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A psychrometric chart is a graph of the thermodynamic parameters of moist air at a constant pressure, often equated to an elevation relative to sea level. The ASHRAE-style psychrometric chart, shown here, was pioneered by Willis Carrier in 1904. [10] It depicts these parameters and is thus a graphical equation of state. The parameters are:
English: This psychrometric chart represents the acceptable combination of air temperature and humidity values, according to the PMV/PPD method in the ASHRAE 55-2010 Standard. The comfort zone in blue represents the 90% of acceptability, which means the conditions between -0.5 and +0.5 PMV, or PPD < 10%.
T g = Globe thermometer temperature (measured with a globe thermometer, also known as a black globe thermometer) T d = Dry-bulb temperature (actual air temperature) Temperatures may be in either Celsius or Fahrenheit; Indoors the following formula is used: = +
The wet-bulb temperature is the lowest temperature that may be achieved by evaporative cooling of a water-wetted, ventilated surface.. By contrast, the dew point is the temperature to which the ambient air must be cooled to reach 100% relative humidity assuming there is no further evaporation into the air; it is the temperature where condensation (dew) and clouds would form.
The psychrometric constant relates the partial pressure of water in air to the air temperature. This lets one interpolate actual vapor pressure from paired dry and wet thermometer bulb temperature readings.
Relative humidity (RH) is computed from the ambient temperature, shown by the dry-bulb thermometer and the difference in temperatures as shown by the wet-bulb and dry-bulb thermometers. Relative humidity can also be determined by locating the intersection of the wet and dry-bulb temperatures on a psychrometric chart. The dry and wet ...
The dry-bulb temperature (DBT) is the temperature of air measured by a thermometer freely exposed to the air, but shielded from radiation. [1] The dry-bulb temperature is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. It is directly proportional to the mean kinetic energy of the air ...
The results are displayed on a psychrometric or a temperature-relative humidity chart and indicate the ranges of temperature and relative humidity that will be comfortable with the given the values input for the remaining four parameters. [44] The PMV/PPD model has a low prediction accuracy. [45]