2 BULLETIN 1013, U. S. DEPARTMENT OF AGRICULTURE. 



tempered to 15 per cent moisture on the invisible losses occurring 

 during the milling, process and on the total yield of mill products ; 

 also the influence of the temperature of the air within the mill upon 

 the total yield of mill products. 



HUMIDITY. 



That the humidity of the atmosphere in the mill influences milling 

 results is commonly known to millers. This influence, however, the 

 miller is able to control to a certain extent by varying the tempering 

 or conditioning process applied to the wheat and the manner of 

 grinding and bolting, provided he can anticipate any change that 

 is likely to take place in the humidity of the atmosphere. Advance 

 information of the weather, however, can not always" be depended 

 upon ; consequently, the miller can not always have his wheat condi- 

 tioned and his mill adjusted properly for obtaining the highest de- 

 gree of efficiency possible. Certain atmospheric conditions are gen- 

 erally recognized as being more conducive to good milling results 

 than others; therefore, the installation of means for controlling the 

 condition of the atmosphere within the mill would give to the miller 

 a greater degree of control over the operation of his plant and there- 

 by improve his results. 



NORMAL MOISTURE CONTENT. 



Evidence that an approximate parallelism exists between the mois- 

 ture content of wheat and the relative humidity x of the atmosphere 

 lies in the well-known fact that the normal moisture content of air- 

 dry wheat is higher when stored in moist climates than when stored 

 in dry climates. The term " normal " as used in this bulletin means 

 that moisture content which is unaffected by continued exposure to 

 the prevailing atmospheric conditions. In other words, it is that 

 point at which equilibrium is established between the moisture con- 

 tent of the wheat and the humidity of the air. According to experi- 

 ments by Stockham, 2 the moisture content normal for different wheats 



1 The amount of water vapor which can be mixed with a given quantity of air varies 

 with its temperature ; the higher the temperature, the greater the amount of water vapor 

 it can hold. When air at a given temperature contains all the moisture possible for it 

 to hold, it is said to be saturated. The percentage or ratio of the actual amount of 

 moisture contained by a cubic unit of air to the amount which the same air would hold 

 at the same temeprature if saturated is called the relative humidity. Relative humidity 

 is usually stated in terms of percentage. 



Since the capacity of the air for water vapor increases proportionately with tem- 

 perature, every increase in temperature, assuming that the quantity of water vapor in 

 the air remains the same, results in a lowering of the relative humidity, and every 

 decrease in temperature results in a proportionate increase of relative humidity until 

 the saturation point is reached, beyond which condensation occurs. 



2 Stockham, W. L., The Capacity of Wheat and Mill Products for Moisture. North 

 Dakota Agr. Exp. Sta. Bui. No. 120, 1917. 



