TOOLS
for the engineer, designer, programmer and technicians to help fine-tune automation systems. Here is a psychrometric calculator
to use when air quality is at most importance. You can
download this calculator here for offline use.
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EMCS Psychrometric Calculator
Definitions
Using this
psychrometric calculator , (traditionally a
Psychrometric Chart ) one can determine several properties of air, if they have a few of the listed values.
With this data engineers and technicians can solve problems or assign values and set points. For example, an economizer system that is
controlled by the building automation system can be controlled by an enthalpy value. This value associated with a controlled output will
only allow the economizer to open under certain conditions that will allow the mechanical cooling systems to shut down and take in free cooling
from outside the structure. The temperature, humidity, and all the properties of the air will be optimum for free cooling using the economizer.
Dry-Bulb Temperature (Tdb )
...is that of an air sample, as determined by an ordinary thermometer, the thermometer's bulb being dry. It is the equilibrium temperature read by an ordinary thermometer when exposed to atmospheric air. It is typically the x-axis, the horizontal axis, of the graph. The SI units for temperature are Celsius and Kelvin, imperial unit is Fahrenheit.
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).
Wet-Bulb Temperature (Twb )
...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 Twb is the same as the Tdb when the air sample is saturated with water. The slope of the line of constant Twb reflects the heat of vaporization of the water required to saturate the air of a given relative humidity.
Dew Point Temperature (Tdp )
...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.
Specific Enthalpy (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 gases, lines of constant enthalpy are parallel to lines of constant Twb . Enthalpy is given in (SI) joules per kilogram of dry air or BTU per pound of dry air.
Humidity Ratio (w ) (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 (Tdb , Twb , Tdp , RH, etc.). It is typically the y-axis, the vertical axis, of the graph. For a given Tdb 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 Volume
...also called inverse density, is the volume per unit mass of the air sample. SI units are cubic meters per kilogram of air; IP units are cubic feet per pound of dry air. Specific volume is shown along the bottom axis of a psychrometric chart , with constant-volume lines slanting upward to the left.
[ Hide Help ]
The following is a simple metric to imperial and reverse unit converter. Select the property, type your known value, select the
to and
from unit, and click convert.
Unit Converter
