The psychrometric chart is a graphical
representation of the thermodynamic properties of moist air. It describes all the possible
combinations of temperature, moisture content, density and energy relating to air at one
time. The usefulness of the psychrometric chart can be seen in that given any two
properties of air, the remaining properties can be determined graphically. It should be
noted that psychrometric charts do differ with respect to altitude (i.e. barometric
pressure). Below is an illustration of a psychrometric chart which is followed by
explanations of the terminology related to its properties.
PROPERTIES OF THE PSYCHROMETRIC CHART AND THEIR DEFINITIONS
DRY BULB TEMPERATURE is the temperature that
we measure with a standard thermometer that has no water on its surface. When people refer
to the temperature of the air, they are commonly referring to its dry bulb temperature.
Dry bulb temperature is represented by vertical lines on the chart and is displayed at the
bottom, increasing from left to right. The unit of measure used for dry bulb temperature
is °C .
WET BULB TEMPERATURE is a temperature
associated with the moisture content of the air. Wet bulb temperature is taken by
surrounding the thermometer with a wet wick and measuring the reading as the water
evaporates. Wet bulb temperatures are always lower than dry bulb temperatures and the only
time that they will be the same is at saturation (i.e. 100% relative humidity). Wet bulb
temperature is represented by lines that slant diagonally from the upper right of the
chart (along the line of saturation) down to the lower left of the chart. The unit of
measure used for wet bulb temperature is °C .
RELATIVE HUMIDITY is the measure of how
saturated the air is with water vapour. If the air is holding all the moisture it can for
a specific set of conditions, then it is said to be saturated. At this saturated state it
is said to have a relative humidity of 100%. Relative humidity is measured as how much
water vapour is present in the air as a percentage of how much water vapour would be in
the same air at saturation. Relative Humidity is represented on the psychrometric chart as
lines that curve from the lower left hand side up and to the right. The unit of measure
used for relative humidity is %.
DEW POINT corresponds to the temperature at
which air becomes completely saturated. At dew point dry bulb temperature and wet bulb
temperature are exactly the same. Dew point is represented along the 100% relative
humidity line on the psychrometric chart.
ENTHALPY is the measure of the total energy
in the air. Energy in air comes in the form of either sensible heat or latent heat.
Sensible heat is the heat (energy) in the air due to the temperature of the air. Latent
heat is the heat (energy) in the air due to the moisture of the air. Following this, air
with same amount of energy may either be dry hot air (high sensible heat) or cool moist
air (high latent heat). Enthalpy lines run almost along the same lines as wet bulb
temperature and in most applications the same lines can used for both properties. The unit
of measure for enthalpy is kJ / KG OF DRY AIR.
MOISTURE CONTENT / HUMIDITY RATIO differs
from relative humidity in that it is the amount of water vapour, by weight, in the air.
Humidity ratio is represented on the chart by lines that run horizontally and the values
are on the right hand side of the chart increasing from bottom to top. The unit of measure
used for humidity ratio is KG MOISTURE / KG OF DRY
SPECIFIC VOLUME is the volume that a certain
weight of air occupies, at a specific set of conditions. Specific volume is represented by
lines that slant from the lower right hand corner towards the upper left hand corner at a
steeper angle than the lines of wet bulb temperature and enthalpy. The unit
of measure used for specific volume is CUBIC METRES /
KG OF DRY AIR.
VAPOUR PRESSURE can also be determined on the
psychrometric chart. Each water molecule in the air exerts pressure on the
surrounding environment. Vapour pressure is represented by lines that run horizontally and
the values are on the far right hand side of the chart increasing from bottom to top. The
unit of measure for vapour pressure is PASCAL.