"Water mist" is not one technology — NFPA 750 recognises low-pressure, intermediate-pressure, and high-pressure water mist systems, distinguished primarily by operating pressure and the droplet size that pressure produces. The distinction matters more than marketing copy usually suggests, because droplet size is the variable that determines how well a system cools, displaces oxygen, and blocks radiant heat.
Pressure determines droplet size
Water mist nozzles atomise water by forcing it through a small orifice at pressure. The higher the pressure, the finer the resulting droplet spectrum:
- Low-pressure water mist typically operates below 12 bar, producing droplets in the 300–1,000 micron range.
- High-pressure water mist operates at 100–140 bar, producing droplets under 200 microns (Dv0.9).
That difference in droplet size is roughly an order of magnitude, and surface area per litre of water scales with the inverse of droplet diameter — so high-pressure mist presents dramatically more surface area for the same water volume.
Why droplet size drives performance
Smaller droplets absorb heat faster, flash to vapour more readily (improving local oxygen displacement at the fire source), and form a denser cloud that attenuates radiant heat more effectively per litre of water discharged. This is why high-pressure systems can achieve comparable or superior fire control to low-pressure systems while using meaningfully less water — a decisive advantage in tunnels, paint booths, and cable galleries where water damage, tank footprint, and retrofit disruption are all real costs.
Where low-pressure mist still has a role
Low-pressure water mist systems are simpler and cheaper to install, and remain a reasonable choice for lower-severity risks or where a facility already has a compatible low-pressure water supply. They are not, however, a substitute for high-pressure mist in the demanding risk categories Mistelix concentrates on — tunnels, paint booths, cable tunnels and transformers — where fire severity and the cost of water damage both justify the finer droplet spectrum.
What to look for in a datasheet
A genuine high-pressure water mist datasheet should state operating pressure (100–140 bar for Mistelix systems), Dv0.9 droplet size (under 200 microns), and full-scale fire test evidence under EN 14972 or an equivalent protocol — not just the words "high pressure" without the numbers behind them. See our Technology page for the complete specification, or Water Mist vs Deluge in Tunnels for how this plays out against the alternative technology.