Which temperature thresholds define Flammable Liquids for closed-cup and open-cup tests?

• A. Not more than 60°C (open-cup) and not more than 65.6°C (closed-cup)
• B. Not more than 60°C (closed-cup) and not more than 65.6°C (open-cup)
• C. Not more than 60°C (open-cup) and not more than 60°C (open-cup)
• D. Not more than 60°C (closed-cup) and not more than 70°C (open-cup)

Answer: (C)

When evaluating whether a liquid is flammable, the method used to measure the flash point matters because the two tests yield different numbers. A closed-cup test traps vapors inside the cup, so vapors accumulate more easily and ignition occurs at a lower temperature. An open-cup test allows vapors to escape, so a higher temperature is needed to reach an ignitable vapor–air mixture.

Because of this, the standard thresholds are: not more than 60°C for the flash point measured by the closed-cup method, and not more than 65.6°C for the flash point measured by the open-cup method. The open-cup value is higher due to vapor escape, hence the larger cutoff.

In practice, the closed-cup result is the primary criterion for classifying flammable liquids in many regulations, with the open-cup threshold serving as a supplementary reference.