The hygrothermograph sensing units monitor and record tem- 

 perature and relative humidity continuously. When it rains, precipi- 

 tation collects in the rain gauge and every 0.25 mm of rain is 

 recorded on a counter (ag, Fig. 2) and on the hygrothermograph 

 chart (rain-intensity line, Fig. 1). The counter indicates the amount 

 of rain that has fallen while the rainfall intensity traced on the chart 

 provides a record of the distribution of rain with time. The length of 

 time (hrs) the leaves remain wet is measured indirectly by the leaf 

 wetness indicator, a string device that simulates leaf wetness. Pre- 

 cipitation increases tension on the string mechanism, causing an 

 electric circuit to be completed which results in a momentary drop in 

 the leaf wetness pen arm every 5 to 7 minutes the string device 

 remains activated. The pattern traced on the chart during wetness is 

 a thick line which is solid during periods of heavy rainfall (Fig. 1). 

 Tension on the sensing mechanism can be adjusted to include con- 

 densation (dew) in addition to precipitation (rain). 



DESCRIPTION OF MAIN COMPONENTS 

 Hygro thermograph 



The hygrothermograph is a portable instrument designed to 

 furnish accurate, continuous records of temperature and relative 

 humidity which are traced by separate pen arms onto a 7-day rec- 

 tangular chart marked-off at two-hour intervals. The chart is 

 mounted on a cylinder which revolves once in 176 hrs (weekly). A 

 spring-wound clock mechanism drives the cylinder. In the model 

 pictured (Fig. 1), the temperature-response unit consists of a Bour- 

 don tube mounted at the right front of the instrument and its asso- 

 ciated lever system. The humidity-sensitive unit consists of a hair 

 element mounted at the right rear of the instrument with the asso- 

 ciated lever system. The instruction manual should be consulted for 

 further details on operation, maintenance, and repair. 



Felt tip pens that snap onto the pen arms should be used in place 

 of the standard tips with ink wells that must be continually filled 

 with ink. The quick response of the leaf wetness indicator and 

 rainfall-intensity pen arms will cause the ink in the standard tips to 

 spatter. Also, the standard tips tend to clog and run dry if not 

 attended to regularly, whereas the felt tip pens require no servicing 

 during the growing season. 



Calibrating the hygrothermograph. It is advisable to check the 

 calibration of the temperature and relative humidity units each time 

 the chart is replaced. Do not remove the hygrothermograph from the 

 shelter, and make sure there is no direct sunlight shining on the 

 sensing elements. 



