PSYCHROMETRIC
CHART
© Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence
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Hello, I would like to show how to
read a psychrometric chart.
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Rather than go through calculations
every time we need some
information about the properties of
air, psychrometric charts have been
devised to graphically represent the
values of those properties.
© Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence
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This is a typical psychrometric chart
© Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence
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This skeleton chart shows
the arrangement of the
various lines and/or
coordinates:
1.
saturation temperature
2.
dewpoint temperature
3.
enthalpy
4.
relative humidity
5.
humidity ratio (moisture
content)
6.
wet bulb temperature
7.
volume of mixture
8.
dry bulb temperature.
The chart is based on a standard barometric
(atmospheric) pressure of 101.3 kPa or 760 mm Hg.
© Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence
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The main coordinates of the
average psychrometric chart are:
 saturation curve (100% RH)
 dry bulb temperature scale
line (0% RH.)
 moisture content or humidity
ratio scale.
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The dry bulb temperature lines
run perpendicular to the base
coordinate. Each line represents
one degree of temperature
change, with the scale ranging
from
-10 °C to 55 °C.
© Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence
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The wet bulb temperature
lines extend diagonally
downward from the
saturation curve at an
approximate angle of 30°
to the base line.
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Each line represents one
degree of temperature
change, with a scale
ranging from 10 °C to
33 °C.
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The temperature scale is
located on the saturation
curve.
© Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence
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The dew point
temperature scale is
the same scale as the
wet bulb scale on the
saturation curve.
However, the dew
point lines extend
horizontally to the
moisture content scale
on the right of the
chart.
© Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence
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The relative humidity
lines follow
approximately the
same curves as the
saturation curve.
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The saturation curve
is actually the line
representing 100%
relative humidity,
with the dry bulb
temperature scale
line representing 0%
relative humidity or
dry air.
© Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence
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The moisture content
or humidity ratio lines
are the same as the
dew point temperature
lines. However, the
scale for the grams of
moisture on the right
of the chart is different
and reads from 0 to 33
grams of moisture per
kilogram of air.
© Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence
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The specific volume
lines run at a steep
angle from top left to
bottom right.
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The numerical values,
along the bottom of the
chart at the ends of
these lines are given in
cubic metres per
kilogram of dry air and
range from 0.75 to
0.95 m²/kg.
© Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence
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Enthalpy is total heat
content and is
designated by the letter
‘h’.
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In psychrometric terms,
enthalpy defines the
heat quantity in the air
and the moisture in the
air.
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It is measured in
kilojoules per kilogram
of dry air.
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The enthalpy lines on a
psychrometric chart are
the same as the wet
bulb lines.
© Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence
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The enthalpy scale is
located in convenient
sections adjacent to the
saturation temperature
curve and ranges from
−10 to 110 kJ/kg of dry
air.
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The scale can be read
by extending the wet
bulb lines until they
meet the scale.
© Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence
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If the value of any two of the
psychrometric properties is
known, the value of any other
property can be determined
from the psychrometric chart.
Normal practice is for the dry
and wet bulb temperatures of
a sample of air to be taken
and then these temperatures
are plotted on the chart. The
two lines representing these
temperatures will always
cross at some point and this
point then represents the
condition of the air in the
sample.
Once this point has been
determined, values for other
properties can be identified.
© Commonwealth of Australia 2010 | Licensed under AEShareNet Share and Return licence