Psychrometry

Psychrometry (sometimes spelled psychrometrics) is the branch of physics dealing with the relationship between air and the water vapor it holds. In restoration, it is the theoretical foundation that turns drying from guesswork into a measurable, predictable process. Every credible drying plan is built on psychrometric principles.

The core insight is that air's capacity to hold moisture changes with temperature. Warm air holds far more water vapor than cold air. By manipulating temperature, airflow, and humidity, a technician can control exactly how fast moisture leaves the structure and where it goes. This is why structural drying is often called applied psychrometry.

Several interrelated values are tracked on every water loss:

• Temperature (dry bulb): the ambient air temperature.
• Relative humidity (RH): the percentage of moisture the air holds relative to its maximum at that temperature.
• Specific humidity (grains per pound, or GPP): the actual weight of water vapor in the air, which unlike RH is not affected by temperature changes.
• Dew point: the temperature at which air becomes saturated and moisture begins to condense.
• Vapor pressure: the force driving moisture from wetter areas toward drier ones.

Grains per pound is especially important because it lets technicians compare the moisture load of different air streams objectively, which is the basis for calculating grain depression.

In practice, a technician uses a thermo-hygrometer to record temperature and humidity at several points: outside the building, in an unaffected area, in the drying chamber, and at the intake and exhaust of each dehumidifier. These readings reveal whether the drying system is working and whether conditions favor evaporation.

For example, if the drying chamber's grains per pound are barely lower than the outside air, the dehumidifiers are not keeping up and adjustments are needed. Psychrometric logs recorded daily also form the evidentiary backbone of the claim, proving to an adjuster that the structure reached a genuine dry standard rather than simply feeling dry to the touch. Understanding psychrometry is what distinguishes a trained restorer from someone merely renting fans.