10 



4. Bond the plexiglass sections using ethylene dichloride. 



a. Begin assembly with plexiglass base secured to the 

 hygrothermograph. 



b. With an eyedropper, coat the bonding surfaces with a thin 

 film of ethylene dichloride. Bonding will occur within 

 minutes. 



c. Bond sides to base. 



d. Bond top to sides. 



e. Coat the edges of the LWI relay plate and the grooves of 

 the stage with ethylene dichloride, then slide the relay 

 plate into the grooves and align so that the plate extends 

 25 mm beyond the front edge of the stage (Fig. 11). 



Constructing the Leaf Wetness Indicator (LWI) 

 Modifying the Sensor Relay 



Materials 



(!) 12 VDC 4 PDT relay, rated 3A at 125 VAC (Radio Shack, 



Tandy Corp., Forth Worth, Texas 76102, U.S.A.; Model #275- 



214) 

 (1) 10 cm length of thin wire (connects to relay plate, for hook 



attachment) 

 (1)10 cm length of thin wire (to attach hook to wire cemented to 



relay plate) 

 (1) 5 cm length of 17 gauge wire (to fashion a hook) 

 (1) tube of silicone cement 



Instructions (refer to Figs. 12, 13) 



1. Temporarily remove plastic case from relay. 



2. Remove the solenoid by drilling through the rivet attaching 

 the solenoid to the relay housing. 



3. Cement a thin wire to the contact plate, and pass the wire 

 through the hole in the relay housing left from the removal of 

 the rivet (a, Fig. 12). 



NOTE: Make sure the wire is attached in such a way that it 

 will not short-out the contacts. 



4. Cut a slit in the plastic relay cover so that the wire may pass 

 through the slit. This will allow the cover to be removed 

 without disturbing the wire (b, Fig. 12). 



5. Fashion a hook from a 2 cm length of 17 gauge wire (a, Fig. 

 13) and attach to the end of the wire cemented to the contact 

 plate. Fasten the hook to the wire from the relay by winding 

 a 10 cm length of wire around the hook and relay wire (b, Fig. 

 13). The hook will later be attached to one end of the sensor 

 element. 



