Humidity & Psychrometrics Calculator

A psychrometric chart is a graphical representation of the thermodynamic properties of moist air at a constant pressure. It plots dry bulb temperature on the x-axis against specific humidity on the y-axis and overlays lines for relative humidity, wet bulb temperature, enthalpy, and specific volume. HVAC engineers use it to design air conditioning, dehumidification, and ventilation systems — any process that changes the temperature or moisture content of air.

Dry bulb temperature is the ordinary air temperature measured by a standard thermometer. Wet bulb temperature is the temperature read from a thermometer wrapped in a wet wick and exposed to airflow — evaporative cooling lowers it below the dry bulb reading. The difference between the two (the wet bulb depression) indicates how dry the air is. When relative humidity reaches 100%, the wet bulb and dry bulb temperatures are equal.

Dew point is the temperature at which air becomes saturated and water vapour begins to condense on surfaces. In industrial settings, unwanted condensation on cold pipes, ductwork, and product surfaces causes corrosion, spoilage, and microbial growth. Knowing the dew point helps engineers set insulation thickness, select dehumidification equipment, and control cold room or cleanroom environments. Heat pump evaporators operating below the dew point must account for condensate drainage and defrost cycles.

ASHRAE Standard 55 recommends indoor comfort conditions of 20 to 26°C dry bulb temperature with 30 to 60% relative humidity. This corresponds roughly to a dew point range of 2 to 17°C and an enthalpy of 40 to 55 kJ/kg of dry air. Outside this range, occupants perceive the air as too dry, too humid, too warm, or too cold. A Karnot heat pump system with reheat and dehumidification can maintain these conditions with significantly less energy than conventional systems.