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Psychrometrics

Calculate the thermodynamic properties of air.

Pa
°C
%
gwater/kgair
°C
°C
Pa
m3/kgdry air
J/kgdry air

Psychrometric Chart

Definitions:

Reference Pressure: Default value of 101325 Pa represents the standard atmospheric pressure (1 atm).

Dry Bulb Temperature: Temperature of air measured by a thermometer freely exposed to the air, but shielded from radiation and moisture. This is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature.

Relative Humidity: Ratio between the amount of water in the air and maximum water the air could potentially hold.

Absolute Humidity: Total mass of water vapor present in a given mass of air. Also sometimes called Humidity Ratio.

Wet Bulb Temperature: Temperature read by a thermometer covered in water-soaked cloth (wet-bulb thermometer) over which air is passed. At 100 % relative humidity, the wet-bulb temperature is equal to the air temperature (dry-bulb temperature). At lower humidity the wet-bulb temperature is lower than dry-bulb temperature because of evaporative cooling.

Dew Point Temperature: Temperature to which air must be cooled to become saturated with water vapor. When cooled further, the water vapor will condense to form liquid water.

Vapor Pressure: Pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system.

Specific Volume: Ratio of a substance's volume to its mass. It is the reciprocal of density.

Specific Enthalpy: Ratio of a system's enthalpy to its mass.

References:

D. Meyer and D. Thevenard (2019). PsychroLib: a library of psychrometric functions to calculate thermodynamic properties of air. Journal of Open Source Software, 4(33), 1137

American Society of Heating, Refrigerating and Air-Conditioning Engineers (2017). ASHRAE handbook fundamentals 2017: SI edition. Atlanta, GA: ASHRAE.