The Assmann Psychrometer is a simple type of hygrometer used by observers to measure relative humidity. Two thermometers are used; one is called the dry bulb thermometer, and the other is called the wet bulb thermometer (so named because it has a wick which is soaked in distilled water snugly fit around its bulb). The bulb is the base of thermometers. The dry bulb has no wick (remains dry).
The photo I took on the right shows two pscyhrometer set ups. The one on the far right is the Assmann Psychrometer; note the hand crank at the top, shaped sort of like a large wingnut (hanging from a bolt). This crank handle is turned; winding up a small spring-loaded motor inside. When the spring tension is maximized, the handle is released and the motor draws air up from the bottom of the unit (note the two chrome tube bases at the bottom); this air passes over the bulbs (both the dry and wick-covered wet bulb) at a steady rate. As air passes over the bulbs, the dry bulb measures the ambient temperature, whereas the wet bulb measures the wet bulb temperature. What’s the difference with wet bulb temperature? Good question! As air passes over the wet bulb, the distilled (distilled to avoid mineral deposits on the wick over time, and for more accurate readings) water begins to evaporate.
The observer records the two readings (dry and wet bulb temperatures). It is recommended that the observer take 3 separate readings and compute the average. The difference between the two readings is called the depression. The depression and ambient temperature (dry bulb) are used in conjunction with a slide rule or graph to determine dew point temperature and relative humidity.
The two thermometers on the left of the Assmann Psychrometer work in the same way as the Assman Psychrometer, however, rather than a mechanical wind-up motor being used to draw air past the bulbs, an electric motor mounted outside the shelter is used. Some Assmann Psychrometers are electric and can be plugged into a wall outlet.
The small glass container at the bottom of the photograph is filled with distilled water for wetting the wet bulb’s wick. – Steve
Top: a better view of the Assmann Psycrometer. The base of the unit has an open design to allow air to be drawn through the tubes and past the bulbs of each thermometer. Photo is from:http://www.thermo-schneider.de/eng_thermoschneider.html
~ Steve Woodruff and Devin Lussier