What is a PID and What Does it Measure?
A Photoionization Detector (PID) is a non-specific sensor that measures a broad range of organic and some inorganic vapors in parts-per-million (ppm) or parts-per-billion (ppb). It is most commonly used for screening "Total VOCs" rather than identifying a specific gas [08:15].
Step 1: Understanding the Ionization Process
The PID works by using an ultraviolet (UV) lamp to "shatter" gas molecules into ions:
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The Process: Light energy hits the gas molecule, knocking off an electron and creating a positive ion. These ions are collected on an electrode, creating an electrical current that the meter converts into a digital reading.
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IP vs. eV: A gas will only be detected if its Ionization Potential (IP) is lower than the electron volt (eV) rating of the lamp. If the IP is higher than the lamp energy, the PID will read zero even if the gas is present [14:30].
Step 2: Choosing the Right UV Lamp
The strength of the UV lamp determines which gases you can "see":
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10.6 eV (Standard): The most common lamp; it is rugged and detects a wide range of VOCs like Benzene and Toluene.
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9.8 eV: More selective; often used to filter out certain gases while focusing on specific aromatics.
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11.7 eV: High-energy lamp; needed for harder-to-ionize chemicals like Methylene Chloride or Formaldehyde, but has a much shorter lifespan [19:45].
Step 3: Applying Correction Factors (CF)
Because PIDs are usually calibrated to Isobutylene, you must adjust the reading for other gases:
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The Formula: $Actual Concentration = Instrument Reading \times CF$.
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Example: If you are measuring a gas with a CF of 0.5, and the meter reads 100 ppm, the actual concentration is 50 ppm. Most modern RAE monitors have these factors built-in for easy selection [28:10].
Step 4: Managing Environmental Interference
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Humidity: High humidity can cause "internal leakage," making the meter read higher than it should or causing "sensor drift."
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Contamination: Dust or oil on the lamp window can block UV light, leading to low readings. Frequent cleaning of the lamp with anhydrous methanol is vital for accuracy [42:00].