Which foam proportioner test measures the velocity of light that travels through foam and compares it to a base reading?

The foam solution refractivity test measures the velocity of light as it travels through foam and compares it to a base reading. This method is used to determine the correct concentration of foam solution, as the refractivity of the foam solution will vary based on the amount of concentrate mixed with water. When light passes through different media, such as air, water, or foam, its velocity changes depending on the medium's density and composition. By measuring these changes in light velocity, this test can effectively assess how well the foam is being formed and whether it meets the required standards for effectiveness in fire suppression.

The refractivity test is particularly important in ensuring that the foam solution is at the optimal concentration to provide the necessary properties for extinguishing fires. Other methods mentioned focus on different aspects of foam testing, such as displacement, conductivity, or pump discharge volume, but do not utilize the principle of light velocity through foam, which is unique to the refractivity test.