Harwil Q-12DS-C2-MK SD6560-857 Flow Switch with Tee Fitting. Since the mid 1980's the Harwil Q-12 flow switch has been used by some of the largest companies in the Pool & Spa and fluid control industries. The Q-12 has several factory adjustable parameters that provide the flexibility to meet the requirements of almost any application. The Q-12 verifies fluid flow, typically in conjunction with another device. For example most of swimming pool salt chlorine generators use the Q-12 to verify sufficient flow before the generator is energized. In a spa or hot tub the Q-12 ensures sufficient flow to prevent the water heater from a meltdown. In a chiller the Q-12 prevents the system from freezing. At home the Q-12 turns on the water disinfection system when someone is getting a glass of water.