Flow switches detect and control media, such as gases, liquids, or vapors. Switches start in a neutral position, such as vertical, extended, or hot. In the neutral position, the switch's circuit is closed. Flowing media moves or cools the sensor, opening the circuit. In low or no flow conditions (or another setpoint condition, such as flow or excess flow), switches revert to a neutral position, closing the circuit, and signaling other equipment within the system to execute functions, such as turning pumps on or off, opening or closing flow control valves, and sounding alarms. Paddle flow switches use mechanical energy to control or monitor flow. Piston flow switches also use mechanical energy to monitor flow but are better in low-flow liquid and gas applications than paddle-style sensors. Thermal-dispersion flow switches use heated sensors and energy differentials created by liquid cooling the sensor to monitor flow. They require less maintenance than paddle and piston-flow switches.