Photo-Ionization Detector (PID)
1. Brief description

This gas sensor applies ultraviolet light to the detectable gas to ionize it. This causes an ion
current to be generated. The sensor measures this current to determine the gas concentration. It
detects a wide range of gases, irrespective of whether they are organic or inorganic. It is generally
used to measure ppb to ppm levels of concentration of volatile organic compounds (VOCs).

Category

Detectable gas

Other methods

Toxic

2. Structure and principles[Structure and principles][Structure]

The sensor consists of an ionization chamber for letting in thedetectable gas, a ultraviolet lamp for applying light, and positive andnegative electrodes for detecting ion currents.

Ionization chamber
Detectable gas

Gas release[Principles]M

The detectable gas enters the ionization chamber and is exposed toultraviolet light from the light source (ultraviolet lamp). This causes
the gas to release electrons, generating cations. The generated
cations and electrons are drawn by the positive and negativeelectrodes, which causes a current to be generated. Since thiscurrent is proportional to the gas concentration, the sensor
measures the current value to determine the concentration of thedetectable gas. Ionizing a detectable gas requires application ofphoton energy larger than the ionization energy specific to that gas.
Photon energy is expressed in the unit electron volt (eV). Thissensor uses a lamp having photon energies such as 10.6 eV and11.7 eV. The larger the photon energy is, the larger amount ofdetectable gas the sensor can ionize.

○ Ultraviolet lamp

The photon energy(eV) of a ultravioletlamp is determined by
the combination of thegas contained in thelamp and the materialof the lamp window.

APowersupplyCarbon tetrachloride
Dichloromethane
Acrylonitrile
Formaldehyde
Ethylene
Isopropyl alcohol
Vinyl chloride
Methyl ethyl ketone
Perchloroethylene
Trichloroethylene
BenzeneStyrene00
100
VOC Concentration (ppm)Gascontained

Window material

Photon energy(eV)

Xenon

Sapphire

8.4

KryptonMagnesium fluoride

10.6

Argon

Lithium fluoride

11.7(for 10.6-eV lamp)

Molecular
formulaDetectable gas(for 11.7-eV lamp)

Molecular
formulaC 2H 4Carbon tetrachloride

CCl4

Isopropyl alcohol

C 3 H 8O

Dichloromethane

CH2Cl2Vinyl chloride

C2H3Cl

Acrylonitrile

C 3H 3N
HCHOEthylene

11.47
11.30
10.91
10.87
10.50
10.10
9.99
9.54
9.50
9.47
9.24
8.4

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４. Detectable gas and molecular formula (examples)
Detectable gas

Ammeter

By applying photon energy larger than the gas-specificionization energy to each gas, the sensor ionizes thegas to determine the gas concentration. The sensor
typically uses a lamp of 10.6 eV or 11.7 eV.

Sensor output(μA)

Ultravioletlight○ Ionization energies of typical substances

10For a gas with a low
concentration of a few hundredppm, the sensor output is almost
proportional to the gas
concentration, increasing linearlywith the gas concentration.

e-M+

Current○ Output characteristics

Electrode ＋

Ultravioletlight

3. Features

IonizedElectrode －Methyl ethyl ketone

C 4H 8O

Formaldehyde

Perchloroethylene

C2Cl4

Acetylene

C 2H 2Trichloroethylene

C2HCl3Chloroform

CHCl3

BenzeneC 6H 6

Carbonyl sulfide

COSStyrene

C8H8Chlorine

Cl2810
Ionization energy (eV)

10.6

11.7

5. Products of this type (examples)○Stationary products... TVOC2○Portable products... GX-6000, Tiger, CUB

GX-6000

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