Top middle or bottom carbon monoxide

[westernman]

I don't think there is any perfect position ... too many factors like airflow etc. can affect it ... all we can hope is that we get it as best as possible.

And have more than one to improve the chances.

 

[wonkywinch]

From the Nereus CO alarm installation manual :

Carbon Monoxide Sensor: Carbon monoxide is similar in density to air and freely mixes with air. It is recommended that the sensor is
sited at a mid to high level.

https://nereusalarms.co.uk/WGxxx-LC_Installation_Instructions.pdf

The CAA have just issued a useful CO safety guide albeit directed at light aircraft pilots -

https://www.caa.co.uk/publication/download/23096

 

[jdc]

The arguments and statements about 'Carbon Dioxide is heavier than air so will move down', which also is said about propane or butane gas, are all complete boll*^£s. There's a difference between a gas and a liquid; in a gas there is no displacement, the molecules are not bonded together and all bounce around in thermal equilibrium and just mix since the distance between molecules is vast compared to the distance over which they interact, ie they just bounce off each other like billiard balls. It's what gas means!

To calculate the change with height you have to apply the Boltzmann distribution, which is that at a height x the number of molecules per volume,

n(x) = n(0) exp(-E(x)/kT)

where k is the Boltzmann constant, T the temperature in Kelvin and E(x) is the energy of a single molecule as a function of height. E = mgx, where m is the mass of the molecule, g the acceleration due to gravity (9.81ms^-2 and x the height. The mass of a molecule of Nitrogen (air is 4/5 Nitrogen) is 28 / N_A grammes where N_A is Avogadro's number, thus 0.028 / 6x10²³ kg.

Putting these into the formula (and assuming temperature doesn't vary much) one gets the reduction in the number of molecules with height as shown here:


which shows the expected reduction to 0.7 atmospheres at a height of 3000 m.

More to the point, the difference in concentrations of CO and O₂ over 2 metres height will only be 3.3x10^-5, or one part in 30,000. Gasses just don't collect in the bilge or rise up - they just disperse in all directions, very fast if small molecules, rather slower for very large molecules.

 

[AntarcticPilot]

The arguments and statements about 'Carbon Dioxide is heavier than air so will move down', which also is said about propane or butane gas, are all complete boll*^£s. There's a difference between a gas and a liquid; in a gas there is no displacement, the molecules are not bonded together and all bounce around in thermal equilibrium and just mix since the distance between molecules is vast compared to the distance over which they interact, ie they just bounce off each other like billiard balls. It's what gas means!

To calculate the change with height you have to apply the Boltzmann distribution, which is that at a height x the number of molecules per volume,

n(x) = n(0) exp(-E(x)/kT)

where k is the Boltzmann constant, T the temperature in Kelvin and E(x) is the energy of a single molecule as a function of height. E = mgx, where m is the mass of the molecule, g the acceleration due to gravity (9.81ms^-2 and x the height. The mass of a molecule of Nitrogen (air is 4/5 Nitrogen) is 28 / N_A grammes where N_A is Avogadro's number, thus 0.028 / 6x10²³ kg.

Putting these into the formula (and assuming temperature doesn't vary much) one gets the reduction in the number of molecules with height as shown here:

View attachment 183993
which shows the expected reduction to 0.7 atmospheres at a height of 3000 m.

More to the point, the difference in concentrations of CO and O₂ over 2 metres height will only be 3.3x10^-5, or one part in 30,000. Gasses just don't collect in the bilge or rise up - they just disperse in all directions, very fast if small molecules, rather slower for very large molecules.

And air is mostly N2 (about 78%), which has the same molecular weight as CO! As far as I can see, the main thing is that CO comes from flames, so it will be hot and rise at least initially. As you say, diffusion will be a relatively slow process.

 

[Barton Seagoon]

Top tip for testing your CO alarm. We managed to set ours off by scorching some toast under our grill. Normal operation of the stove or grill has never triggered it, but burning toast seems to produce enough CO. Not surprising really, since the toast goes black in the process!

 

[B27]

Mine is mid level in the cabin, relatively forward, so near where we are breathing.
Nearer the stove would be more prone to moisture.

 

[Baddox]

All answers so far seem to ignore the way carbon monoxide is formed. It comes from incompleted combustion which indicates that the CO produced will be hot and therefore rise initially. I keep carbon monoxide alarms near the galley and also at about bunk level.

 

[MisterBaxter]

I did a fairly extensive risk assessment exercise around CO at one point and a couple of things came up. One is that it mixes freely with air and disperses fairly fast and evenly through a space, so placement isn't super-critical in that sense.
The other was that once it has dispersed through a space that you're in you're in very big trouble already .
So for the alarm, close to any possible source; somewhere dry and visible so that you can see the alarm and know if anything happens to it (eg it gets knocked off or damaged); and if in doubt, install more than one.