At what atmospheric pressure is the flash point testing defined for liquids?

• A. One atmosphere
• B. Two atmospheres
• C. Zero atmospheres
• D. Three atmospheres

Answer: (A)

Flash point testing uses standard atmospheric pressure so results are comparable and repeatable. The defined pressure is one atmosphere (about 101.3 kPa). At this pressure, the liquid can vaporize enough to form an ignitable mixture with air when an ignition source is present, and the temperature at which that happens is the flash point. If you raised the pressure, vapor formation becomes less favorable at a given temperature, often pushing the flash point higher. If you lowered the pressure toward a vacuum, there would be insufficient oxygen for ignition. Higher or lower pressures would thus change the measured flash point, which is why the standard reference is one atmosphere.