Dewpoint meters: Selection matters
November 1st 2010 Dewpoint is among the most important measurements to
ensure a clean, dry compressed air system. There are plenty
of meter options available in the market but it pays to
consider your selection process carefully, as Vaisala exp
Careful choice influences the dewpoint
meter's stability and accuracy,
eventually affecting process efficiency
and the finished product's quality. Vaisala
experts have identified four questions that
they believe are helpful in
narrowing down the options.
Portable or fixed?
Buyers needs to decide
whether they require a
portable or fixed platform.
Portable battery powered
devices vary in size and
function but are typically
used for verifying fixed
mount transmitters in the
field or for spot-checking
various points in a
compressed air system.
Fixed mount transmitters
usually require an external
power source and can greatly range in
complexity.
The most sophisticated units have
displays, alarm relays, datalogging, ethernet
connections and more. These can be
installed in the supply side, for example for
dryer monitoring and control, or in the
demand side connecting to PLCs, dataloggers
and panel meter displays.
Dryer type dictates range
Vaisala recommends the buyer check the
type of dryers being used in the system. In
most compressed air systems the type of
dryer will dictate the dewpoint operating
range required from the dewpoint sensor.
Sensing technologies are optimised to excel
in specific ranges - very dry conditions, midrange
ambient levels, or high humidity
condensing environments. Choosing the
instrument with the right measurement
range will impact both the short and long
term accuracy and stability of the dewpoint
measurement.
Temperature and pressure
tolerance
Temperature doesn't affect the dewpoint value
but may impact the sensor's performance. If
the air is very hot at the intended
measurement location, installation of a
sampling system may be required. Pressure
conditions need to be understood as well.
Changes in pressure have a dramatic effect on
dewpoint. Also, the mechanical pressure
rating of the probe needs to withstand the
pressure demands of the process.
In-the-line or via a sample cell
Last but not least, the decision on whether
the measurement should be in-situ or
outside the system using a sampling line
narrows down the dewpoint transmitter
options. According to Vaisala, in-the-line
measurements can be less costly to install
and have potentially faster response time.
Drawbacks can include fluctuating line
pressures resulting in changing dewpoint
values and difficulty in removing the probe
conveniently from the compressed air system
when it's time for service or calibration. The
alternative approach involves the use of a
sample cell and stainless steel tubing
connected to the main line. The advantages
are better control of air pressure minimising
main line fluctuations, ability to isolate the
sensor from the system and the option to
cool the air sample if necessary.
Help available
As dew point measurements can be made
almost anywhere on the supply or demand
side of a system, the application conditions
can vary significantly thereby affecting
instrument performance requirements.
Buyers should also make sure that sensors
withstand and recover from exposure to
many contaminants, such as compressor oil,
water spikes, ambient humidity and chemical
impurities. If you have any doubts about
selection, manufacturers of dewpoint meters
should be able to advise. Vaisala, for
example, offers help 24/7 (www.vaisala.com). More articles from Vaisala Ltd: | 
