The Hygrodeik.
—In Fig. 158 is shown a form of hygrometer known as a hygrodeik, by means of which atmospheric humidity may be determined without the use of the tables. In the figure the wet-bulb and dry-bulb thermometers are easily recognized. A glass water bottle W is held to the base of the instrument by spring clips which permit its removal to be filled with water. Between the thermometers is a diagram chart from which the atmospheric humidity is taken. An index arm, carrying a movable pointer P, permits the instrument to be set for any observed thermometer readings.
The index is really a graphical method of expressing the figures given in the table on [pages 202-203]. In the picture the wet-bulb thermometer reads 65°, the dry-bulb thermometer 77°. To determine the relative humidity under these conditions the movable arm is swung to the left and the pointer P placed on the left-hand scale at the line 65°. The arm is then swung to the right until the pointer touches the downward curving line beginning at 77°, the dry-bulb reading. The lower end of the arm H now points to the relative humidity, where 52 per cent. is indicated by the scale at the bottom of the index.
The same result is obtained from the table of Relative Humidity. The readings of the thermometers in the figure give a difference in temperature of 12°, the dry-bulb thermometer reads 77°. Referring this data to the humidity table, the column marked 12, for the depression of the wet-bulb thermometer and opposite 77° in the air temperature column, is found 53 which indicates the per cent. of saturation. The hygrodeik gives further the temperature of the dew-point, on the scale to the right; and the absolute humidity may be found by following the upward curving line nearest the pointer, at the end of which line is given the value in grains of moisture per cubic foot. The hygrodeik or other instrument of the kind is very largely used in places where relative humidity is regularly observed by those of limited experience, as in school-rooms, auditoriums, etc. Such records are not intended to be perfectly accurate and the readings of the hygrodeik are very well-suited for the purpose.
Fig. 159.—Psychrometer of U. S. Weather Bureau type; for accurate determination of atmospheric humidity.
In using the hygrometer and the hygrodeik the instruments are stationary; they are usually hung on the wall in a convenient location for observation and are placed to avoid accidental drafts in order that the conditions surrounding the observation may be the same at all times. The evaporation which takes place from the wet bulb is due to natural convection and does not always reach the maximum amount. The evaporation is furthermore influenced by accidental variations and consequently the results cannot be considered exact.
Under conditions that demand more exact humidity records than are obtainable with hygrometer, the psychrometer furnishes means of making more accurate observation. The psychrometer shown in Fig. 159 is of the form used by the U. S. Weather Department. Like the hygrometer, it is composed of a wet-bulb and a dry-bulb thermometer but no water cup is attached to the instrument for moistening the wick of the wet bulb. When ready for use the wick is wet with water before each observation.
The greater accuracy to be attained by the use of this instrument is on account of the maximum evaporation which is obtained from the wet bulb for any atmospheric condition. The evaporation which takes place from the wet-bulb thermometer in quiet air is not so great as that which occurs if the same air is in motion. In moving air, however, there is a certain maximum rate beyond which no further evaporation will take place.
The motion of the air may be produced either by blowing on the bulb with a fan or air blast, or by whirling the thermometer. With the psychrometer the latter method is used. This instrument is provided with a handle which is pivoted to the frame and about which it is swung to produce a maximum evaporation from the wick. When a motion of the air is attained sufficient to produce a saturated atmosphere about the bulb, the temperature will remain constant.
Fig. 160.—Dial hygrometer.
A velocity of air or the motion of the wet-bulb thermometer 10 feet per second is that usually taken as the rate for observation and the swinging is kept up 3 or 4 minutes or until the temperature of the wet-bulb thermometer remains stationary.
Then the temperature of each thermometer is read and the humidity found in the table. Relative humidity determinations may be made at temperatures below the freezing point if sufficient precaution is taken in the observations. When the instrument is not in use, it is kept in the metallic case shown in the picture, to protect it from injury.