What method is used to determine the foam percentage in an apparatus foam system test?

The method used to determine the foam percentage in an apparatus foam system test involves using a refractometer. A refractometer is specifically designed to measure the degree of refraction of light as it passes through a liquid, which changes based on the concentration of the solution. In the context of foam solution, the refractometer can provide a precise reading of the foam concentrate's concentration in the water, effectively translating to the foam percentage. This method is reliable and allows for accurate measurement, which is essential in ensuring that the foam meets the necessary standards for firefighting effectiveness.

Other methods listed, such as using a flow meter, visual inspection, or a density gauge, do not provide the same level of accuracy or direct measurement of foam percentage. A flow meter measures the volume of liquid passing through, but it does not give information about the foam solution's concentration. Visual inspection is subjective and can lead to inconsistent results. A density gauge could give some insight into the physical properties of the foam solution, but it does not directly measure the foam percentage in the same way a refractometer does.