These devices activate a boat alarm (not included) upon loss of raw cooling water flow to the engine.
Made of PVC and designed for marine applications, they use a 2-wire, non-polarity switch that is normally closed (normally open models are available).
The switch opens, and the alarm shuts off, as long as the flow of water exceeds the minimum flow rate.
While heat exchanger cooled engines will run several minutes without raw water flow before overheating, during that time significant damage can be done to the engine, raw water pump and exhaust system. Monitoring raw water flow is a prudent step toward the safe operation of your engine.

Monitors the flow of raw cooling water to engines, generators, or bait tanks
Prevents costly engine damage when raw water intake is blocked by debris in the water
Sets off alarm when flow rate falls below minimum - flow rate to activate the alarm is adjustable, and is shown in the Table
Note that #13209 is non-adjustable and is factory set to activate at flows less than 1.5 gpm; it is designed for small engines, gensets, etc.
Can be used with bell alarm (AQA#20100) or buzzer alarms (AQA#20106), etc., but if the alarm draws more than 0.5 amps, you must use a relay switch to activate the alarm
Multi-voltage - works with 12, 24, and 32 volt systems