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

2. Molecule is ionized

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

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

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 Sensing and Counter Electrodes

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 Lamp	Major Emission Lines		Relative Intensity	Window Crystal	Cystal transmittance λ Range (nm)
Nominal Lamp Photon Energies	Gas in Lamp	eV	(nm)	Relative Intensity	Window Crystal	Cystal transmittance λ Range (nm)
11.7eV	Argon	11.83 11.62	104.8 106.7	1000 500	Lithium fluoride (LiF)	105 - 5000
10.6eV	Krypton	10.64 10.03	116.5 123.6	200 650	Magnesium fluoride (MgF2)	115 - 70000
9.8eV	Krypton	10.03	123.6	650	Calcium fluoride (CaF2)	125 - 8000