ods, the tracing on the chart will be too congested to allow an 

 accurate determination of the amount of rain that has fallen (note 

 the extended solid rain-intensity line in Fig. 1-A). 



The rain collector should be inspected periodically to make sure 

 that no debris has collected in the orifice. It is also recommended 

 that the collector and the tipping-bucket be washed periodically 

 with a mild detergent to maintain a smooth, clean surface for the 

 rain to drain properly. Dirty and greasy surfaces will affect the 

 accuracy. 



Leaf Wetness Indicator 



The leaf wetness indicator (LWI) apparatus does not measure 

 leaf surface wetness directly; rather, it provides an indication of the 

 time interval when leaf surfaces are wet through the wetting and 

 drying of a sensing element. The LWI is not attached directly to the 

 hygrothermograph, and this allows considerable flexibility in locat- 

 ing the sensing element within the canopy or cover of a wide range 

 of crops. Thus, it can be placed within a field of wheat or the canopy 

 of an apple tree (see cover) or a potato plant. The only major 

 requirement is that no plant parts, soil, or other objects contact the 

 sensing element. The LWI is comprised of the following components: 



1. Wetness sensing element (a. Fig. 4) 



2. Sensor relay (b, Fig. 4) 



3. Sensing element tension-adjustor (c, Fig. 4) 



4. LWI housing (d. Fig. 4) 



5. Sensor indicator assembly (e, f, Fig. 4) 



6. Unijunction assembly (circuit board and components; b, 

 Fig. 16) 



7. Unijunction accelerator switch (a. Fig. 16) 



8. Leaf wetness pen arm and pen arm relay (Fig. 1) 

 Sensing element. The wetness sensing element is a length of 



cotton chalk line with a solderless ring tongue lug crimped to each 

 end for attachment to the sensor relay and the sensing element 

 tension adjustor. Cotton chalk line is used by carpenters to mark 

 lines on wood surfaces. It was selected because it is responsive to 

 wetting and drying but does not need to be degreased. Other leaf 

 wetness indicators have used degreased hemp. This material works, 

 but hemp is hard to find and even harder to degrease since extrac- 

 tion apparatus is necessary. 



Operation of Sensing Element. The sensing element constricts 

 when it becomes moistened by rain or dew, causing the contact plate 

 in the sensor relay to be pulled toward copper contact Cj (Fig. 5-A). If 

 the contact plate (Cp) and contact Ci do make contact (Fig. 5-B), then 

 the electrical circuitry to the unijunction and LWI relay and pen arm 

 is 'closed'. The unijunction circuitry is now activated and the LWI 



