62 MANUAL OF MARINE METEOROLOGICAL OBSERVATIONS 



bulbs is at least 15 feet per second (9 knots). Psychrometer tables are 

 based upon this rate of ventilation, which must be maintained to obtain 

 accurate measurements. 



7131. PSYCHROMETER (portable, aspirated, electric). The aspirated 

 psychrometer was designed primarily for shipboard use. The instrument 

 consists of a pair of matched wet and dry-bulb thermometers, a ventilating 

 fan which is powered by three standard size "D" flashlight batteries. The 

 illumination and the "on- off" switching system is operated by a knob on the 

 side of the psychrometer case. Thermometers are shielded from external 

 heat radiation by means of a removable plastic air intake shield. (See Fig. 10.) 



7131.1. Care should be exercised when replacing the air intake ther- 

 mometer shield. Slide the shield in place with the wide open nozzles point- 

 ing outward from the instrument. Ventilation will be limited if replaced in 

 a reverse position. 



7132. Care of the Psychrometer. The bulb of the wet-bulb thermometer 

 is covered with muslin, which must be saturated with water before the 

 psychrometer is ventilated and a reading of the wet- bulb temperature is 

 made. Use only clean, fresh water, preferably distilled water or cooled 

 water from the condenser, to moisten the muslin. Change the muslin when- 

 ever it becomes soiled or encrusted with lime or salt, 



7133. Sling Psychrometer. The sling psychrometer is readily adapt- 

 able to shipboard use since it is easily carried to the most suitable exposure. 

 The dry and wet- bulb thermometers are mounted on a single frame that is 

 attached to a handle in such a way that the thermometers can be whirled in a 

 circle (see Fig. 9). Errors in reading arising from exposure to rain or 

 direct sunlif^ht are overcome, to a large extent, by the strong ventilation to 

 which the thermometers are subjected. When not in use, the psychrometer 

 should be stored at outdoor temperature in a safe, sheltered place (e.g., in 

 a padded box secured to the bulkhead). 



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Figure 9. --Sling psychrometer 



