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CHAPTER 17. OPERATION OF EQUIPMENT SECTION 7. TEMPERATURE AND HUMIDITY MEASURING EQUIPMENT When rain or snow is occurring, the observer shall dry the bulb, if wet, and shield it from the precipitation as long as necessary to permit dissipation of extraneous heat before reading it again. This reading shall be used for psychrometric purposes rather than the reading normally made when the lowest wet-bulb reading is taken. When frost forms on the thermometer, the observer shall remove the frost with a warm cloth and allow sufficient time for the dissipation of extraneous heat before reading the thermometer. The procedure used in moistening the wet-bulb varies according to whether the dry-bulb temperature is above freezing, near, or below freezing, and whether the relative humidity is high or low. 17-60. WET-BULB THERMOMETER - TEMPERATURE ABOVE FREEZING The observer shall moisten the wet-bulb with clean water just prior to ventilating the psychrometer (even though the humidity is high or the wick already appears wet). If, however, the temperature is high and the relative humidity is low, or it is expected that the final temperature of the wet-bulb will be 0 degrees Celsius or less, the observer shall moisten the wet-bulb thoroughly several minutes before taking a reading so that a drop of water will have formed on the end of the bulb. This procedure will reduce the temperature of the wet-bulb without danger of the wick drying out before the temperature reaches its lowest point. 17-61. WET-BULB THERMOMETER - HIGH TEMPERATURE AND LOW HUMIDITY In areas where the temperature is high and the relative humidity low, the observer shall use pre-cooled water for moistening the wet-bulb to avert premature drying of the wick. Water may be pre-cooled for this purpose by storing it in a porous jug. To avoid altering moisture conditions in the shelter, the observer shall not keep this jug in the shelter. If this method is not effective, the observer shall extend the wick from the wet-bulb to an open container of water and keep the end of the wick immersed in water between observations. When the psychrometer is ventilated, the observer shall remove the wick from the water until the wet-bulb thermometer has been read. Regardless of the methods used, the observer shall ventilate the psychrometer in accordance with paragraph 17-64, Psychrometric Ventilation, before determining the wet-bulb temperature. 17-62. WET-BULB THERMOMETER - TEMPERATURES BELOW FREEZING At wet-bulb temperatures below 0°C, if the wick is not frozen, the observer shall touch it with clean ice, snow, or another cold object to induce freezing. If freezing of the wick cannot be induced, the observer shall use the low temperature range of the psychrometric calculator for the computation of psychrometric data. (See paragraph 17-83, Using the Calculator.) 17-63. WET-BULB THERMOMETER - DRY-BULB TEMPERATURE BELOW 2.8 DEGREES CELSIUS At dry-bulb temperatures of 2.8°C or below, the observer shall use water that has been kept at room temperature in order to melt completely any accumulation of ice on the wet-bulb. The observer shall moisten the bulb thoroughly, at least 15 minutes before ventilating the psychrometer to permit the latent heat released if the water freezes to be dissipated before ventilation is begun. The observer shall not allow excess water to remain on the wet-bulb, since a thin, thoroughly cooled coating is necessary for accurate data. 17-64. PSYCHROMETRIC VENTILATION The observer shall ventilate the psychrometer for about 10 seconds. The minimum speed of air passing over the psychrometer bulbs should be 15 feet per second. This is approximately one revolution per second of the geared (2 to 1 ratio) whirling psychrometer crank, two revolutions per second of the sling psychrometer, and three and one-half revolutions per second of the crank of the psychrometer fan or motor (direct-drive) whirling psychrometer. 17-65. SLING PSYCHROMETER VENTILATION The observer shall ventilate the sling psychrometer as follows:
17-66. OBTAINING READINGS FROM SLING PSYCHROMETER After proper ventilation has been achieved, the observer shall quickly read both thermometers, wet-bulb first. The observer shall repeat this procedure until two successive wet-bulb readings are the same, indicating that the wet-bulb temperature has reached its equilibrium point. If the wet-bulb temperature rises between successive readings, the observer shall re-moisten the wick and reventilate. Accurate readings are especially important at low temperatures, where a given wet-bulb depression has a greater effect on the accuracy of psychrometer computations.
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