想問"濕球温度”的原理

Wet bulb temperature (濕球溫度) is measured by a wet-bulb thermometer (濕球溫度計).

The principle of a wet-bulb thermometer is based on the evapouration of water during which latent heat is absorbed. A wet-bulb thermometer has a piece of cotton wrapped around the temperature sensitive region of the thermometer (this is the mercury bulb in a mercury-in-glass thermometer or the platinum sensor in a platinum thermometer. In fact, these are the two types of thermometer used by the Hong Kong Observatory in wet-bulb temperature measurement). The piece of cotton is dipped into a small vessel containing water, which wets the cotton piece.

The rate of evapouration of water in the cotton piece depends very much on the humidity of the surrounding air. When the humidity is high, the evapouration rate is slow. But when the surrounding air is dry, the evapouration rate is fast.

Because latent heat (潛熱) is absorbed by water in it changes from liquid to gas state. Hence during evapouration, much heat is absorbed from the cotton piece and lowers its temperature. The thermometer then records this temperature. This is the wet-bulb temperature.

The difference between the wet-bulb temperature and the dry-bulb temperature (乾球溫度)(measured by an ordinary thermometer) gives a measurement on which the relative humidity (相對濕度) of air is estimated.


2014-12-10 15:31:26 補充：
Q:棉花入面水分蒸發，不是應該係棉花吸收潛熱嗎？
A: It is the water that requires latent heat to vapourize. The heat is absorbed by the water and which comes from the cotton wrap.

2014-12-10 15:34:33 補充：
Q:既然吸收了潛熱，不是應温度上升嗎？為何棉花反而温度下降？
A: As said above,the heat comes from the cotton and is given to the water to make it vapourize. Thus the cotton wrap loses heat and its temperature decreases. The thermometer records such decrease of temperature.

水分蒸發(即是水份增加左動能)，帶走左棉花內之潛熱；既然如此，棉花點解會吸收了潛熱??又點解會温度上升????
棉花在此祇是一種媒介，提供水薄膜包裹濕球、同時提供最大與空氣的接觸面、提供最大的蒸發效果。

CLAP.



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你錯了，正名是乾濕球濕度計，結構上有兩支相同溫度計，一支被濕棉花包裹，叫做濕球，另一支冇好自然叫做乾球，濕球量度溫度會較低，所以兩支讀數有相差，如果大氣濕度高，濕球棉花水份揮發慢，乾濕球溫差大，相反你會明白，不必畫公仔。