﻿16 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE. 



HUMIDITY MEASURING INSTRUMENTS. 



Since the humidity in the air determines the drying characteristics 

 at any given temperature, the control of humidity in the kiln is of 

 prime importance* It is essential that the moisture be removed 

 from the wood surface at the maximum safe drying rate. If the 

 humidity is too low the wood will dry too fast and will be injured; 

 if the humidity is too high the drying will be slow and expensive. 



Humidity is measured by an instrument variously named a 

 hygrometer, a psychrometer, or a wet and dry bulb thermometer. 

 Modifications of the instrument adapted to dry-kiln use bear trade 

 names, but the principle underlying all modifications is the same. A 

 wet surface exposed to a breeze of nonsaturated air will be cooled a 

 certain amount by the evaporation of water from the surface. The 

 amount is constant for any given temperature and humidity. 

 Knowing the amount of cooling, called wet-bulb depression, and the 

 temperature of the air, the humidity can be determined by formula 

 or by reference to a humidity chart. The wet and dry bulb ther- 

 mometer consists of two separate thermometers, mounted on a panel. 

 One, the dry bulb, registers the temperature of the air; and the 

 other, the wet bulb, register's the air temperature minus the wet-bulb 

 depression. The wet bulb is equipped with a silk or muslin wick 

 dipped in a water reservoir. The wick surrounds the bulb of the 

 thermometer and keeps it Avet by drawing water from the reservoir. 

 The evaporation of this water from the bulb produces the cooling or 

 wet-bulb depression. 



To obtain accuracy it is essential that the wicks be clean and that 

 there be a brisk circulation of air over the wet bulb. A velocity of 

 at least 15 feet per second is recommended by various authorities, 

 but this is more than is needed, except for the most accurate work. 

 With certain types of wet and dry bulb thermometers circulation is 

 produced by whirling the entire instrument. Such instruments are 

 known as sling psychrometers. Other instruments are provided with 

 maximum-reading thermometers, so that they can be removed from 

 the kiln and read outside. The mercury or fluid column in these 

 thermometers must be shaken down before they are replaced in the 

 kiln. They record only the maximum wet and dry bulb temperatures 

 since they were last shaken down. If the temperature and humidity 

 variations have been reasonably great during this time the readings 

 will be deceptive. 



Table 1 is a humidity chart for use with wet and dry bulb ther- 

 mometers. It is based on the difference between the wet-bulb and 

 dry-bulb temperatures. The dry-bulb temperatures are in the left- 

 hand column and the difference between wet and dry bulb tem- 

 peratures in the top row. The relative humidity is given at the in- 

 tersection of the row and column. Suppose the dry bulb reads 140° 

 F. and the wet bulb 130° F., the difference between them is 10°. By 

 reading across the 140 row to column 10 the relative humidity will 

 be found to be 75 per cent. 



Most of the kiln humidity recorders are wet-bulb instruments with 

 extension tubes. They differ from dry -bulb recorders (recording 

 thermometers) in that the sensitve bulb in the kiln is provided with 

 a wick and a water reservoir. Any type of recording thermometer 



