User’s Guide ________________________________________________ HM40
9
Quantities Explained
The table below describes the quantities measured by
the HM40. All of the quantities are measured or
calculated when the meter is on, independent of what is
currently displayed.
Quantity Symbol
Unit(s) Description
Relative
humidity
RH
%
Ratio of the partial pressure of
water vapor in the air to the
saturation vapor pressure of air
at the current temperature.
Dewpoint Td °C
°F
Temperature at which the
water vapor in the air will
condense into water at the
current pressure. When the
dewpoint is below 0 °C, the
meter outputs frostpoint
instead of dewpoint.
Wet bulb
temperature
Tw °C
°F
The minimum temperature
that can be reached by
evaporative cooling in the
current conditions.
Absolute
humidity
a g/m
3
gr/ft
3
Quantity of water in a cubic
meter (or cubic foot) of air.
Mixing ratio
x
g/kg
gr/lb
Ratio of water vapor mass per
kilogram (or pound) of dry air.
Enthalpy h
kJ/kg
btu/lb
Sum of the internal energy of a
thermodynamic system.
Temperature T
°C
°F
Temperature in Celsius or
Fahrenheit scale.