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, 
