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A psychrometric chart is a graph of the physical properties of moist
air at a constant pressure (often equated to an elevation relative to
sea level). The chart graphically expresses how various properties
relate to each other, and is thus a graphical equation of state. The thermophysical properties found on most psychrometric charts are:
- Dry-bulb temperature (DBT) is that of an air sample, as determined by an ordinary thermometer, the thermometer's bulb being dry. It is typically the x-axis, the horizontal axis, of the graph. The SI units for temperature are Celsius; other units are Fahrenheit.
- Wet-bulb temperature (WBT)
is that of an air sample after it has passed through a
constant-pressure, ideal, adiabatic saturation process, that is, after
the air has passed over a large surface of liquid water in an insulated
channel. In practice, this is the reading of a thermometer whose
sensing bulb is covered with a wet sock evaporating into a rapid stream
of the sample air. The WBT is the same as the DBT when the air sample
is saturated with water. The slope of the line of constant WBT reflects
the heat of vaporization of the water required to saturate the air of a
given relative humidity.
- Dew point temperature (DPT)
is that temperature at which a moist air sample at the same pressure
would reach water vapor saturation. At this saturation point, water
vapor would begin to condense into liquid water fog or (if below
freezing) solid hoarfrost, as heat is removed. The dewpoint temperature
is measured easily and provides useful information, but is normally not
considered an independent property. It duplicates information available
via other humidity properties and the saturation curve.
- Relative Humidity (RH)
is the ratio of the mole fraction of water vapor to the mole fraction
of saturated moist air at the same temperature and pressure. RH is
dimensionless, and is usually expressed as a percentage. Lines of
constant RH reflect the physics of air and water: they are determined
via experimental measurement. Note: the notion that air "holds"
moisture, or that moisture dissolves in dry air and saturates the
solution at some proportion, is an erroneous (albeit widespread)
concept.
- Humidity Ratio
(also known as Moisture Content, Mixing Ratio, or Specific Humidity) is
the proportion of mass of water vapor per unit mass of dry air at the
given conditions (DBT, WBT, DPT, RH, etc.). It is typically the y-axis,
the vertical axis, of the graph. For a given DBT there will be a
particular humidity ratio for which the air sample is at 100% relative
humidity: the relationship reflects the physics of water and air and
must be measured. Humidity Ratio is dimensionless, but is sometimes
expressed as grams of water per kilogram of dry air or grains of water
per pound of air.
- Specific Enthalpy symbolized by h,
also called heat content per unit mass, is the sum of the internal
(heat) energy of the moist air in question, including the heat of the
air and water vapor within. In the approximation of ideal gasses, lines
of constant enthalpy are parallel to lines of constant WBT. Enthalpy is
given in (SI) joules per kilogram of air or BTU per pound of air.
- Specific Volume, also called Inverse Density,
is the volume per unit mass of the air sample. The SI units are cubic
meters per kilogram of air; other units are cubic feet per pound of dry
air.
The versatility of the psychrometric chart lies in the fact that by
knowing three independent properties of some moist air (one of which is
the pressure), the other properties can be determined. Changes in state,
such as when two air streams mix, can be modeled easily and somewhat
graphically using the correct psychrometric chart for the location's
air pressure or elevation relative to sea level. For locations at or
below 2000 ft (600 m), a common assumption is to use the sea level
psychrometric chart.
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