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Placing the probe of an valuable PID
in contaminated, moist soil isn't the most ideal situation.
However by following a few simple steps you can keep your readings
accurate and your PID safe from damage. These notes,
RAE
Systems Technical Note TN-118, provide useful tips on using RAE PIDs
for soil headspace measurements.
Measuring organic compounds emitted from
potentially contaminated soils requires special attention beyond that
needed for typical ambient air monitoring. Soils are often dusty and
humid, conditions that can cause high, drifting readings on MiniRAE PIDs
if not properly maintained. Interferences are usually traceable to
condensation in the sensor, causing a current leakage across the
electrodes and thus a false positive signal. The situation is
exacerbated when the sensor is contaminated by soil dust or condensed,
high boiling organic compounds.
Preparing a Sample
Place the sample into a clean sample
container or bag. Ensure that enough air space
("headspace") above the sample is present for sampling.
Shake the sample container to thoroughly mix the soil sample with the air
in the headspace. Let the sample equilibrate to room temperature of
approximately 25 deg C. Sample through a septum in a hard container
or through the sample bag wall. A septum can easily be formed in
virtually any hard container with aluminium foil held in place by a rubber
band around the rim of the vessel. When sampling with the PID,
ensure that dirt and moisture are not sucked into the PID probe.
To ensure optimum performance:
1) Keep the sensor clean using high-purity
methanol, preferably using an ultrasonic bath.
2) Keep the lamp clean using high- purity
methanol. Never use acetone.
3) Use the C-filter to absorb moisture and
dust. Perform frequent changes of the C-filter and membrane filters
(daily to weekly depending on usage and dirtiness).
4) Use the water trap filters as an extra
precaution, especially in dusty or moist environments where water mist may
be present.
5) Avoid situations in which the PID is
colder than the soil being sampled, such as heating the soil samples to
increase headspace organic concentration, or bringing a cold PID into a
warm room without allowing time for temperature equilibrium. If
anything, try to keep the PID warmer than the soil samples.
6) To obtain more stable readings, plumb
the effluent flow from the MiniRAE PID back into the sample container to
reduce the losses. Use Teflon or metal tubing for this purpose so as
to prevent adsorption to Tygon or other plastic tubing. Losses will
not be stopped altogether but will be greatly reduced. |
