
8
Mixing Ratio
The mixing ratio (x) is the ratio of
water vapor mass to the mass of dry
gas. It is dimensionless but often
expressed in grams per kilogram of
dry air. The mixing ratio is mainly
used in drying processes and HVAC
applications for calculating water
content when the mass flow of air is
known.
Wet Bulb Temperature
Traditionally, the wet bulb
temperature (T
w
) is the temperature
indicated by a thermometer wrapped
in a wet cotton sheath. The wet bulb
and ambient temperatures can be
used together to calculate relative
humidity or dew point. For example,
the wet bulb temperature is used
in air conditioning applications
where it is compared to the dry bulb
temperature to determine the cooling
capacity of evaporative coolers.
Absolute Humidity
Absolute humidity (a) refers to the
mass of water in a unit volume of
moist air at a given temperature and
pressure. It is usually expressed
as grams per cubic meter of air.
Absolute humidity is a typical
parameter in process control and
drying applications.
The Effect of Environmental Conditions
on Humidity Measurement
Environmental conditions can have
a significant effect on humidity and
dew point measurements. Take the
following environmental factors into
consideration to achieve the best
possible measurement result:
Select a Representative
Measurement Location
Always choose a measuring
point that is representative of the
environment being measured,
avoiding any hot or cold spots. A
transmitter mounted near a door,
humidifier, heat source, or air
conditioning inlet will be subject to
rapid humidity changes and may
appear unstable.
As relative humidity is strongly
temperature dependent, it is
very important that the humidity
sensor is at the same temperature
as the measured air or gas. When
comparing the humidity readings
of two different instruments, the
thermal equilibrium between the
units/probes and the measured gas is
particularly crucial.
Unlike relative humidity, dew point
measurement is independent
of temperature. However, when
measuring dew point, pressure
conditions must be taken into
account.
Beware of Temperature
Differences
When mounting a humidity probe
into a process, avoid temperature
drops along the probe body.
When there is a large temperature
difference between the probe and
the external environment, the whole
probe should be mounted within the
process and the cable entry point
should be insulated.
When there is a risk of condensation,
the probe should be mounted
horizontally to avoid water dripping
down the probe/cable and saturating
the filter (see figure 1).
Ensure that air is allowed to
flow around the sensor. Free air
flow ensures that the sensor is
in equilibrium with the process
temperature. At 20°C and 50%RH, 1°C
difference between the sensor and
the measurement zone will cause an
error of 3%RH. At 100%RH the error is
6%RH (see figure 2).
Figure 1: Mounting a humidity probe
in a condensing environment.
Figure 2: Measurement error at
100%RH at various temperatures when
the temperature difference between the
ambient air and the sensor is 1°C.
Water Activity
Water activity (aw) is similar to
equilibrium relative humidity and
uses a scale of 0 to 1, instead of
0% to 100%.
Enthalpy
Enthalpy is the amount of energy
required to bring a gas to its current
state from a dry gas at 0°C. It is used
in air conditioning calculations.
Recommended
Not
recommended
if risk of
Mounting for
duct or chamber
Probe
Insulation
Insulation
Seal
Seal
-40 -20 0
0
1
2
3
4
5
6
7
8
9
10
20
Temperature ( C)
dRH (%RH)
40 60 80 100
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