Dept. Bull. 1136, U. S. Dept. of Agriculture PLATE 4 
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EXTENSION-T UBE RECORDING PSYCHROMETERS 
A.—A recording psychrometer is a 2-pen recording thermometer having a water box or a porous sleeve 
for one of the bulbs. Theinstrument illustrated, the vapor-filled type, is provided with an overflow-type 
water box and a wet wick. The wick, which incloses the wet bulb and dips into the water in the water 
box, is directly behind the dry bulb in the illustration. 
B.—This instrument, which is intended for the same use as the one in A, is equipped with a porous 
sleeve in place of the water box. In the illustration the porous sleeve, which incloses the wet bulb, is 
shown directly below the dry bulb. Water under slight pressure is fed to the inside of the sleeve, filling 
the annular space between the sleeve and the wet bulb. Then, seeping out through the porous wall, it 
evaporates from the outer surface, thus producing the drop in temperature commonly called ‘‘wet-bulb 
depression.’’ The flow of water through the sleeve must be sufficient to keep its outer surface wet. 
