Psychrometrics is the study of moist air thermodynamics, and the conditions and processes involving moist air.

Some of the terms:

<B>Dry Bulb:</B> Actual (sensible) temperature of the air sample.
<B>Dew Point:</B> Temperature at which the air sample reaches saturation.
<B>Relative Humidity:</B> Ratio of the partial pressure of water vapor in the air sample to the pressure of saturated water at the temperature of the air sample.
<B>Pressure of Water:</B> Yes, water exerts a pressure. This explains why water will evaporate at a temperature below its boiling point. In fact, even ice exerts a pressure!
<B>Partial Pressure of Water Vapor:</B> Yes, the moisture in air also exerts a pressure. It is referred to as a partial pressure because it is part of the total pressure of the moist air sample, i.e., partial pressure of water vapor + partial pressure of the dry air pressure equals the total pressure of the moist air sample.
<B>Humidity Ratio:</B> Ratio of the mass of water vapor to the mass of dry air in the air sample.
<B>Specific Humidity:</B> Ratio of the mass of water vapor to the mass of the air sample.
<B>Absolute Humidity:</B> Ratio of the mass of water vapor to the total volume of the air sample.
<B>Degree of Saturation:</B> Ratio of the humidity ratio of the air sample to the humidity ratio of the air sample at saturation.
<B>Wet Bulb:</B> Thermodynamic wet bulb is referred to as the temperature of adiabatic saturation. It is the temperature at which water, by evaporating into air, can bring the air to saturation. Interestingly, it is a property of air which cannot be measured, only calculated. Ordinary wet bulb is the temperature read using a psychrometer, and it can only approximate thermodynamic wet bulb. For the purposes of psychrometric calculations, however, ordinary wet bulb is sufficiently close enough to thermodynamic wet bulb.

Typical problems which psychrometrics is used to solve:

1. Cooling and dehumidification loads
2. Mixing of two or more airstreams
3. Heating and humidification loads

A rule of thumb used by many in the a/c business is 400 cfm of airflow requires 1 ton of air conditioning. Shall we attempt to proved this rule?