High expansion ratio foam is typically used in confined spaces such as shipboard compartments basements mines and enclosed aircraft hangars at expansion ratios of:

• A. 2:1 to 10:1
• B. 10:1 to 20:1
• C. 200:1 to 1000:1
• D. 5:1 to 9:1

Answer: (C)

When foam is used in confined spaces, the goal is to fill the entire volume quickly and create a continuous foam blanket that cools, separates the fuel from the air, and restricts oxygen access. High expansion foam achieves this by generating a huge amount of foam from a relatively small amount of concentrate, producing extremely large expansion ratios. These very large ratios—hundreds to one, up to about a thousand to one—allow the foam to rapidly flood a space like a ship’s compartment, a basement, a mine, or an enclosed aircraft hangar, reaching around corners and behind equipment where water or nozzle reach alone couldn’t. The result is a blanket that blankets the fire, cools surfaces, and suppresses the fire more effectively in a closed volume.

Lower expansion foams produce denser, less volumetric foam that doesn’t fill large enclosed spaces as readily, and they’re better suited to direct application on a fire in open or semi-open areas or on surfaces. Medium expansion foams fall somewhere in between but still don’t flood a confined volume as effectively as high expansion foam. So for confined spaces, the ability to rapidly fill the room with a foam blanket makes the very high expansion ratios the best fit.