Psychrometrics

Relative Humidity (rH)

Relative humidity is a percentage that describes the saturation of the air mixture with water vapor. It compares the current amount of water vapor in the air to the maximum amount of water vapor that can exist in that air mixture at the same temperature and pressure.

• What it measures: The percentage of saturation. At 100% rH, the air is considered saturated. This is the point where the rate of water molecules evaporating into the air is equal to the rate of water molecules condensing out of it.

• Supersaturation: While the scale is typically described as being between 0−100%, it is possible for a gas mixture to be supersaturated, meaning it has a relative humidity greater than 100%. However, this is an unstable state, and condensation will rapidly occur to bring the rH back down. For practical purposes, 100% rH is the key threshold where phase change begins.

• Comfort: The scale from 0−100% is often used to describe human comfort. A relative humidity of around 50% is generally considered comfortable, whereas higher percentages can feel sticky and lower percentages can feel excessively dry.

• The common metaphor of air "holding" water vapor can be misleading. A more precise way to describe the relationship between temperature and relative humidity involves the rates of evaporation and condensation.

  • As temperature decreases, water molecules in the air have less kinetic energy. This causes the rate of condensation to increase, as the slower-moving molecules are more likely to be captured and form liquid water. At the same time, the rate of evaporation decreases. When the condensation rate surpasses the evaporation rate, the air mixture approaches saturation (100% rH), and liquid water begins to form.

  • Conversely, as temperature increases, water molecules have more kinetic energy. This increases the rate of evaporation from any liquid water source, while the rate of condensation decreases. This dynamic leads to a lower relative humidity because the amount of water vapor in the air mixture is well below the point of saturation.

Dew Point

The dew point is an absolute measurement of the amount of water vapor in the air, expressed as a temperature.

• What it measures: The temperature to which an air mixture must be cooled (at a constant pressure) for it to become saturated and for water vapor to begin condensing. If the dew point is 55∘F and the air temperature drops to 55∘F, the air becomes 100% saturated. If it drops further, condensation will form as dew on surfaces.

• Key characteristic: The dew point is not dependent on temperature. It is a direct indication of the actual amount of moisture in the air. This makes it a more reliable measure of how "muggy" a room will feel, as a high dew point always indicates a high amount of moisture, regardless of the air temperature.

Vapor Pressure and Vapor Pressure Differential

Every gas in an air mixture exerts its own pressure. The pressure exerted by the water vapor alone is called vapor pressure.

• What it measures: The pressure contributed by the water vapor in the air mixture. The higher the vapor pressure, the more water vapor is present.

• Vapor Pressure Differential: This is the difference in vapor pressure between two locations. Water vapor naturally moves from areas of high vapor pressure to areas of low vapor pressure. This principle is crucial in drying; a wet material has a higher vapor pressure than the air around it, causing moisture to move from the material into the air.

Humidity Ratio and Grain Depression

Humidity ratio is another absolute measure of moisture, often used in professional drying and restoration.

• Humidity Ratio: This is a direct measure of the weight of water vapor per unit weight of dry air. It is typically measured in grains per pound (GPP). A "grain" is a very small unit of weight, with 7,000 grains equaling one pound of water.

• Grain Depression: This term is used to measure the performance of a dehumidifier. It is the difference between the humidity ratio of the air entering the dehumidifier and the air leaving it. A larger grain depression indicates that the dehumidifier is removing more water from the air, signifying a more effective drying process.