Good evening, Gents:

OK, let’s approach it from a different angle.

Go back to our chamber, but this time let’s just use two gases: “air” and toluene. Toluene is heavier than air, in fact, it is immensely heavier than air; about three times heavier than air.

So let’s take our chamber filled with air, and fill the bottom portion with just toluene gas. Now we have a one foot deep layer of toluene in the bottom, right? Good. Now, if we hook up a manometer and read the pressure differential across the shell of the chamber, we would see that the gases in the chamber are at equilibrium – no net pressure between the top and the bottom, and since the chamber is not pressurized (it’s just holding the gases, like a cup holds air), there is no net pressure differential across the wall of the chamber.

See below:



Now let’s carefully suck out the air above the layer of toluene. If the toluene is resting on the bottom of the chamber because it has settled there (since it is heavy), it should just remain there, right?

See the chamber below:



But look what has happened to the pressure differentials. The toluene is still exerting the same pressure as before, as expected, but now, there is a vacuum in the top part of the chamber. How could you possibly have a pressurized chamber at the bottom, and a perfect vacuum at the top?

Answer: You can’t.

The toluene will expand into the top layer to fill the vacuum. In fact, the toluene doesn’t know what the pressure is at all … because it, like all perfect gases, will perfectly fill any volume in which it is placed and it will extert the EXACT same pressure on the walls of the container regardless of the original pressure in the container – this is known as Pascal’s Law.

The actual pressure in the chamber is described by an equation known as the ideal gas law: PV=nRT. (pressure times volume equals the number of moles of gas times the ideal gas constant times the temperature).

If this were not the case, then remember this: “Air” itself is a mixture of gases that are lighter-than-air, and heavier-than-air: Taken as an whole, “air” would have an equivalent gram molecular weight of about 28; diatomic oxygen (heavier than air at 32 gmw) is 20.9%; CO2 (44 gmw, at 0.03%); diatomic nitrogen is 28 gmw at 78%, and argon at 40 gmw contributes about 1%.

So if a chamber were filled with “air,” and gases separated out according to their “weights,” an enclosed chamber would quickly partition out in layers, with CO2 settling to the bottom, upon which a layer of argon would rest; which would be covered by a layer of oxygen, capped by a layer of nitrogen. There is not a single reader of this board who would expect to see that happen – so why would any of you expect that CO, with a gmw of 28 would “settle out?’

DavidR – the answer to your observations lies in the last part of my post “The proper placement of a CO meter in a property has more to do with occupancy, proximity to a combustion source, topography of the structure, thermal by-passes, and so forth. Each of these parameters vastly overwhelms any consideration of molecular weight.” Similarly, the measurements you observe also depend on those parameters.

Ted: In fact, my ignorance is overwhelming. Presume I have absolutely no idea of what I am talking about. So take the info on its own merits – it will either stand or fall on its own merits, the credibility of the poster notwithstanding!

Dang – I need another beer!

Cheers!
Caoimhín P. Connell
Forensic Industrial Hygienist
Forensic Industrial Hygiene

(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)

AMDG