[SML] Smoke vs Heat vs Rate of Rise detectors

[Dale Farmer]

On 3/9/2022 11:30 AM, Stephen Lee via Stagecraft wrote:
> George Davidson wrote:
> 
>> One note we were always given is that the mist systems displace the 
>> oxygen, so get the hell out if it goes off.
> 
> 
> I think whoever said that was conflating water mist systems with 
> something else. Yes, Carbon Dioxide extinguishers (and many other gases, 
> for that matter) will displace oxygen. Yes, Halon and it's newer flavors 
> will bind to oxygen and render it useless. Fires, of course, will 
> consume oxygen.
> 
   Part of the firefighting effect is that the boiling of the water 
droplets at the actual fire location converts that water into steam, 
expanding I forget how many thousand percent.  That will cause a 
localized displacement of the air. A person close enough to be bothered 
by the lack of oxygen is probably more concerned with being incinerated 
by the fire.
    As the fire gets knocked down, it stops boiling as many droplets, 
and fresh air will get sucked in as the heated air and steam rise up. 
Thus it will flash back up as the oxygen becomes available.  Thats why 
in that fire video you see the flames flare up and die down several 
times before the fire gets extinguished.
   Maritime trivia.  Older ships used to use steam from the boilers as a 
firefighting agent in cargo and bilge fires.   That would indeed 
displace all the air in a confined space, But no worries, any people 
would be cooked by the live steam before they asphyxiated.
   Some dry bulk cargo ships now use a nitrogen gas inerting system in 
cargos that are prone to auto ignition.  Grain carriers especially, as 
this also has a side effect of killing off any unwelcome critters that 
came along with the grain, satisfying the international trade 
requirement that cargos be free of vermin.
    Water mist systems are replacing halon and CO2 systems, which are 
much more immediately lethal to people without breathing apparatus.

Dale

> But water mist displacing oxygen? Not buying it. I can't remember 
> getting light-headed the last time I walked through a fog bank or a rain 
> shower. If fog displaced oxygen, then most of San Francisco would be an 
> uninhabitable wasteland. ;)
> 
> However, I have seen several movies where bad creatures would emerge 
> from the fog.
> 
> As an interesting aside, some very benign things will also consume 
> oxygen, rust being one of them. The mention of ships reminded me of 
> this: There have been more than a few sailors who have died from 
> asphyxiation when they climbed down into the chain locker (a large 
> storage compartment in a ship which holds the anchor chain when it's not 
> deployed). The air inside the compartment turned out to have very little 
> oxygen because the rusting process on the chain had slowly consumed it. 
> It was a cautionary tale to never enter a confined space without 
> ventilating it thoroughly first.
> 
> 
> ____________________________________________________________
> For list information see <http://stagecraft.theprices.net/>
> Stagecraft mailing list
> Stagecraft at theatrical.net
> http://theatrical.net/mailman/listinfo/stagecraft_theatrical.net