - something about which I know little
other than what I have read, mostly on this list. I found that I was
confused by certain numbers, ratios and claims for different stoves.
Having Tami around is a blessing because she thinks about these things
and does much to boot. After a number of emails passed back and forth
between the members of this mutual appreciation club, I am venturing out
into the sunshine to start making reports on useful aspects of my work.
I am writing this for all the people who have never used fancy equipment
to measure what is going on in a stove - I have just started too!
First, something that I was confused about: I have heard different
numbers quoted for the 'amount of carbon-monoxide (CO) a stove produces'
and while it was illogical in certain respects, it sounds simple and
thus a good method of evaluating the effectiveness of combustion and the
usefulness of the stove design or application.
There are two places where CO is important: in the stove, and in the
room. If there is a lot of CO in parts per million (ppm) in the room,
it is bad, but if it is in the stove (I will say 'in the chimney'), it
is not necessarily bad. It depends on how much CO2 there is mixed with
it. This I didn't understand initially. I thought it was analogous to a
heat transfer efficiency - a better number means a better stove.
CO2 in the chimney is evidence of the combustion of a carbon-containing
fuel in the combustion chamber. The contentration of that CO2 in ppm is
irrelevant. If you burn carbonaceous fuel there will be CO2 in the
chimney at some concentration or other. If you let in a LOT of excess
air, the CO2 will be diluted so there is no value in comparing the CO2
concentrations of different stoves. Unless you know exactly how much
unneeded air is passing through the stove, the CO2 concentration is just
a number. For example, if you have a stove and measure 5.5% CO2 in the
chimney, it only means that 94.5% of the gases are something else. It
does not tell anything useful about the fire or the stove.
CO in the chimney is evidence of incomplete combustion of a
carbon-containing fuel in the combustion chamber. Again. The
concentration of the CO in the chimney is not a meaningful figure.
There is nothing you can decide about one stove that has 650 ppm CO in
the chimney and another that has 225 ppm CO using that figure on its
own. The one with the lower figure might be drafting large amounts of
air through the stove and diluting the CO.
What is very useful is to know how much of the carbon that is being
burned is emerging from the combustion chamber as CO and how much as
CO2. If the CO is high and the CO2 is low, this indicates incomplete
combustion. For example when lighting a coal fire with newspaper and
wood, I got CO figures of as much as 15% of the CO2 level. This is
expressed by my meter as a proportion of CO to CO2, for example the CO2
might be 6.23% of the chimney gases and the CO level might be 0.023 of
that. The South African Bureau of Standards (SABS) wishes CO from
paraffin stoves to be at a 0.02 of the CO2 level.
Collecting the gases with a hood and extracting them and measuring the
absolute CO level in ppm tells you almost nothing unless you are
measuring other things as well. If the exact dilution is not known, the
figure has no value. It is the RATIO that is important as it tells us
what the efficiency and completeness of combustion is.
The CO level in a room in ppm is a totally different matter altogether.
The health risk from exposure to CO from breathing smoke in a room is
given a danger rating based on the concentration and the exposure time.
Briefly, you can live in 9 ppm, get by for 8 hours at 50 ppm and you
have to leave the room at 100 ppm. At 1000 ppm you will not live more
than a few minutes.
All this CO in a room story bears no direct relationship to the stove CO
numbers because you might be burning a large clean fire or a small dirty
fire and the result for the room would be the same. An example of this
would be burning a rather large amount of wood in a room in a fire with
an average 3% (0.03) CO level or exactly 1/2 as much charcoal at a 6%
(0.06) CO level. One might say the charcoal is twice as polluting. It
is no more polluting if the amount of fuel used to cook the same meal is
Dr Karve's 100gm-per-time charcoal cooker could be very polluting in
terms of CO per Kg of fuel burned, but it would not matter if the only
alternative was burning 500 gm of wood, even if the total CO output was
more than 1/5 as much per gram. If someone measures the CO levels in
his stove at 1250 ppm CO it tells us exactly nothing about the
combustion. He might be burning all the charcoal in a very small amount
of air quickly and efficiently with very little excess air. The
combustion products might be very concentrated. If you harassed him
about the CO level he could make the air inlet door a little larger and
get a 'better CO level' without improving the combustion at all, or
perhaps even making it worse. The only way to know is to compare the
With this background, I will report on three different types of
experiment that I have measured so we can talk about how to standardize
stove emission testing in a meaningful way.
