Which of the following is not considered an apparatus mounted foam proportioning system?

The correct choice highlights that fire extinguishers are not considered an apparatus mounted foam proportioning system. Generally, foam proportioning systems are integrated into fire apparatus specifically designed for firefighting, where the foam concentrate is mixed with water under controlled conditions to create a foam solution that can effectively combat certain types of fires.

Engine-mounted systems represent a key example of these, as they are built into fire engines to proportion foam automatically during firefighting operations. Water tanks, although essential components of fire apparatus, are primarily for water storage and do not function as proportioning systems. Portable mixers, while they can mix foam with water, are not mounted systems and are instead used in various non-vehicle applications.

Thus, fire extinguishers stand apart as they serve as standalone firefighting tools rather than as integrated systems for foam proportioning on firefighting apparatus. They may contain foam agents but do not incorporate the automatic mixing process characteristic of apparatus-mounted systems.