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PID Sensor Operating Principles

PID - Operating Principle

  • PIDs use ultraviolet light as source of energy to remove an electron from neutrally charged target molecules creating electrically charged fragments (ions)
  • This produces a flow of electrical current proportional to the concentration of contaminant
  • The amount of energy needed to remove an electron from a particular molecule is the ionization potential (or IP)
  • The energy must be greater than the IP in order for an ionization detector to be able to detect a particular substance

Operation of PID lamp, sensing and counter electrodes

1. Neutrally charged molecule diffuses into glow zone

Operation of PID lamp
Operation of PID lamp

2. Molecule is ionized

3.Free electron is electrostatically accelerated to positively charged sensing electrode where it is counted

Operation of PID lamp
Operation of PID lamp

4.Positively charged fragment (ion) is electrostatically accelerated to counter electrode, where it picks up a replacement electron and regains neutral charge

Ionization Potential

  • IP determines if the PID can detect the gas
  • If the IP of the gas is less than the eV output of the lamp the PID can detectthe gas
  • Ionization Potential (IP) measures the bond strength of a gas and does not correlate with the Correction Factor
  • Ionization Potentials are found in the NIOSH Pocket Guide and many chemical texts

Ionization Potential Values

Substance Ionization Energy (eV)
carbon monoxide 14.01
carbon dioxide 13.77
methane 12.98
water 12.59
oxygen 12.08
chlorine 11.48
hydrogen sulfide 10.46
n-hexane 10.18
ammonia 10.16
hexane 10.13
acetone 9.69
benzene 9.25
butadiene 9.07
toluene 8.82

Technical Advances in PIDs

Technical Advances in PIDs
  • Miniaturization
  • Ruggedness
  • EMI/RFI resistance
  • Lower humidity interference

PID Components

  • Detector assembly
  • Electrodes: sensing, counter and (in some designs) fence
  • Lamp: most commonly 10.6EV, 11.7eV or 9.8 eV
PID Components

PID Sensing and Counter Electrodes

PID Components

Characteristics of PID Lamps

Characteristics of PID Lamps
  • Sealed borosillicate glass body
  • Window of specific crystalline material
  • Filled with specific noble gas or mixture of noble gasese
  • 10.6 eV lamp should last 10,000 operating hours or three years or longer

Characteristics of PID lamps

Nominal Lamp Photon Energies Gas in LampMajor Emission LinesRelative IntensityWindow CrystalCystal transmittance λ Range (nm)
eV(nm)
11.7eV Argon11.83
11.62		104.8
106.7		1000
500		Lithium fluoride (LiF)105 - 5000
10.6eV Krypton10.64
10.03		116.5
123.6		200
650		Magnesium fluoride (MgF2)115 - 70000
9.8eV Krypton10.03123.6650Calcium fluoride (CaF2)125 - 8000