Introduction to Psychrometry Science and Calculations

The psychrometric chart is the primary analytical tool for understanding the relationship between air temperature, moisture content, and energy in a drying environment. Reading and applying this chart is a core technical competency for any restoration technician.

Chart axes: dry bulb temperature (horizontal axis, degrees Celsius) and humidity ratio or specific humidity (vertical axis, grams of water vapour per kilogram of dry air)

Saturation curve: the curved upper boundary of the chart — at this line, air is holding the maximum possible moisture at that temperature. Above this line, condensation occurs.

Relative humidity (RH) lines: curved lines running parallel to the saturation curve — each represents a constant percentage of the maximum moisture capacity at any given temperature. The target for structural drying is sustained RH below 55 percent.

Dew point temperature: the temperature at which air becomes saturated and condensation begins — read from the saturation curve at the current humidity ratio. Critical for preventing secondary moisture damage during drying.

Wet bulb temperature: the temperature read by a thermometer whose bulb is covered by a wet wick — used with dry bulb to locate the air state on the psychrometric chart. The closer wet bulb and dry bulb are, the more humid the air.

Enthalpy lines: lines of constant total heat content in the air — used to calculate the energy required for dehumidification and to assess the efficiency of drying equipment

Field Action: Practice plotting the air state on a psychrometric chart using readings from a job site. Take dry bulb temperature and relative humidity, convert RH to humidity ratio using the chart, and identify dew point and wet bulb temperature. This skill is the foundation of all drying strategy decisions.

Standard Ref: IICRC S500 Section 7 — Psychrometrics: provides the authoritative reference for psychrometric analysis in water damage restoration. All drying calculations on S500-governed projects must reference this framework.