Hot-Wire Liquid Water Sensor (LWC-300)

The hot-wire liquid water sensor features a coiled wire that is maintained at 150° C. This coil acts as a variable resistance in one arm of a Wheatstone Bridge circuit. The resistance of the sensing coil decreases as the wire temperature decreases. Temperature decreases can be caused by vaporization of water droplets or by convective heat losses to air that flows past the sensor. Heat losses by conduction from the ends of the master coil are minimized by slave coils that are maintained at the same temperature as the master coil and are located at each end of the master. The resistance of the sensing coil is directly proportional to its temperature; therefore, the control circuit maintains the sensor at constant temperature by maintaining it at constant resistance. A Wheatstone bridge is formed of four resistances, of which the master coil sensor is one. The power dissipated by the sensor is the product of the current through the sensor and the voltage drop across it. Once the power dissipated by the sensor is known, the effects of convective heat losses are estimated. The power dissipation due solely to vaporization can then be estimated, which in turn gives an estimate of liquid water content.