The psychometric chart is representative snapshot of the properties of wet air at sea level. Understanding this chart and the relationship of temperature to humidity is the key to effectively drying buildings. If your contractor or even more so his employees don't understand these relationships then you had better get him away from your drying project.

The main date points in the chart are.

DRY BULB temperature the temperature of the air using a thermometer such as the one in your house the out put may be in Celsius or Fahrenheit.

Wet-bulb temperature is measured using a standard mercury-in-glass thermometer, with the thermometer bulb wrapped in muslin, which is kept wet. The evaporation of water from the thermometer has a cooling effect, so the temperature indicated by the wet bulb thermometer is less than the temperature indicated by a dry-bulb (normal, unmodified) thermometer. The rate of evaporation from the wet-bulb thermometer depends on the humidity of the air - evaporation is slower when the air is already full of water vapour. For this reason, the difference in the temperatures indicated by the two thermometers gives a measure of atmospheric humidity.

Dew point temperature  The dew point temperature is defined as the temperature at which the air becomes saturated with water vapor when the air is cooled by removing sensible heat.

Relative Humidity

The amount of water vapor in the air at any given time is usually less than that required to saturate the air. The relative humidity is the percent of saturation humidity, generally calculated in relation to saturated vapor density.