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What is dew point ?
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Dew point indicates the amount of moisture in the air, the temperature at which water vapor is condensed on the surface of a material when atmospheric temperature is down under a given humidity and pressure.

At the higher locations, atmospheric pressure lowers but volume of air increases, thus lowering water vapor pressure. So the dew point in the air is lowered at about 0.2¡É per 100m.

Dew point is measured for dew condensation test of twofold glass. The lower dew point the difficult a dew condensation on the surface of a material.

On a float glass (in thickness of 5mm), dew condensation occurs when outdoor temperature is down to 8¡É at relative humidity of 60% and indoor temperature of 20¡É. Relative humidity at dew point is 100 % when vapor pressure is equal to saturated vapor pressure.

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Absolute humidity indicates the content of moisture in the air of 1m3, and is expressed in the unit of g/m3.
Relative humidity is defined as the ratio of water vapor pressure (content) to saturated vapor pressure. The higher the temperature, the more the relative humidity.
John Frederic Daniell : 1790 - 1845
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He was a chemist, meteorologist and inventor, born in London and interested in meteorology and chemistry from his childhood. He attracted public attention since he published meteorological essays covering the invention of dew point hygrometer while serving as a fellow of Royal Society at the age of 23.

Dew point hygrometer is a humidity sensor working on dew point, consisting of a cooling device that forms dew by condensing water vapor in the air and a thermometer measuring the temperature at which condensation occurs. Many metal plates are used for the cooling device, which includes various types, for example, one which dew point can be observed in naked eyes and the other which reflective light is observed through a photo cathode tube after projecting a beam of light onto the surface at which dew is formed. It is, however, difficult to correctly determine dew point with a dew point hygrometer that has less accuracy in the measurement. The hygrometer is used in measuring humidity in the air of relatively higher temperature. Nevertheless, it was widely used as an innovative invention.

He was interested in meteorological effect of solar radiation and the cooling of the Earth and discussed the movement of air in the trade wind zone by using an equipment he developed for himself.

Meanwhile, he invented a higher quality daniel cell than voltaic cell used in the early 1836, during his service as the first professor of chemistry in King's College, London. Especially, he studied the reason why the voltaic cell is powered down after current discharge and invented an improved cell that can provide current for a sustained period of time. Daniel cell is a cell where two electrolytes, copper sulfate and zinc sulfate, were fused, with zinc at anode and copper at cathode, and gives constant electromotive force of 1.10 V, thus providing an advantage of long-time use and not generating fluctuation in voltage and giving off noxious fumes, previously used for power source for telegraphic use. Daniel's cell paved the way to electrical research, thus enabling the inventor to be awarded the highest Copley medal from Royal Society.

In addition, he wrote down "Meterological Essay" (1823) and " Introduction to chemical philosophy researches" (1839).