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surfaces are observed to be wet. Leaf wetness may be due to conden- 

 sation (dew) or precipitation (rain), and the sensing apparatus can 

 be adjusted to record only during periods of precipitation on leaf 

 surfaces or to record periods of condensation in addition to precipi- 

 tation. The degree of sensitivity required, of course, will be deter- 

 mined by the type of data that must be recorded. Calibration is 

 accomplished by altering the tension of the sensing element. Cali- 

 bration can be achieved through actual observations of rainfall or 

 'artificially' by spraying water on the sensing element and foliage. 



Calibrating for precipitation. 



Rainfall procedure. In the rainfall procedure, tension of the 

 element is adjusted when a given amount of rain (such as 0.25 mm) 

 is recorded by the digital rain gauge. Before the designated amount 

 of rain has fallen, press the sensor indicator switch. The LED should 

 not glow. If it does, turn the sensing element tension adjustment 

 knob counterclockwise until the LED stops glowing. When the 

 designated amount of rain has fallen, turn the tension-adjustor 

 knob clockwise slowly until the light glows, indicating that the 

 sensor has made electrical contact and the unijunction assembly 

 and LWI pen arm relay are activated. The sensor is now calibrated 

 to detect the beginning of a precipitation period. 



The sensor should be checked throughout the wet period to make 

 sure that electrical contact is maintained until the plant parts are 

 dry. This is done by pushing the sensor-indicator switch and check- 

 ing the LED to make sure it is glowing. If it isn't glowing, turn the 

 tension-adjustor knob clockwise until it does glow. The precipitation 

 period ends when the foliage or other plant parts are considered dry, 

 and this may be determined visually or through touch, i.e. running 

 your hand along the plant and checking the palm for moisture. At 

 the time it is determined that the leaves are dry, the LED should not 

 glow when the switch is pressed. The sensor is now calibrated to 

 detect the end of a wetting period initiated by precipitation. 



It is recommended that the LWI be checked the next few times 

 precipitation occurs to make sure the sensor is functioning properly, 

 adjusting tension on the sensing element when necessary so that 

 the trace on the chart corresponds to actual periods of leaf wetting. 



Artificial wetting procedure. During dry weather, calibration 

 can be achieved by using an atomizer to thoroughly wet the sensing 

 element and parts of the plant and then adjusting tension on the 

 sensing element to correspond to the wetting and drying time of the 

 plant parts. This procedure may not be as accurate as the rainfall 

 procedure, so the sensor should also be checked during periods of 

 actual precipitation and adjustments made when necessary. This 

 procedure is useful, however, when establishing the instrument in 



