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Glossary of Eco-Build-Related Terms

I find a good place to find defimitions of abbreviations is dictionary.reference.com alternatively type appreviation into Google and clink on the 'definition link' in the top right hand corner after the search results have displayed.


Thermal Dynamics Terms


BTU: British thermal units. Generally speaking, a BTU is the amount of heat required to raise the temperature of one pound (0.45 kg) of water 1 degree Fahrenheit (0.56 degrees Celsius). Specifically, 1 BTU equals 1,055 joules. In heating and cooling terms, 1 "ton" equals 12,000 BTU.

Conduction: The molecule-to-molecule transfer of kinetic energy (one molecule becomes energized and, in turn, energizes adjacent molecules). A cast-iron skillet handle heats up because of conduction through the metal.

Convection: The transfer of heat by the physical movement of molecules from one place to another. Hot air rises and heated water thermosiphons. Through convection a tank of water heated from a single source will advance towards reaching an even temperature throughout its mass.

Heat: A form of energy associated with the motion of atoms or molecules which is capable of being transmitted through solid and fluid media by conduction, through fluid media by convection, and through empty space by radiation.

Insulation: A material that reduces or prevents the transmission of heat or sound or electricity

Radiation: The transfer of heat through space via electromagnetic waves (radiant energy). A campfire can warm you even if there is wind between you and the fire, because radiation is not affected by air.

R-value: A measure of the capacity of a material, such as insulation, to impede heat flow, with increasing values indicating a greater capacity.
The R-value is a non-SI measure of thermal resistance used in housing insulation. It is defined as R = ft2 F h / BTU. For comparison, the SI unit of thermal resistance is R = K.m/W, and 1 ft F h / Btu = approx. 0.1761 K.m/W. R-values are measured by testing laboratories, usually in something called a guarded hot box. Heat flow through the layer of material can be calculated by keeping one side of the material at a constant temperature, say 90F (32C), and measuring how much supplemental energy is required to keep the other side of the material at a different constant temperature, say 50F (10.C)--all this is defined in great detail in ASTM (American Society of Testing and Materials) procedures. The result is a steady-state R-value ("steady-state" because the difference in temperature across the material is kept steady).



Fan and Moving Air property Terms


CFM: cubic feet per minute



Humidity Terms


Absolute humidity: The mass of water vapor in a given volume of air( i.e., density of water vapor in a given parcel, usually expressed in grams per cubic meter

Actual vapor pressure: The partial pressure exerted by the water vapor present in a parcel. Water in a gaseous state (i.e. water vapor) exerts a pressure just like the atmospheric air. Vapor pressure is also measured in millibars.

Condensation: The phase change of a gas to a liquid. In the atmosphere, the change of water vapor to liquid water.

Dewpoint: the temperature air would have to be cooled to in order for saturation to occur. The dewpoint temperature assumes there is no change in air pressure or moisture content of the air.

Dry bulb temperature: The actual air temperature. See wet bulb temperature below.

Freezing: The phase change of liquid water into ice.

Evaporation: The phase change of liquid water into water vapor.

Melting: The phase change of ice into liquid water.

Mixing ratio: The mass of water vapor in a parcel divided by the mass of the dry air in the parcel (not including water vapor)

Relative humidity: The amount of water vapor actually in the air divided by the amount of water vapor the air can hold. Relative humidity is expressed as a percentage and can be computed in a variety of ways. One way is to divide the actual vapor pressure by the saturation vapor pressure and then multiply by 100 to convert to a percent. Click here for text explaining this in more detail.

Saturation of air: The condition under which the amount of water vapor in the air is the maximum possible at the existing temperature and pressure. Condensation or sublimation will begin if the temperature falls or water vapor is added to the air.

Saturation vapor pressure: The maximum partial pressure that water vapor molecules would exert if the air were saturated with vapor at a given temperature. Saturation vapor pressure is directly proportional to the temperature.

Specific humidity: The mass of water vapor in a parcel divided by the total mass of the air in the parcel (including water vapor)

Sublimation: In U.S. meteorology, the phase change of water vapor in the air directly into ice or the chance of ice directly into water vapor. Chemists, and sometimes meteorologists, refer to the vapor to solid phase change as "deposition."

Wet bulb temperature: The lowest temperature that can be obtained by evaporating water into the air at constant pressure. The name comes from the technique of putting a wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place. Wet bulb temperatures can be used along with the dry bulb temperature to calculate dew point or relative humidity



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