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The representation is made on a temperature-relative humidity, instead of a standard psychrometric chart. The comfort zone in blue represents the 90% of acceptability, which means the conditions between -0.5 and +0.5 PMV, or PPD < 10%.
The graphic method utilizes an overlay on a psychrometric chart to indicate the operative temperatures and humidity at which thermal comfort is achieved in the winter (1.0 clo) and summer (0.5 clo). It is based on the Predicted Mean Vote (PMV) model. [4]
The recommended level of indoor humidity is in the range of 30–60% in air conditioned buildings, [34] [35] but new standards such as the adaptive model allow lower and higher humidity, depending on the other factors involved in thermal comfort.
By comparison, thermal comfort standard ASHRAE 55 requires systems designed to control humidity to maintain a dew point of 16.8 °C (62.2 °F) though no lower humidity limit is established. [ 44 ] Water vapor is a lighter gas than other gaseous components of air at the same temperature, so humid air will tend to rise by natural convection .
If all the other factors influencing humidity remain constant, at ground level the relative humidity rises as the temperature falls; this is because less vapor is needed to saturate the air. In normal conditions, the dew point temperature will not be greater than the air temperature, since relative humidity typically [5] does not exceed 100%. [6]
There are six tables; four are used for entering climatic data, for comparison with the requirements for thermal comfort; and two for reading off appropriate design criteria. [6] A rough outline of the table usage is: Air Temperatures. The max, min, and mean temperatures for each month are entered into this table. Humidity, Precipitation, and Wind.
In the recent past, it was common for house temperatures to be kept below the comfort level; a 1978 UK study found average indoor home temperatures to be 15.8 °C (60.4 °F) while Japan in 1980 had median home temperatures of 13 °C (55 °F) to 15 °C (59 °F).
For locations at not more than 2000 ft (600 m) of altitude it is common practice to use the sea-level psychrometric chart. In the ω-t chart, the dry bulb temperature (t) appears as the abscissa (horizontal axis) and the humidity ratio (ω) appear as the ordinate (vertical axis). A chart is valid for a given air pressure (or elevation above sea ...