Practical Tips for Taking Accurate Humidity Measurements with Hand

Practical Tips for Taking Accurate Humidity Measurements with Hand
Practical Tips for Taking Accurate Humidity
Measurements with Hand-Held Meters
Guidelines for Selecting an
Appropriate Location for
Humidity Measurement
Relative humidity is an important parameter for human and
animal comfort. Controlling relative humidity is also a vital
part of climate control in a wide variety of applications in the
process and storage industries.
Using a hand-held meter to measure
humidity and temperature provides
valuable information about the
conditions in the surrounding
environment. Measurements
are performed to collect basic
information about the environmental
conditions, to check the operation
and calibration of fixed humidity
instruments, and to verify the
operation of humidifying and/or
dehumidifying equipment.
Careful Selection
of Measurement
Environment –
The Key to Successful
Humidity Measurement
The key to successful environmental
measurements starts with the
selection of a measurement spot that
is representative of the surrounding
environmental conditions. After
selecting an optimal measurement
location, the humidity reading
should be allowed to stabilize
The measurement location
should be representative of the
conditions in the measurement
Avoid locations close to the
discharge of supply air ducts,
near exterior doors and
windows, and inside walls that
are exposed to solar radiation.
Locate humidity sensors
away from heat and moisture
Ensure that air flows freely
around the humidity sensor.
Avoid situations where water
might condense on the probe
– the humidity sensor will not
operate correctly when wet.
before taking the measurement.
This is best done by following
the stabilization of the humidity
reading from the graphical display
of the humidity meter. Alternatively,
the humidity meter can be left
to stabilize in the measurement
environment and the reading can be
checked after a proper stabilization
period – the larger the temperature
difference between the probe and
the measurement environment, the
longer the stabilization time required.
When performing a humidity
measurement it is good to remember
that the measurement is highly
temperature dependent. For example,
in conditions of 50 %RH (relative
humidity), a 1 °C (1.8 °F) change in
temperature changes the humidity
reading by 3 %RH. In environmental
conditions of 50 %RH and 20 °C, a
3 °C (5.4 °F) rise in temperature (to
23 °C/73.4 °F) causes the humidity
reading to drop to 41.6 %RH. Avoiding
non-representative sources of heat
and cold is therefore essential if an
accurate humidity measurement is to
be obtained.
Reliability for Humidity
A reliable humidity meter provides
valuable support when making
decisions on equipment investments
or maintenance, for example. In order
to ensure reliable measurements,
it is essential to carry out regular
instrument maintenance, including
instrument calibration and filter
Calibration of the hand-held meter
should be performed regularly to
guarantee optimal measurement
performance. Vaisala recommends
a one-year calibration interval for
all its humidity meters. Calibration
can be performed on site with a
humidity calibrator such as the
Vaisala Humidity Calibrator HMK15.
Alternatively, the instrument can be
sent to a local Vaisala Service Center
for calibration.
Some hand-held meters include
exchangeable measurement probes
or modules which enable the old
probe to be easily replaced by a new
one with a fresh factory calibration.
In addition to regular calibration,
maintaining the condition of the
sensor’s protective filter is critical for
accurate humidity measurements. If
the filter gets dirty, there is a risk of
inadequate gas exchange between
the sensor and the surrounding
environment, resulting in a micro
environment around the sensor that
is independent of the surrounding
humidity conditions. A quick filter
change is enough to fix this problem.
Please note that to avoid damage
when changing the filter, the sensor
element should not be touched.
Follow Humidity Trends
In many cases it is more informative
to observe the humidity and
temperature readings over a longer
period of time instead of performing
short spot measurements. For
example, a 24-hour measurement
cycle for the humidity and
temperature data better supports
the examination of the control
cycle and environmental conditions
in the space than random spot
polymer absorbs and releases water
vapor as the relative humidity of
the surrounding air increases and
decreases. The dielectric properties
of the polymer film depend on the
amount of absorbed water. A change
in the relative humidity of the
surrounding environment changes
the capacitance of the sensor. The
instrument’s electronics measure
this capacitance and convert it into a
humidity reading.
to learn more about our complete
humidity offering.
Relative Humidity
Relative humidity is the ratio
of the partial pressure of water
vapor in the air to the saturated
vapor pressure of water at a
defined temperature.
The relative humidity of indoor
air is influenced by both air
temperature and the water
content of the air. The warmer
the air, the more water it can
contain and, conversely, the
cooler the air, the less water it
can contain.
Relative humidity is normally
expressed as a percentage
(0–100 %).
Additional humidity
parameters – such as dewpoint,
wet bulb temperature, absolute
humidity, mixing ratio, and
enthalpy – can be calculated
from relative humidity and
temperature readings.
Humidity Under Control
Since introducing the Vaisala
HUMICAP® thin-film capacitive
humidity sensor in 1973, the sensor
has developed from one company's
innovation into a global industry
Today, capacitive thin-film polymer
humidity sensors are widely used
in both industrial and commercial
applications. The sensor consists of
a substrate on which a thin film of
polymer is deposited between two
conductive electrodes. The thin-film
For more information, visit or contact
us at [email protected]
Ref. B211097EN-B ©Vaisala 2011
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