Operator Manual: Liquid Water Content Sensor (LWC)

The LWC-300/301 is the DMT implementation of an instrument originally developed by Warren King at the Australian CSIRO (see Appendix A: References). It is used for measurement of cloud liquid water content. This sensor, oftentimes referred to as the “King Probe”, is used primarily for the study of cloud micro-physical processes, aircraft icing, aircraft icing certifications, and cloud seeding.

The LWC-300 operates under the principle that liquid water content can be calculated from measuring the heat released when water droplets are vaporized. A heated cylinder is exposed to the airstream and intercepts oncoming droplets. The electronics maintain this sensor at a constant temperature (approximately 150o C) and monitors the power required to regulate the temperature as droplets vaporize. This power is directly related to the amount of heat taken away by convection plus the heat of vaporization. The convective heat losses are known empirically and vary with airspeed, ambient temperature and ambient pressure. The liquid water content is calculated from total power requirements minus convective power losses.

The system consists of a sensing coil held in the external air stream by a heated strut and an electronic control box with digital signal processor. Customers who have purchased the optional Particle Analysis and Display System (PADS) also receive a computer with PADS software installed. The electronic control box uses a serial RS-422 protocol to deliver Analog, Digital, or a simultaneous delivery of both Analog and Digital via a 10-pin connector to communicate with the PADS software, which displays the amount of power consumed in keeping the wire at the constant temperature, the calculated LWC, and several housekeeping parameters.