
9
60°C the transmitter electronics
should be mounted outside the
process and only a suitable high
temperature probe should be
inserted into the high temperature
environment. Moreover, built-
in temperature compensation is
required to minimize the errors
caused by large temperature
swings or operation at temperature
extremes.
When measuring humidity in
processes operating at around
ambient pressure, a small leak may
be tolerable and can be reduced by
sealing around the probe or cable.
However, when the process needs
to be isolated, or when there is a
large pressure difference between
the process and the external
environment, a sealed probe head
with appropriate mounting must be
used. Pressure leaks at the point of
entry will alter the local humidity and
result in false readings.
In many applications it is advisable
to isolate the probe from the process
with a ball valve to enable the
removal of the probe for maintenance
without shutting down the process
(see figure 3).
When is a Sampling
System Needed for
Dew Point Measurement?
Wherever possible, the probe
should be mounted in the actual
process to achieve the most accurate
measurements and a rapid response
time. However, direct installations
are not always feasible. In such
situations, sample cells installed
in-line provide an entry point for a
suitable measurement probe.
Note that external sampling systems
should not be used to measure
relative humidity because the
change in temperature will affect
the measurement. Sampling systems
can instead be used with dew point
probes. When measuring dew point,
sampling systems are typically used
to lower the temperature of the
process gas, to protect the probe
against particulate contamination,
or to enable easy connection and
disconnection of the instrument
without ramping down the process.
The simplest dew point sampling
setup consists of a dew point
transmitter connected to a sampling
cell. Vaisala has several models
suitable for the most common
applications and sampling needs.
For example, the easy to install
DSC74 sampling cell is designed for
the flow and pressure conditions in
compressed air applications.
In demanding process conditions,
sampling systems must be designed
carefully. As dew point is pressure
dependent, a flow meter, pressure
gauge, special non-porous tubing,
filters, and pump may be needed. As
an example, a flow chart showing the
Vaisala DRYCAP
®
Portable Sampling
System DSS70A for DM70 is shown in
figure 4.
Figure 3: A ball valve installation in a
process pipe line.
The Right Instrument
for High Humidity
Environments with >90%RH are
defined here as high humidity
environments. At 90%RH a
difference of 2°C can cause water to
condense on the sensor, which in an
unventilated space may take hours
to dry. Vaisala humidity sensors
will recover from condensation.
However, if the condensed water
is contaminated, the instrument
accuracy can be affected due to
deposits on the sensor, especially
salt deposits. Even the life of
the sensor may be shortened. In
applications with high humidity
where condensation can occur, a
warmed sensor head probe such as
the Vaisala HUMICAP
®
Humidity and
Temperature Transmitter HMT337
should be used.
The Right Instrument for
Low Humidity
Environments with <10%RH are
defined here as low humidity
environments. At low humidities, the
calibration accuracy of instruments
measuring relative humidity may not
be adequate. Instead, measuring dew
point will provide a good indication
of humidity. For example, Vaisala
DRYCAP
®
products are designed for
measuring dew point.
If a dryer fails in a compressed air
system, water condensation may
appear and the instrument will need
to recover. Many dew point sensors
are damaged or destroyed in such
situations, but Vaisala DRYCAP
®
dew point sensors withstand high
humidity – and even water spikes.
The Right Instrument for
Extreme Temperature and
Pressure Conditions
Continuous exposure to extreme
temperatures may affect sensor
and probe materials over time. It is
therefore very important to select
a suitable product for demanding
environments. In temperatures above
Installation with the ball valve set
Humidity
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