Psychrometric Online Calculator

Overview

Engineering reference data for Psychrometric Online Calculator in air psychrometrics.

Key Formulas

Humidity Ratio

$\omega = 0.622 \frac{P_v}{P_a}$

Mass of water vapor per mass of dry air.

Relative Humidity

$\phi = \frac{P_v}{P_{vs}} \times 100\%$

Ratio of actual to saturation vapor pressure.

Wet Bulb Temperature

$T_{wb} = T_{db} - \frac{P_{vs} - P_v}{\gamma}$

Temperature measured by wet-bulb thermometer.

Enthalpy of Moist Air

$h = c_p T + \omega h_g$

Sensible + latent heat per unit mass of dry air.

Variables

Using the Online Calculator

For direct calculation of moist air properties, you can use the psychrometric tool available at the following link:

Psychrometric Calculator: http://www.sugartech.co.za/psychro/index.php

This tool applies the perfect gas relations for moist air, based on the methodologies established in the 1989 ASHRAE Fundamentals Handbook. It allows you to input known properties (e.g., dry-bulb temperature, wet-bulb temperature, or relative humidity) to determine a full set of state points on the psychrometric chart.

Common Psychrometric Calculations

While the online tool provides a comprehensive solution, the core relationships can be expressed with fundamental formulas. For example, the humidity ratio ($W$) can be calculated from partial pressures, and relative humidity ($\phi$) is defined as the ratio of the actual vapor pressure ($P_w$) to the saturation vapor pressure ($P_{ws}$) at the dry-bulb temperature.

$\phi = \frac{P_w}{P_{ws}}$

The relationship between dry-bulb temperature ($T_{db}$), wet-bulb temperature ($T_{wb}$), and humidity ratio is more complex but is handled automatically by the linked calculator using iterative methods consistent with the ASHRAE formulations.

References

• Original Source