112 MODERN FRUIT MARKETING 



To determine the relative humidity in the storage 

 rooms two methods can be employed. The first and most 

 common one is the use of the whorl psychrometer (Fig. 

 88). This consists of a wet and dry bulb thermometer 

 similar to the ones used in the observation stations of the 

 Government. These are arranged so a whirling motion 

 can be given them, and then by noting the different 

 temperatures between the two bulbs, and referring to a 

 table, the amount of moisture can be calculated. Such an 

 instrument costs about $5, and has to be used carefully 

 and skillfully to give satisfaction. 



The other method is to use a self-recording machine 

 known as the hydrograph. The principle of this instru- 



Fig. 88. SLING OR WHORL PSYCHROMETER 



ment is to make a continuous record on a revolving disk, 

 recording during the day the humidity at any given time. 

 The principle of the marker is the expansion of horse 

 hairs due to the amount of moisture in the air. An 

 instrument of this type would cost about $60, and where 

 temperatures run below zero would be of no value. 



Controlling Humidity. It is not always easy to 

 control the humidity in storage houses. If it runs too 

 high certain absorbents are used to take up the surplus 

 moisture. The most common of these are lime and cal- 

 cium chloride. This is the same form of calcium chloride 

 as is used in the pipes for the ice-brine cooling system. 



