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Wet & Dry Thermometers
A Wet & Dry Bulb thermometer consists of two thermometers, one of which is dry and one which is kept moist with distilled water on a sock or wick is called wet. The two thermometers are thus called the dry-bulb and the wet-bulb. At temperatures above the freezing point of water, evaporation of water from the wick lowers the temperature, so that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer. When the air temperature is below freezing, however, the wet-bulb is covered with a thin coating of ice and may be warmer than the dry bulb. Relative humidity is computed from the difference in temperatures as shown by the wet-bulb and ambient temperature as shown by the dry-bulb thermometers. Relative humidity can also be determined by locating the intersection of the wet and dry-bulb temperatures on a psychrometric chart. Psychrometers are commonly used in meteorology, and in the HVAC industry for proper refrigerant charging of residential and commercial air conditioning systems.
The whirling psychrometer whirling hygrometer, where the thermometers are attached to a handle or length of rope and spun around in the air for a few minutes, is sometimes used for field measurements. It consists of two thermometers, one which is dry and one which is kept moist with distilled water on a sock or wick uses the same principle as that of WET & DRY thermometers , however the two thermometers are fitted into a device that resembles a ratchet or football rattle
The Assmann Psychrometer consists of two similar sensitive mercury-in-glass thermometers having range of -10 to + 50°C mounted side by side in highly polished metal frame . The bulb of one of the thermometers is covered with thin muslin which is moistened with distilled water before use . The thermometers are suspended in the frame with their bulbs surrounded by two thin coaxial metal tubes which are chromium plated and highly polished , they are thermally insulated from the rest of the frame by retaining pieces and protect the bulbs from the effects of exposure of solar radiation . The tubes may be removed from the frame by unscrewing the retaining pieces. Ventilation of the bulbs is provided by the fan driven by the clock work mechanism encased in the housing at the top,air being drawn past the bulbs and up the hollow central column. The clockwork mechanism runs for about 10 minutes on one winding and provides adequate ventilation of the bulbs for about 5 minutes . The frame complete with thermometers may be unscrewed from the housing whenever required .