Schneier on Security

Roger • November 22, 2008 2:22 AM

It frankly does not seem unreasonable to suggest that if a display containing 10,000 gallons of liquid be separated from the public by a thin transparent barrier, then that liquid should be neither highly flammable nor carcinogenic. If it really took 9/11 to promulgate such a rule (which I frankly doubt), then thank goodness for 9/11 injecting a note of common sense for a change, instead of being an excuse to conceal foolishness.

@monopole:

Actually given the low boiling point of Novec and it’s fire suppression properties a movie plot terrorist would use an incendiary explosive to vaporize the fluid and asphyxiate the people in the hall.

a) Since Novec seems to be almost completely non-toxic, this would require that so much of the material evaporate as to completely exclude oxygen, for long enough to cause asphyxia. This is very unlikely because
b) It’s not that volatile; the boiling point is 49°C, so it would take immense energy to vapourise a large amount quickly. In fact it will not be all that much different to releasing an immense amount of water, just evaporating somewhat faster, and reaching a somewhat higher equilibrium “humidity.” The idea that the terrorist’s device could boil off enough to cause asphyxiation is not much more plausible than suggesting that they could asphyxiate people by boiling off a large amount of water. Of course once released, it will all evaporate eventually, but there will be plenty of time to get some ventilation going.

Roger • November 23, 2008 2:57 AM

@Clive Robinson:
Yes, asphyxiation by displacement of oxygen is possible for any gas or gas mixture (other than one containing adequate oxygen), if it is present at a high enough concentration that the residual oxygen concentration is too low to support life.

This is true of nitrogen, Novec, and the halons. The difference between Novec and halons (any halon, but especially 1211 or 1301) is that Novec is much less volatile. Those halons have boiling points below room temperature, so at equilibrium they can (under the right circumstances) completely displace all air. Novec has a boiling point of 49°C, so at typically room temperatures, even under pessimal ventilation conditions there will still be some air left. Exactly how much depends on the vapour pressure curve, the exact temperature, and just how bad the ventilation is; but it seems pretty unlikely that it will build up to asphyxiating concentrations.

One of the only reasons inhaling “condensing” water vapour (ie fog/mist) does not kill you is that your lungs are lined with it and it quickly condenses out whilst still alowing oxygen to cross over

Sort of; the key point is that the boiling point of water is much higher than either typical ambient temperatures, or body temperature. Thus it cannot normally be present at concentrations high enough to exclude sufficient oxygen to cause asphyxiation; if it does reach such a high concentration (e.g. by being pre-heated), then the excess will, as you say, condense out. But this is just as true for Novec, with the difference that the STP / b.p. difference is not as large, so it can get somewhat closer.

(Having said that, at sufficiently reduced air pressures, such as the tops of the highest mountains where the boiling point of water approaches human body temperature, it would be possible to asphyxiate someone with water vapour.)