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These notes, from
RAE
Systems, provide advice on how to reduce condensation on your PID
sensor.
Question: How do I prevent humidity
condensation
in my PID?
Answer: Keep the PID clean and warmer
than the air sample.
When humidity condenses in a PID sensor, it
causes a false positive reading that drifts and
may saturate the detector. Such a situation can
occur when sampling highly humid streams
such as in soil vapor extractions or steam lines.
It also can occur when a monitor is calibrated
in a cool, air-conditioned lab or office, and
then taken outdoors into a hot, humid area. A
clean, dry PID sensor should not give a significant
response in humid air when the unit is at
the same or higher temperature as the gas.
Therefore, good sensor cleaning and protection
with filters is critical in such operations.
Also make sure that the sensor electrodes are
not bent or corroded.
However, if the PID is colder than the sample
gas, no amount of maintenance will prevent
condensation. Once condensation occurs, it
may take several minutes to clear out even after
the unit warms up to the outside or sample gas
temperature. Therefore, a soil sample should
never be heated above ambient temperature to
drive off VOCs, because this also drives off
water, causing condensation. In some cases it is
helpful to even warm the PID slightly before
testing soil samples.
When moving from a cool, dry indoor
environment
to a hot, humid outdoor area, the
PID should either be turned off or fitted with
a Humidity Filtering tube (p/n 025-2001-
010, box of 10). The filtering tube allows the
unit to be kept running which helps it warm
up faster to the outdoor temperature. That
way the PID can be used sooner. The tube is
able to maintain <2 percent RH downstream
humidity for a 500 mL/min sample flow at 99
percent RH and 45oC (113oF) for over 10 minutes.
The tube must be removed before measurements
are taken because it absorbs many
VOCs in addition to water vapor.
For more information, please refer to RAE
Technical Note TN-175 at
http://www.raesystems.com/technotes.html. |
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