Water Damage – Terms to know – Psychrometry

What is psychrometry and what role does it play in a water damage scenario? Webster’s Dictionary defines psychrometry as follows: The art of determining the amount of moisture in the air by the use of the psychrometer (sling hygrometer)

Water Damage

Photo of a sling hygrometer or psychrometer

from the Greek “psychro” meaning cold, it refers to the combination wet and dry thermometers in the sling hygrometer that are used to determine relative humidity. As the wet bulb is swung through the air, water evaporates from the wet bulb, and there is a cooling effect from evaporation. This accelerates the cooling effect, and by plotting the difference in temperature from the wet bulb to the dry bulb on a psychrometric chart, it is possible to determine the relative humidity of the surrounding air.

Psychrometric Chart

Although the sling hygrometer (psychrometer) is still in use today, many restoration technicians find using a hand held instrument called a hygrometer far easier to use. This instrument uses an electronic sensor to evaluate the temperature and relative humidity of the air it is sensing.

The ability to measure the relative humidity and temperature of the air will help the restoration technician determine how to set up an adequate drying system. The technician needs to use the driest air with the largest capacity to carry moisture, to allow the fastest rate of evaporation from wet materials and prevent further water damage. This translates into the hottest, driest air available. This may be outside air, and if so, that is known as an open drying system. If the outside air is too cool or humid, then it may be necessary to set up a closed drying system.

Psychrometric Chart

The setting up of a closed drying system is going to require the technician to have the appropriate equipment. An understanding of psychrometrics, balancing temperature, humidity, and air flow of both open and closed drying systems is a must. It will be necessary to measure the relative humidity and temperature and have the right types and capacity of dehumidifiers to handle the water load in the structure.

Moisture is first removed from materials (like carpet) by physical action. Then the remaining water is allowed to evaporate into the air. Air movers help push away moisture laden air that is adjacent to the wet carpet. This allows drier air to come in contact with the wet carpet so that evaporation can continue. The act of evaporation, which is the change from a liquid state to a vapor state, requires energy and that use of energy will lower the air temperature as more and more of the air in a structure approaches the saturation point (the point where it will not hold any more moisture). Obviously if the air has a high relative humidity to start with, this will slow the rate of evaporation.

The water in the air tries to reach an equilibrium point: dry air and moist air mix, and dry components in a structure start to absorb moisture out of the air. These components are called Hygroscopic, or having the property of absorbing or being affected by the moisture in the air. These materials normally have a moisture content of about 10%, however when the relative humidity of the air exceeds 60% the rate of absorption by the hygroscopic material is greatly accelerated. These items swell as they absorb water, any damage that occurs from water that is introduced in this fashion is referred to as secondary damage. If we allow the humidity of the air to stay high for too long a period than these hygroscopic items may become damaged. Items like drywall become very weak when wet.

The normal range of humidity found in a home is in the 30-50% relative humidity range. When the relative humidity of the indoor environment exceeds 60% it becomes unhealthy to inhabit and the hygroscopic materials in the structure will start absorbing moisture. This is because the moisture tries to reach an equilibrium in the home. Moisture travels from areas of high humidity to areas of low humidity. This occurs because the extra weight of the water in the air increases the pressure exerted on the surrounding environment. This is called vapor pressure.


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