This CR21 version of the 237 Leaf Wetness Grid is retired. The 231 (pn 2375) primarily measured the percentage of time that a surface was wet versus dry. It was intended to emulate a leaf surface. The measurement was made by determining the electrical resistance on the surface (that is, a wet surface is less resistant). The 231 was shipped unpainted; the customer determined the appropriate paint to apply to the sensor's surface. The wet/dry transition point for the application was also determined by the customer.