Dew Point Temperature and Humidity Ratio
Dew Point Temperature
"The true and accurate definition of dew point temperature is the temperature at which the rate water vapor leaves the gas is equal to the rate water vapor enters the gas at a given pressure." (Leadership In Restorative Drying, page 89)
"Dew point is a metric derived from an evaluation of the environmental condition. It is important to recognize that if the water vapor in the air is allowed to cool below the dew point temperature through the mechanism of losing its thermal energy to a cold surface, condensation shall form on it. Therefore, the restoration professional must evaluate the temperature of the materials exposed to this air in order to control the formation of dew on these surfaces. To prevent condensation in a closed structure a close evaluation of the contact between cool structural surfaces in conjunction with environmental dew point temperatures should be evaluated and documented." (Leadership In Restorative Drying, page 90)
The dew point calculation can be a very helpful piece of data in a drying strategy. First we calculate the dew point of the temperature in the air and then use a lazer thermometer to determine the temperature on the surface of the hygroscopic material we are trying to dry. We then take the temperature of the hygroscopic material and subtract the dew point temperature we calculated. The number we derive is called the delta dew point temerature. The larger this number the better it is for an effective drying strategy.
Early in the development of the restoration industry, restorers struggled with finding a way to get an accurate measurement of the actual moisture in the air. Using relative humidity measures did not take into account the energy born in the water molecules. After some research it was discovered that there was a way to quantify the moisture content of the environment. It was called "specific humidity".
One of the common terms that has come to be used in the restoration industry is 'specific humidity'. It was defined as, 'the amount of water vapor that a volume of air contains by weight. This is expressed within our industry as 'grains per pound of dry air'.
"A logical approach to quantifying the volume of water in an air sample would be to measure the weight of the water in a given weight of air. If we are to quantify moisture in a given sample of air, we are speaking of extremely small measurements of water/weight. As cientific approach to these small measures of weight is to measure in measures of grains." (Leadership In Restorative Drying, page 91)
The given weight of the sample of air was 1 pound, which at between 77 degrees F and 96 degrees F is about 14 cubic feet of air. The ratio that was to be used was grains of water per pound of air. 1 Pound of wet air weighs 7000 grains. The critical point that needs clarification lies around the two terms, specific humidity and humidity ratio. While the restoration community has been using the correct metric in their calculations, we have been calling it by the wrong name.
Specific Humidity = Grains per pound of wet air
Humidity Ratio = Grains per pound of dry air
"Note: On a psychrometric chart, the metrics of vapor pressure, humidity ratio (inaccurately referred to as specific humidity), and dew point temperatures are absolute expressionsof the quanity of vapor and energy held by the vapor. They are directly related to each other. As environmental vapor pressure increases, dew point temperature shall rise as well as the humidity ratio. These metrics express a defined value; a quantitative expression of the condition or energy held within the gas. This contrasts with 'relative humidity' which is a qualitative expression where the environment's vapor pressures are comparable with the equilibrium vapor pressure of the air over the surface of pure water at that temperature. Relative pressure is expressed as a percentage." (Leadership In Restorative Drying, page 93)