The PM-224 and PM-824 are used to detect snowfall and low temperature to activate the ProMelt Mats or Cables. The PM-224 has a moisture sensor "fixed" to the top. The PM-824 has a remote moisture sensor, which allows mounting the control in a more convenient location. It can be mounted on a wall or inside the building and the sensor outside exposed to the snowfall, adding convenience of operation and adjustment. Both models offer automatic activation, rain/snow detection, adjustable trigger temperature, and a drying cycle, as well as "manual on" operation. Either control will operate in conjunction with our Contactor Pro series of ProMelt controls, which should be ordered separately. The unit measures approximately 5" x 7" x 3".