Low expansion ratio foam is typically described as being effective for controlling and extinguishing most Class B fires at an expansion ratio of:

• A. 2:1
• B. 20:1
• C. 50:1
• D. 200:1

Answer: (B)

Low expansion foam works by creating a dense, cohesive blanket that blankets the fuel surface, excludes air, and provides cooling and vapor suppression. The expansion ratio describes how much foam solution expands into foam; for most Class B fires, a density and stability achieved around 20:1 gives a thick, persistent layer that clings to the liquid and slows evaporation long enough to knock down the flame. That’s why 20:1 is the typical choice for effective control and extinction of these fires with low expansion foam. A much lower ratio like 2:1 would produce a thinner foam that doesn’t blanket well, while much higher ratios (50:1 or 200:1) produce lighter foams that are less effective at sealing the surface and cooling, and are used in different, less common scenarios.