User Contributed Dictionary
Noun
psychrometry the measurement or study of the thermodynamic properties of moist air
Usage notes
Not to be confused with: psychometry.Extensive Definition
Psychrometrics or psychrometry are terms used to
describe the field of engineering concerned with the determination
of physical and thermodynamic properties of gasvapor mixtures. The
term derives from the Greek psuchra (cold)
and metron (means
of measurement).
Common applications
The principles of psychrometry apply to any physical system consisting of gasvapor mixtures. The most common system of interest, however, are mixtures of water vapor and air because of its application in heating, ventilating, and airconditioning and meteorology.Psychrometric ratio
The psychrometric ratio is the ratio of the heat transfer coefficient to the product of mass transfer coefficient and humid heat at a wetted surface. It may be evaluated with the following equation: r = \frac
 where:


 r = Psychrometric ratio, dimensionless
 h_c = convective heat transfer coefficient, W m2 K1
 k_y = convective mass transfer coefficient, kg m2 s1
 c_s = humid heat, J kg1 K1

Humid heat is the constantpressure specific heat
of moist air, per unit mass of dry air.
The psychrometric ratio is an important property
in the area of psychrometrics as it relates the absolute humidity
and saturation humidity to the difference between the dry bulb
temperature and the
adiabatic saturation temperature.
Mixtures of air and water vapor are the most
common systems encountered in psychrometry. The psychrometric ratio
of airwater vapor mixtures is approximately unity which implies
that the difference between the adiabatic saturation temperature
and wet bulb temperature of airwater vapor mixtures is small. This
property of airwater vapor systems simplifies drying and cooling
calculations often performed using psychrometic
relationships.
Psychrometric chart
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: Drybulb temperature (DBT) is that of an air sample, as determined by an ordinary thermometer, the thermometer's bulb being dry. It is typically the xaxis, the horizontal axis, of the graph. The SI units for temperature are Celsius; other units are Fahrenheit.
 Wetbulb temperature (WBT) is that of an air sample after it has passed through a constantpressure, 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 (see Relative humidity for further details).
 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 yaxis, 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.
The relationship between DBT, WBT, and RH is
given by the Mollier
diagram (pressureenthalpy) for water in air, developed by
Richard
Mollier. Willis
Carrier, considered the 'father' of modern airconditioning,
rearranged the Mollier diagram for moist air (its Ts chart) to
allow such graphical solutions. Many variations and improvements to
the psychrometric charts have occurred since, and most charts do
not show the specific entropy (s) like the Mollier diagram.
ASHRAE now
publishes what are considered the modern, standard psychrometric
charts, in both IP and SI units, for a variety of elevations or
air pressures.
How to read the chart
The most common chart used by practitioners and students alike is the "ωt" (omegat) chart in which the dry bulb temperature (DBT) appears horizontally as the abscissa and the humidity ratios (ω) appear as the ordinates.In order to use a particular chart, for a given
air pressure or elevation, at least two of the six independent
properties must be known (DBT, WBT, RH, humidity ratio, specific
enthalpy, and specific volume). This gives rise to \left(\right) =
15 possible combinations.
DBT: This can be determined from the abscissa on
the xaxis,
the horizontal axis
DPT: Follow the horizontal line from the point
where the line from the horizontal axis arrives at 100% RH, also
known as the saturation curve.
WBT: Line inclined to the horizontal and
intersects saturation curve at DBT point.
RH: Hyperbolic lines drawn asymptotically with
respect to the saturation curve which corresponds to 100% RH.
Humidity ratio: Marked on the yaxis.
Specific enthalpy: lines of equal values, or hash
marks for, slope from the upper left to the lower right.
Specific volume: Equally spaced parallel family
of lines.
Drybulb temperature
Common thermometers measure what is known as the drybulb temperature. Electronic temperature measurement, via thermocouples, thermistors, and resistance temperature devices (RTDs), for example, have been widely used too since they became available.Wetbulb temperature
The thermodynamic wetbulb temperature is a thermodynamic property of a mixture of air and water vapor. The value indicated by a simple wetbulb thermometer often provides an adequate approximation of the thermodynamic wetbulb temperature.A wetbulb thermometer is an instrument which may
be used to infer the amount of moisture in the air. If a moist wick
is placed over a thermometer bulb the evaporation of moisture from
the wick will lower the thermometer reading (temperature). If the
air surrounding a wetbulb thermometer is dry, evaporation from the
moist wick will be more rapid than if the air is moist. When the
air is saturated no
water will evaporate from the cloth wick and the temperature of the
wetbulb thermometer will be the same as the reading on the
drybulb thermometer. However, if the air is not saturated water
will evaporate from the wick causing the temperature reading to be
lower.
The accuracy of a simple wetbulb thermometer
depends on how fast air passes over the bulb and how well the
thermometer is shielded from the radiant temperature of its
surroundings. Speeds up to 5,000 ft/min (60 mph) are best but
dangerous to move a thermometer at that speed. Errors up to 15% can
occur if the air movement is too slow or if there is too much
radiant heat present (sunlight, for example).
A wet bulb
temperature taken with air moving at about 12 m/s is referred
to as a screen temperature, whereas a temperature taken with air
moving about 3.5 m/s or more is referred to as sling
temperature.
A psychrometer is a device
that includes both a drybulb and a wetbulb thermometer. A sling
psychrometer requires manual operation to create the airflow over
the bulbs, but a powered psychrometer includes a fan for this
function.
References
See also
External links
 Psychrometric Chart  Detailed psychrometric chart including curves for enthalpy, air mass, and water mass.
 American Society of Heating, Refrigerating and AirConditioning Engineers, Inc.
 Universal Industrial Gases, Inc. webpage  Links to physical properties tables, to psychrometric charts that depict interrelationships among the various physical properties of air, to online psychrometric properties calculators for easy calculation of properties, and to individual gas information pages.
 Corwin's Calculators Calculator for humidity, dew point.
psychrometry in Czech: Mollierův diagram
psychrometry in German: Psychrometrie
psychrometry in French: Psychrométrie
psychrometry in Serbian:
Psihrometrija