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Foam (Expansion Ratio)

Foam (Expansion Ratio)

The term "expansion ratio" or expansion rate refers to the volume of finished foam produced by a given volume of foam solution.

Expansion ratio is calculated by dividing the volume of finished foam by the volume of foam solution used to create it. For example, an expansion ratio of 5:1 means that one litre of foam solution, after aeration, will fill an empty 5-litre container with expanded foam.

Different types of fires and fire-fighting scenarios may require different expansion rates for optimal effectiveness.

  • Low expansion foams have an expansion ratio between 2:1 and 20:1 and are commonly used for fires involving flammable liquids, such as Class B fires.
  • Medium expansion foams have an expansion ratio between 20:1 and 200:1 and are often used in situations where a higher expansion ratio is required, such as in shipboard applications.
  • High expansion foams have an expansion ratio above 200:1 and are typically used in confined spaces, such as underground mines, where a higher expansion ratio is needed to fill the space quickly.

Below is a table that summarises the expansion rate and typical applications for each type of foam:

There are 3 expansion ratios depending on the foam proportioning and discharge devices usedBio-ex, Foam Concentrate in Low, Medium and High Expansion, September 5, 2022, https://www.bio-ex.com/en/our-expertises/low-medium-high-expansion/.

Foam Type Expansion Ratio Features Applications
Low-expansion foam < 20 Less than 20:1 Bubbles are very small, dense and heavy, and the foam is less sensitive to atmospheric conditions such as wind or rain. Low expansion foam is commonly used on hydrocarbon fires such as in tank storage, refineries, airports and onbroad ships.
Medium-expansion foam ≥ 20 to < 200 Between 20:1 to 200:1 The thick foam blanket quickly spreads above the fuel Medium expansion foam is used by fire brigades, for small surfaces such as road traffic accident, solvent storage or to control hazardous chemicals spill.
High-expansion foam ≥ 200 Greater than 200:1 In high expansion foam, bubbles are very light and large, and contain a high proportion of air. High Expansion is used on dry product fires or enclosed areas to drawn large volume surfaces and fill cavities such as warehouse, cellars, aircraft hangar or engine/pump rooms aboard ships. 


In preparing this definition, we have drawn from various sources including Legislation, Codes, Standards and industry information, research and knowledge.  Like the english language, these definitions may subtly change from time to time. As such these definitions are provided solely on the basis that users will be responsible for making their own assessment of the definition and and are advised to verify all relevant representations, statements and information.