1.2.2 Psychrometry 102 Specific Humidity Readings

Definitions

Specific humidity is a measure of the amount of water in the air by volume. The most common measurements used in water restoration are:

Vapor Pressure is the pressure exerted by a vapor on its surrounding environment. Vapor pressure is also measured in liquid at the water temperature and 100% rh.

Dew Point is the temperature at which humidity in air reaches saturation and will condense on surfaces. Surfaces that are cooler than dew point will condense moisture and surfaces warmer than dew point will evaporate water from the surface.

Grains Per Pound (GPP) is a unit used to measure specific humidity or weight of moisture in air (7000 grains equals one pound of water or approximately one pint)

How to Calculate

Psychrometric Chart

In order to calculate specific humidity a number of tools may be used. During an IICRC water restoration class you will be asked to use a Psychrometric Chart. You use this by taking the temperature and the relative humidity of the air and crossing to the specific humidity measurements. (This form can be printed from ReetsTV.)

Psychrometric Calculator Wheel

A Psychrometric calculator is often used on site to calculate GPP and dew point. It is limited in that it does not calculate vapor pressure. You simply align the temperature of the air with the relative humidity of the air and the calculations are shown in the boxes within the calculator.

ReetsDryCalc

This is a smart phone app that does all of the calculations based on temperature of the air, relative humidity of the air and surface temperature. It will calculate GPP, dew point, vapor pressure and evaporation potential. (A link to this app can be found at ReetsTV.)

1.2.2 - Psychrometry 102 Hygrometer

The readings for air temperature and relative humidity are taken with a hygrometer. Make sure that when you take readings with a hygrometer you allow the meter enough time to acclimate or adjust to the environment you are in. This may take from a few seconds to several minutes based on the model you are using. Watch the numbers on the meter. When they stabilize (although they are changing slightly they are not continuing to either rise or fall) record the temperature and relative humidity. If the meter that you are using does not calculate specific humidity then calculate for specific humidity with one of the above tools.

Recording Psychrometric Readings

Specific Humidity readings must be both calculated and recorded on every job. A job is not complete unless these readings are documented. A generic form is available at ReetsTV. On that form record the following 3 items:

1. temperature of the air,

2. relative humidity of the air and

3. specific humidity calculations.

(ReetsTV Episode Psychrometry 102)

1.2.2 - Psychrometry 102