Introduction to Psychrometry

" What exactly is psychrometry?  'Psychrometrics deals with thermodynamic properties of moist air and uses these properties to analyze conditions and processes involving moist air'.  (1997 ASHRAE Handbook, 6.1 p1)

In simpler terms, psychrometry is the science and practice associated with measuring the properties of air, namely temperature and humidity, and analyzing these properties to better understand their effects on the surrounding environment." (New Guide to Restorative Drying, Dri-Eaz Education Series, page 80)

"There are three basic properties evaluated in psychrometrics:

• Humidity
• Airflow
• Temperature

This is often referred to as the drying pie and each has a direct influence on the rate of evaporation (or condensation) that will occur when moisture and materials interact.  In order for evaporation to occur at a rate that will bring the "body of water" to a net loss, there must be sufficient energy generated within the body of water itself so that the water molecules move rapidly and break their bonds with one another and their state is altered. 

"While water temperature and air moisture content define whether condensation or evaporation will win the battle, airflow determines the rate at which the battle will play out.  Where no airflow exists, the net change will be slow.  Where airflow does exist, the net change will occur at a more rapid rate."  (New Guide to Restorative Drying, Dri-Eaz Education Series, page 81) 

"Humidity,airflow and temperature directly affect the sate in which water exists - solid (ice), liquid (water) or gas (vapor) and the rate at which the change of state occurs.  It is critical to have a clear understanding of the methods for measuring psychrometric properties and of how to use them appropriately when evaluating a restorative drying environment.

Ultimately, the goal in restorative drying is to remove excess water from affected building materials after a water intrusion.  This process requires restorers change liquid water into vapor (evaporation).  Once water is evaporated, it must then be removed.  This is most often done by changing vapor (humidity) back into a liquid (condensation) using refrigerant dehumidifiers.  Therefore, the goal in restorative drying involves changing water's state.  (New Guide to Restorative Drying, Dri-Eaz Education Series, page 81) 

The Influence of Temperature

There are several changes of state that can occur, depending upon whether energy is being added or removed.  The amount of energy required to change water's state is quite phenomenal.  In fact, it requires more energy to change water from one state to the next than is required with almost any other molecule.  

Energy is measured in the restorative drying industry using dry bulb temperature, or simply degrees Fahrenheit (Celsius).  where temperature is higher, more energy is present.  It is important to note that temperature measurement is valuable, but is not a specific measurement of heat.  Heat energy is measured by other means, commonly in British Thermal Units, or BTUs (United States, Canada) and in Calories (Metric)."    (New Guide to Restorative Drying, Dri-Eaz Education Series, page 81-82)  

The Influence of Humidity

The Relative Relationship

"The amount of moisture in materials is most directly related to the relative humidity (RH) of the surrounding air.  RH is the amount of moisture in air expressed as a percentage of the air's total holding capacity.  It is an expression of how much water the air is holding compared to what it could hold... Materials will seek equilibrium with the surrounding relative humidity.  For example, if an environment was to become more humid, and the relative humidity increased to 70%, hygroscopic materials would absorb moisture until they were holding approximately 70% of their holding capacity.  This is referred to as a relative equilibrium."

The Specific Relationship

 "The rate at which moisture will move is most related to the actual amount of water in the air.  In addition to relative humidity, humidity can be measured as a weight.  This measurement is much more specific to the amount of water in air.  It also directly affects how quickly water will move.  Where water vapor in air weighs more, it exerts more pressure.  This pressure pushes back against water leaving materials, or adds force to water entering materials.

The weight measurement used in the restorative drying industry (United States, Canada) is grains per pound of dry air, abbreviated as GPP.  It is a ratio, expressing the weight of water in a given weight of air (a pound of air)... Because this measurement is of the water vapor weight, it tells the restorer where the moisture is in greatest concentration.  Greater weight means more moisture.  Higher GPP (g/kg) in the air surrounding a material will retard the evaporation process because the weight of water vapor resists vapor leaving the material.  GPP (g/kg) therefore influences the rate at moisture moves toward equilibrium".  (New Guide to Restorative Drying, Dri-Eaz Education Series, page 83-84)

The Influence of Airflow

"Air movement in the drying environment speeds the effect of humidity and temperature.  It provides a blending effect, ensuring that the warm, dry air created by drying equipment (e.g., dehumidifiers) is quickly and continuously placed into contact with wet materials.  It also ensures that evaporated water vapor is removed from the same surfaces.

Another important purpose of air movement is to create air exchange between adjacent affected areas.  Without airflow, humidity and temperature would greatly vary from one area to the next.  Restorers would need to install drying equipment in every affected air space within the structure.  This would be impractical for most applications... The more moisture a material contains, the faster water will evaporate.  Greater evaporation rates require more airflow to maintain low humidity along the surface.  As materials dry, less airflow is necessary.  This relationship is directly related to the type of water present.  Where surface water is present, the greatest amount of airflow is necessary.  Where only bound water is present, the least amount of airflow is necessary".  (New Guide to Restorative Drying, Dri-Eaz Education Series, page 84-85)

The Psychrometric Chart

To properly evaluate temperature, relative humidity and humidity ratio (GPP, g/kg), restorers must use a psychrometric chart.  Other tools are available to calculate GPP (g/kg) and can be useful (e.g., electronic psychrometric calculators).  Typically, a restorative drying technician will acquire a dry bulb temperature (Fahrenheit or Celsius) and a relative humidity using a thermo-hygrometer.  These two readings establish the size of the air mass (temperature) and the relative amount of moisture in the air (relative humidity).

Using these two values, the technician will use the psychrometric chart to calculate GPP (g/kg).  Other calculations are also made if necessary, including vapor pressure, dew point temperature and thirst... Once these readings are established, the technician compares them to surrounding conditions or previous conditions to evaluate drying potential".  (New Guide to Restorative Drying, Dri-Eaz Education Series, page 85)

For a look at some of the equipment available for restorative drying, click on this link which will take you to the Dri-Eaz home page which displays a link at the bottom for their Virtual Training Center.  An excellent resource to understand how drying equipment works.  Take note of the MMS Plus which is the measuring instrument that Super Bee uses for determining many of the psychrometric calculations during our drying projects.