additional pen arm (Fig. 1-B). The four-pen arm design has the 

 advantage of being adaptable to almost any hygrothermograph; 

 however, one disadvantage is that more time and labor are required 

 for its construction compared to the three-pen arm design. The three- 

 pen arm design has a major disadvantage in that only the hygro- 

 thermograph pictured, or one of similar construction, can be used, 

 since it must be constructed to accommodate the solenoid attach- 

 ment shown in Fig. 20. An advantage of this design is that it is 

 easier to construct since only one additional pen arm assembly is 

 required. 



The step-by-step instructions for construction of the two designs 

 are very specific, but they apply only to the hygrothermograph 

 model, rain gauge, and materials listed. The hygrothermograph 

 model is one we have found to be reliable and durable, but several 

 makes are available that can be substituted, especially when con- 

 structing the four-pen arm design. The digital rain gauge was 

 selected because it was relatively inexpensive, durable, and accu- 

 rate for our purposes; but again, similar instruments are available 

 that may be substituted. The materials listed were selected because 

 they were readily available at local hardware and electrical supply 

 stores. It is not likely that identical materials will be available in 

 other locations, and even different makes of hygrothermograph and 

 rain gauge may be selected. If so, then the detailed instructions and 

 materials specified here will serve primarily as a useful framework 

 from which modifications will be necessary for constructing this 

 instrumentation. 



PURPOSE 



The purpose of this bulletin is to provide written and illustrated 

 instructions for constructing and calibrating this instrumentation 

 and an explanation of its operation. 



BRIEF DESCRIPTION OF FIELD OPERATION 



T-he major components are a digital rain gauge and a leaf wet- 

 ness indicator coupled to pen arms on a hygrothermograph which 

 also monitors and records temperature and relative humidity (see 

 cover). The hygrothermograph must be placed within an instrumen- 

 tation shelter to shield the sensing elements from direct sunlight 

 and precipitation. The rain gauge must be located in an open area 

 free from any obstruction that would hinder rain from collecting in 

 the rain gauge. The leaf wetness indicatof should be placed within 

 the canopy of the crop, preferably in a location that would allow the 

 longest drying period following precipitation or dew. 



