Psycrometry is used to work out different properties of moisture in air. A tool used for many years by heating engineers is to refer to Psycrometric chart. A typical application may be to work out what happens when air of a particular temperature and humidity from one part of an air conditioning system mixes the air from another part that has a different moisture content. There are loads of these charts available for downloading from the internet, as well as online calculators to save having to manipulate charts by hand.
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You would not think, by the look of them that these charts make it easy for people working with moisture parameters. This perhaps gives you indication of the complexity of the physical chemistry and mathematics of the subject.
In the Psycrometric chart, there is two sections, (winter comfort zone) and (summer comfort zone). These zones cover the range of temperature and relative humidity set out in guidelines from the American society of heating, refrigerating and Air-Conditioning Engineers (ASHRAE)  for air quality within building and homes for human comfort. Although not included in ASHRAE title, engineers also have to consider ventilation for larger buildings (HVAC) and for designing homes. The external temperature and humidity have a massive influence on the challenges faced to the maintain a satisfactory living and working environment.
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If you pick a temperature say 84°F, let us follow the reddish brown line going left until you reach the (10% relative humidity) line as indicated by the white arrows, drop a vertical line down and you can read (approximately) that the temperature is 76°F. back up again and keep following the reddish brown until you reach the 20% line and drop down again. The temperature is 70°F.
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I’ve heard in Ireland, and particularly on the west coast, buildings are in front line of moist air driving in off the Atlantic in the prevailing westerlies. These buildings are at least twice as challenged by humidity than buildings in central England.
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A building in London
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An interesting new approach is to build intelligent buildings that will monitor the internal relative humidity and adjust the level to within a defined range. This possible now by development of accurate and more precise digital humidity measurement that can function to feed back to humidity controlling devises.
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