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detector location -gas--bedtime reading- gets heavy! - Ross - 02-14-2008 16:11
Ask A Scientist
Chemistry Archive
Carbon Monoxide and Density
Question - Does carbon monoxide rise or sink if produced inside a
house? Why? Does natural gas rise or sink?
(I am trying to figure out where to position gas detectors).
-----------------------------------------------------------
Although the molecular weights of these gases differ, convection and not
density differences dominate the distribution of CO, methane etc. The
release of gases associated with combustion tend to rise because they are
hotter than ambient. For this reason smoke detectors and CO detectors are
usually placed high on the walls of hallways etc.
Vince Calder
====================================================
Michael,
High or low in a room; It makes no difference where you put the detector
-- but not because carbon monoxide and air have approximately the same
density. Both are gases and as a result, diffusion alone and the entropy
effect will take care of mixing them. Any convection currents and/or
forced-air turbulence present will also enable the gases to mix and
affect
the detector.
It is popularly misconceived that light (low-density) gases will somehow
float atop heavy (higher density) gases. Indeed, if the higher density
gas
was admitted low in the room, and done so in a manner that would not
cause
much mixing, it would take a while for the gases to become thoroughly
mixed by the mechanisms mentioned above. Even so, they would
eventually mix.
That said, since time to detection may be of the essence, and in
circumstances where the contaminant gas (such as radon -- a gas of very
high density) gets into the room through entry points more or less at
floor level, where one puts the detector can become important.
Regards,
ProfHoff 909
=====================================================
Air is 1/4 oxygen O2, (weight 16) and mainly nitrogen N2 (weight 14).
CO is weight 12+8 = 20, so heavier than "air"
Methane (natural gas) is CH4 = 12 + 4(1) = 16, so mixes with air fairly
well, hard to say.
I would put the CO and methane detectors near the furnace.
Steve Ross
=====================================================
Neither, for the most part.
Carbon monoxide has nearly the same density as air.
You'll notice that the manufacturer's instructions that come with the
detector don't say high or low either.
The density of a gas is proportional to the weight of a single
molecule of
that gas.
So you figure relative buoyancy for yourself if you start learning a few
atomic weights:
H=1, He=4, C=12, N=14, O=16.
Add them up for the molecular weights of pure gasses:
H2 = (1+1) = 2, very light
He = (4) = 4, very light
N2 = (14+14 ) = 28, about neutral
O2 = (16+16) = 32, slightly heavy
CO2= (12+16+16)= 44, heavy
CO = (12+16) = 28, about neutral
CH4= (12+4*1) = 18, light (majority part of natural gas)
H2O= (2*1+16) = 18, light (steam)
C2H6=(2*12+6*1)= 30, about neutral (minority part of natural gas)
C3H8=(8 + 3*12)= 42, heavy (propane)
C4H10=(10+4*12)= 58, (butane)
C5H12=(12+5*12)= 70, pentane, lightest part of gasoline
For mixed gasses just take a proportionate average:
Air is 80% N2 + 20% O2 .
air = 0.8(28) + 0.2(32) = 29 (exactly neutral, by definition)
So pure carbon monoxide is actually about 3% lighter than air.
But usually it is made in modest concentrations, mixed in with the normal
combustion products: CO2, H2O.
Which are always mixed with the 80% Nitrogen that never participates in
burning.
Then that mixes with room air, making an even smaller concentration...
And there are uncertainties...
Some fuels make light exhaust (more H2O), some make heavy (more CO2).
Then when the exhaust cools the light part, H20 (steam), may condense and
drop out.
Not to mention that the exhaust gas was expanded when hot, and it
contracts as it cools.
No rule can predict which way it is going to go in most circumstances.
Because it travels in whatever directions your air normally
circulates, which varies,
it is difficult in most homes to find an advantageous position where CO
will enter or concentrate.
So we settle for any convenient position, or one associated with that
which we wish to protect,
namely us, breathing in air at medium heights.
My CO detector is at about waist-level in a hallway, partly because there
was a convenient power socket there.
Also the only air-intake to the house heater is there, so virtually all
the air in the house will pass that position regularly.
Doing a really good job needs more than one detector, but they are not
cheap enough for that yet.
One for each potential source of monoxide, plus one for each
protectee. Maybe someday.
Meanwhile, if I had a wall heater, I wouldd definitely place my one
detector near there, probably high up.
Jim Swenson
PS- natural gas rises.
=====================================================
Up-date 2/4/2005
=====================================================
Just read your reply to Michael K. on "Carbon Monoxide and Density" The
furnace room location may be okay if you have more than one alarm.
However
of more prime concern is that the occupants be able to hear the alarm
while asleep. National Fire Protection Association has a code , number
720, 2-1.1.2* 1998 states that " A carbon monoxide alarm or detector
should be centrally located outside of each separated sleeping area in
the
immediate vicinity of the bedrooms. Where bedrooms are separated and the
audibility of the alarm or detector to occupants within the bedroom area
could be seriously impaired, more than one unit could be needed. Each
alarm or detector should be located on the wall, ceiling, or other
location as specified in the installation instructions that accompany the
unit." . In addition more than CO will be emitted with furnace failure or
with the use of an unvented heaters. Carbon dioxide a heavier gas will
collect on the floor building up and possibly preventing carbon monoxide
from ever reaching the alarm if located at a floor outlet. That is why it
is recommended that CO alarms be placed high around eye level where the
higher concentrations would be concentrated. You are correct that natural
gas rises, however Michael K. did not state if his question on the gas
detector if it was about natural gas or propane gas. If his residence is
using propane then he would need to place the gas detector on the floor
next to the furnace and gas water heater. It is my hope that you ammend
your answers to Michael K.
Gary R.
=====================================================
--- In WanderlodgeForum@yahoogroups.com, "John McGinnis"
>
> I thought the CO detector was mounted close to the floor in those
> coaches that have the built in detector. Isn't it to detect the CO
> from sources such as exhaust fumes from the engine & fumes from the
> Suburban furnances, & propane frig. I just set my First Alert CO
> detector on the bedside cabinet, no screws. I just feel safer with is
> close by.
> John 88FC35
>