The Humidity Diagram
Fig. 93. The United States Forest Service Humidity Diagram for determination of Absolute Humidities. Dew Points and Vapor Pressures; also Relative Humidities by means of Wet and Dry-Bulb Thermometer, for any temperatures and change in temperature.
Some simple means of determining humidities and changes in humidity brought about by changes in temperature in the dry kiln without the use of tables is almost a necessity. To meet this requirement the United States Forestry Service has devised the Humidity Diagram shown in [Figure 93]. It differs in several respects from the hydrodeiks now in use.
The purpose of the humidity diagram is to enable the dry-kiln operator to determine quickly the humidity conditions and vapor pressure, as well as the changes which take place with changes of temperature. The diagram above is adapted to the direct solution of problems of this character without recourse to tables or mathematical calculations.
The humidity diagram consists of two distinct sets of curves on the same sheet. One set, the convex curves, is for the determination of relative humidity of wet-and-dry-bulb hygrometer or psychrometer; the other, the concave curves, is derived from the vapor pressures and shows the amount of moisture per cubic foot at relative humidities and temperatures when read at the dew-point. The latter curves, therefore, are independent of all variables affecting the wet-bulb readings. They are proportional to vapor pressures, not to density, and, therefore, may be followed from one temperature to another with correctness. The short dashes show the correction (increase or decrease) which is necessary in the relative humidity, read from the convex curves, with an increase or decrease from the normal barometric pressure of 30 inches, for which the curves have been plotted. This correction, except for very low temperatures, is so small that it may usually be disregarded.
The ordinates, or vertical distances, are relative humidity expressed in per cent of saturation, from 0 per cent at the bottom to 100 per cent at the top. The abscissae, or horizontal distances, are temperatures in degrees Fahrenheit from 30 degrees below zero, at the left, to 220 degrees above, at the right.
Examples of Use
The application of the humidity diagram can best be understood by sample problems. These problems also show the wide range of conditions to which the diagram will apply.