Test Type 1
I lit a downdraft coal stove using a dreadful oily coal from Witbank
using 1 sheet of newspaper and 200 gm or wood. In the initial lighting
phase the CO level was 15% of the CO2 level and there was dense white
smoke in the chimney.
Once the wood and paper had mostly burned away and the coal was lighting
(short, dark, red flames with a lot of black smoke from the flame tips)
the CO level dropped to 4.4%. The reason that is was that low is that
is just the nature of a downdraft stove. The flames are drawn over the
hot coals and the smoke is burned.
As the coal got going (still lots of black coals on fire) the CO level
dropped lower and lower until it stabilized at 1.5 to 1.8%. This means
that the whole smoke volume could have been vented into a kitchen and it
would still be 7.5/1.5 = 5 times cleaner than a Panda paraffin stove.
When the coal finished 'coking' and was nothing but red hot coals, the
chimney gasses were completely clear and one would expect that the stove
would finish burning the fuel over the next couple of hours without
'smoke'. Well after the coal finished going from black to red coals,
the CO level steadily rose. The heat output also increased (double to
triple) and the absolute CO concentration in ppm increased. This means
that more carbon was being burned per minute, and that it was not
getting enough air. The CO stabilized at 9% when the fire appeared
visually to be in its 'cleanest' phase. It is this condition that the
South African mbaula (open-topped paint can stove) is taken indoors for
space heating! Yikes!
In general, except for the initial lighting, the visibility of smoke
(visible PIC's) did not correlate to CO levels at all.
When I added a few new lumps of coal to the combustion chamber to creafe
more red flames, the CO level dropped to 7% as it ignited. This bears
Test Type 2
I lit a wood fire in a Vesto and got all sorts of figure depending on
how much fuel was in it, what type of wood, how small it was split how
short it was cut and whether or not the air controller was closed or
open. As one might expect, dry hard wood in chunks that did not heat
and gasify too quickly was the best performer in terms of emissions with
1-2% being achievable for long periods of time. In the 'dying embers'
stage the CO rises to 10% or more unless it is allowed extra air by
raising the can slightly (as the manufacturer recommends). The worst
figures (12-15%) are seen when overloading the combustion chamber with
very dry or small or smokey branches with bark. There simply isn't
enough air getting in to burn the smoke properly.
In general, there was a good correlation between visible smoke and CO
except at the end when there is only a small amount of charcoal left.
The absolute concentrations of CO in the Vesto are high compared with
open fires because it does not allow in a large amount of excess air (it
remains undiluted) but the combustion efficiency is very high.
Test Type 3
I use one of the newest gasifier conversions of the Vesto to check the
CO level when it was in 'gasifying mode' as described in an earlier
message to this list. Paul A must take notice of this test!
Once the fire was established and the wood was burning well, the primary
air was closed off and the stove thus turned into a wood gasifier. A
deck of bluish-yellow flame is established at the level of the secondary
air inlets (a circle of holes above the fuel). In this condition the CO
level dropped to 0.1% to 0.3% (0.003) of the CO2 level - a ratio of
1:333. The sample was drawn 'hot' from a point about 100mm above the
flames in the centre of the combustion chamber. This is extraordinarily
clean. (BTW the meter discounts the background CO and CO2 levels and is
When the wood finally burned to charcoal, the CO began to rise. It
still continued to burn as a 'charcoal gasifier' but the CO level rose
to 5-6%. I lifted the stove to allow air to flow freely underneath and
up through the combustion chamber. The fire immediately jumped up in
intensity, the flames shortened and became intense, and the CO portion
nearly doubled! This bears further investigation. The Vesto burning as
a charcoal gasifier was cleaner than as a free-flowing-air charcoal
There was no meaningful correlation between visible smoke and CO
Truly minimizing CO emissions seems to required one or more of these
1. Gasify the wood and burning the gas - close to the wood in the
case of the Vesto (to maintain the ignition). Total CO levels and CO
ratios are extremely low in wood gasifiers.
2. Quench the charcoal and saving or selling it to avoid the high
CO phase altogether - an approach favoured by some. It could perhaps be
burned in a more appropriate device with lower CO.
3 Add fresh fuel to the wood gasifier just as it reaches the
charcoaled stage. It works by mixing the CO from the charcoal with the
gases from the wood volatiles. Ideally refuel it a couple of minutes
before the volatiles run out when the flames change to mostly blue.
This can only be done in a stove designed to be refuelled 'on the fly'.
The batch load gasifiers are a problem in this regard.
4. Learn what causes CO to be high as visible smoke is nearly
useless as an indicator of CO levels except in the very beginning.
Without a meter you are whistling in the dark.