Re: Nest Protect
@Tim99 "Er, no." - No to what ? Your post agrees with what I said - the convection current you mention is directly caused by the lower density of the warm gas from the incendiary source which is the reason for installing detectors in the ceiling - nothing to do with the relative densities of the gases, everything to do with the temperature of the combustion products.
The mixing of the room air with the combustion gases (of whatever kind) does occur but this still takes time - not much but enough that the detector has long since activated and you have the opportunity to escape. Try it - light the bin on fire on your room now, watch as the smoke fills the room from the top down - granted the room fills completely with smoke in a few minutes but you will easily have time to walk out before it does.
As an aside, I did not refer to CO2 in any of my posts (probably because the whole thread is about carbon monoxide detection) nor at any point have I been confused between carbon monoxide and carbon dioxide. I am well aware of the reduction in density of any ideal gas relative to the temperature*, unfortunately your assertion that the height of the fire above the floor is the reason why a floor based escape plan is best is only true if you disregard your own argument relating to the mixing of the convection current into the room rapidly (which you should, because it's wrong - the height of the fire above the floor in no significant way alters the mixing effect of the convection currents, the combustion gases arrival at floor level is not significantly different for a couch 2 feet off the floor than it is for a fire on the floor itself).
To cut to the chase - detectors are ceiling mounted because hot gas rises. Doesn't matter what gas or how dense it is relative to other gases at room temperature, the density reduction due to temperature differentials is far greater than the difference in density of any plausible combustion product against the prevailing room air.
*Although I note as a further side point that you have failed to take into account the changes in pressure that would accompany a fire in an enclosed space, so the density only halves with a doubled temperature if the pressure remains constant, a situation that only occurs in a wide open space - inside a building the restricted air flow out of doors and windows would lead to a rise in pressure and concommitant rise in density along with the rise in temperature until the pressure was able to equalise with the outside world -this change in pressure and so density would, of course, be the same for all the gas products in the room, though.