Which device mixes foam concentrate in the water stream in the proper percentage?

The correct choice is the proportioner, as it is specifically designed to accurately mix foam concentrate with water at predetermined ratios. This process is essential for ensuring that the foam produced has the correct consistency and effectiveness for extinguishing fires, particularly those involving flammable liquids.

A proportioner can operate in various configurations, such as fixed or variable, and can be installed in fire suppression systems or used in mobile firefighting equipment. Its functionality is critical because improper mixing could lead to ineffective foam or insufficient fire suppression capabilities.

Other devices mentioned, like mixers and aspirators, serve different purposes within foam and fire protection systems. Mixers are often used for blending substances, aspirators are designed to draw foam concentrate into the water stream through a venturi effect, and aerators focus on introducing air into the foam solution to create a foam blanket. However, none of these devices specifically focus on achieving the precise percentage mixing of foam concentrate and water as effectively as a proportioner does.