- 6- 



A time -pattern transmitter controls humidity in the same manner 

 as that for temperature. This instrument passes a signal corresponding 

 to any desired humidity. As the follower arm on the cam cannot climb 

 a gradient of more than 70 percent and natural humidity patterns often 

 present greater rises, the linkage has been changed to give a full range 

 of signals using only the outer third of the regular cam. This signal is 

 transmitted to a humidity controller-recorder directly above it. A 

 humidity- sensing element in the aspirator box within the bioclimatic 

 cabinet measures the relative humidity therein and transmits it to the 

 control panel. This element, an electric hygrometer, is accurate to 

 within +2°F. of dew point through a range of 9 to 95 percent. 



Should the humidity be below that desired, the air signal from the 

 humidity controller will build up. This air signal is then transmitted 

 to a pneumatic -electric relay, which makes contact at 13.5 p.s.i. and 

 operates the steam solenoid valve. Should the humidity be above that 

 desired, the transmitted air signal decreases, and since this signal is 

 interconnected with the air signal from the temperature controller - 

 recorder through a diverting relay which transmits the lower output 

 from either instrument, the refrigeration cycle is brought into opera- 

 tion exactly as for temperature control. Complete saturation is possible 

 within a few minutes at temperatures lower than 100° F., and the 

 humidity can be lowered as much as 60 percent per hour. 



A steam-control valve that modulates the quantity of vapor flowing 

 through the steam solenoid is positioned by the transmitted air signals 

 from the time-pattern transmitters. 



A pneumatic -electric relay, connected into the air line from the 

 temperature transmitter, controls the compressor speed. High speed 

 is obtained when the temperature drops to 30° F. or below, and low 

 speed when it rises to 49°. 



Cycle Changing 



To provide for alternate operation and defrosting of the two air- 

 handling units, two time clocks are provided (fig. 6). The main con- 

 trol time clock alternates the operation from one air-handling unit to 

 the other after it has been in operation for any selected period. These 

 periods may extend from 1 to 12 hours. An electrical time -delay 

 relay in the circuit to the blower prevents it from starting for approxi- 

 mately 10 seconds after each cycle change. This permits the refrig- 

 erant to chill the coil or the heaters to warm up before the blower 

 starts, thereby eliminating undesired disturbances in chamber-air 

 control due to delays in performance of these units. 



The second time clock controls the operation of the defrost heaters 

 and auxiliary fans. Approximately 15 minutes after a cycle change this 



