Hi Andy,
Maybe use a flow meter from Mantech. A little bit of electronics, such as a small transistor connected to the output pulse train, charging a small value cap would switch a relay on, enabling the UV circuit.
Very simple and cheap. Simply place a diode in series with the output pulse train, charging a 1uF capacitor, in turn connected to the base of a transistor like a BC337, and the emmiter to 0V supply, and the collector to the one side of the coil, and the other side of the coil connected to 12V supply. When the flow meter runs, the pulses would charge the capacitor up, and switching the transistor on, which in turn will switch the relay on. When the water stops flowing, the capacitor would discharge through the base of the transistor, switching the transistor off, and in turn switching the relay off.
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