What method is used to test foam solution concentrate?

The refractometer is the correct tool for testing foam solution concentrate because it measures the concentration of the foam solution based on how light refracts through the liquid. This instrument provides a quick and accurate way to determine the percentage of foam concentrate present in the solution. Accurately measuring the concentration is crucial for ensuring that the foam operates effectively during firefighting situations, as incorrect concentrations can impact performance.

The other methods listed do not serve this specific purpose effectively. A thermometer would be used to measure temperature, which does not provide information about the concentration of the foam solution. A hydrometer measures the density of liquids, which can indicate concentration but is not the ideal tool for foam solutions specifically, as it can be impacted by other factors. Finally, a pressure gauge measures the pressure within a system, which is unrelated to the concentration of the foam solution. Hence, the refractometer stands out as the appropriate choice for this application.