What method of foam proportioning involves using an external pump to force foam concentrate into the fire stream?

The method of foam proportioning that involves using an external pump to force foam concentrate into the fire stream is known as injection. This technique is particularly effective because it allows for precise control of the foam concentrate being added to the water stream, ensuring a consistent mixture as it is delivered to the fire.

Injection systems typically utilize a pump to create pressure that pushes the foam concentrate into the main water stream. This is advantageous in scenarios where high foaming efficiency is required, especially in situations demanding a rapid response to a fire. By using an external pump, the system can ensure that the foam is proportioned accurately regardless of fluctuations in water supply or pressure.

Other methods, like batch mixing or direct application, do not utilize an external pump for mixing the concentrate into the stream. Batch mixing combines a set volume of foam with water in a tank before delivery, while direct application involves applying foam directly to the surface of the burning material without any mixing process. Induction involves aspirating air into the foam concentrate, mixing it with water without a dedicated pump. Thus, injection stands out as the correct answer for its efficiency and controlled application in fighting fires.