A Quick Guide to Maintaining your MiniRae 3000 and MiniRae Lite Photoionisation Detector (PID)
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Soil and water headspace sampling requires special attention beyond that needed for typical ambient air monitoring. Stripper effluents and soil vapour extraction streams are typically near 100% RH (relative humidity), and soil samples are often dusty and humid. Such conditions can cause high, drifting readings on many PIDs if not properly maintained. Interference is usually traceable to condensation in the sensor, causing a current leakage across the electrodes and thus a false-positive signal.
This situation is exacerbated when the sensor is contaminated by soil dust or condensed, high-boiling organic compounds.
The following steps will help you to maintain your PID:
a) Keep the sensor clean using high-purity methanol, preferably using an ultrasound bath. Flush the residual solvent from the sensor with a rapid stream of clean air, and clean the lamp housing area that contacts the sensor when in place.
b) On sensors with interdigital fingers, check that the metal electrode fingers do not contact the Teflon sensor walls. Bend them out carefully if necessary. Replace the sensor if the electrodes are corroded.
c) Keep the lamp clean using high-purity methanol. Never use acetone on 11.7 eV lamps.
d) Perform frequent changes of the dust filters (daily to monthly, depending on usage and dirtiness).
e) Use additional external filters (e.g., Teflon “water trap”) as an extra precaution, especially in dusty or moist environments. Once a - e are complete you will need to re-calibrate your unit. Please see the video below
f) Start sampling by using a dilution attachment, especially for highly contaminated soils. This minimises the amount of dust, water, and high-boiling organics condensing on the sensor and lamp. If the concentrations are too low to give a reading, remove the attachment to obtain an undiluted reading. Its important to remember, if you see anything around or over 1000ppm you are likely to have condensed VOCs on and in your PID and it'll need cleaning.
g) Avoid situations in which the PID is colder than the soil being sampled, such as heating the soil samples to increase the headspace organic concentration, or bringing a cold PID into a warm room without allowing time for temperature equilibration. If anything, try to keep the PID warmer than the soil samples. Otherwise you will create a condensing surface inside your PID and moisture will form on the sensor and lamp.
h) To obtain more stable readings, plumb the effluent flow from the PID back into the sample container to prevent diluting the sample. 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.
i) If humidity problems persist, use a humidity filtering tube to absorb moisture.
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