PID Operating Characteristics

PID Operating Characteristics

• Detects Total Volatile Organic Compounds
• Accurate, Sensitive to PPM levels
• No External Fuel Needed
• Minimal Training Needed to Operate
• Limitations:
  • Non-specific
  • Subject to signal loss from:
    • High RH
    • High CH₄
    • High O₂

Effects of Methane on PID Output

• High concentrations of methane can "quench" PID signal

% Methane	Volume % LEL Methane	Reading when exposed to 50 ppm hexane in the presence of Methane
2.5%	50% LEL	26 ppm
1.0%	20% LEL	45 ppm
0.5%	10% LEL	48 ppm
0.25%	4% LEL	49 ppm

PID TVOC Applications

• Rapid screening technique for initial assessment
• Detect wide range of toxic VOCs
• Sensitive to PPM levels
• Accurate and linear over wide range
• Low Cost
• Multiple applications:
  • PEL/TLV compliance
  • Hazardous threshold indication for toxic / combustible
  • Hazmat / Emergency response
  • IAQ
  • WMD / CWA

PID detectable compounds

• Most VOCs with:
  • Boiling Point < 200° C
  • SVapor Pressures (Pv) > 1.0 mm Hg at 20° C
• Detect some inorganics (e.g. NO, NO₂, NH₃)
• Hydrides (arsine, phosphine)
• Do Not Detect:
  • CO, CO₂, SO_x,
  • Metals
  • Semi-Volatiles - PAH, higher phenols
  • Non-Volatiles - PCBs, pesticides

Compounds Detectable by PID

Organics: Compounds with carbon

Aromatic compounds (containing benzene ring): Benzene, Toluene, Xylene

Ketones and aldehydes (containing C=O bond): Acetone, MEK

Amines & amides (compounds containing nitrogen): Diethyl amine

Chlorinated hydrocarbons: Perchlorethylene, Trichloroethylene (TCE)

Alkanes (saturated hydrocarbons C3 and higher): Pentane, Hexane

Unsaturated hydrocarbons (double or triple carbon-carbon bonds): Butadiene, Isobutylene

Alcohols (-OH): Ethanol, Isopropanol

Sulfides and compounds containing sulfur: Mercaptans, Hydrogen sulfide

Inorganics (compounds without carbon): Ammonia, Chlorine

Hydrides: Arsine, Phosphine

Compounds not detectable by PID

Compounds normally present in air: Oxygen, Nitrogen, Carbon dioxide, Argon

Inorganic toxics: Carbon monoxide, Hydrogen cyanide, Ozone (O3)

Hydrocarbons and VOCs with ionization energies higher than 11.7eV: Methane, Natural gas

Acids: Sulfuric acid (H₂SO₄), Hydrochloric acid (HCl), Nitric acid (HNO₃)

Radiation

Aerosol droplets and particulates

PID as "BroadRange" Sensor

• VOCs usually detected by means of broad-range sensors
• Broad-range sensors provide overall reading for general class or group of chemically related contaminants
• Cannot distinguish between different contaminants they are able to detect
• Provide single total reading for all detectable substances present

PID instruments are nonspecific

• Reading is sum of signals of all detectable substances present, also:
• Reading is function of their varying ionization potentials and other physical properties
• PID readings always relative to gas used to calibrate detector
• Equivalent concentrations of gases other than the one used to calibrate the instrument may not produce equivalent readings!

Response is Relative to Gas Measured

• Reading of 10 ppm only indicates ion current equivalent to that produced by 10 ppm concentration calibrant
• Amount of different contaminant needed to produce same current may be larger or smaller than concentration of calibrant
• Since PID readings always relative to calibrant, should be recorded as ppm-calibration gas equivalent units, or PID units, never as true concentrations unless:
  • Contaminant being monitored is same as one used to calibrate instrument, or
  • Reading is corrected to account for difference in relative response

PID Correction Factors

• Correction Factor (CF) is measure of sensitivity of PID to specific gas
• CFs do not make PID specific to a chemical, only correct the measurement scale to that chemical
• CFs allow calibration on inexpensive, non-toxic "surrogate" gas (like isobutylene)
• Most manufacturers furnish tables, or built-in library of CFs to correct or normalize readings when contaminant is known
• Instrument able to express readings in true parts per million equivalent concentrations for the contaminant measured

CF measures sensitivity

• Low CF = high PID sensitivity to a gas
• More toxic the gas, more desirable to have low correction factor :
  • If Exposure limit is < 10 ppm, CF should be ≦ 1
• If chemical less toxic, higher CF may be acceptable
  • If Exposure limit is > 10 ppm, CF ≦ 10
• When CF > 10 use PIDs as gross leak detectors only
  • High correction factor magnifies effects of interfering gases and vapors

PID readings only quantifiable if measuring a known substance

Decision making with a PID

• Two sensitivities must be understood to make a decision with a PID
  • Human Sensitivity: as defined by AGCIH, NIOSH, OSHA or corporate exposure limits
  • PID Sensitivity: as defined through testing by the manufacturer of the PID

Correction Factors (10.6 eV Lamp)

Actual response of PID (Isobutylene scale) to 100 ppm Toluene