The PID uses a UV light source to ionize organic molecules into positive and negative ions (ionization) that can be detected by the detector. The detector captures the positive and negative charges of the ionized gas and converts it into a current signal to measure the gas concentration.

When the gas to be tested absorbs high-energy ultraviolet light, the gas molecules are excited by ultraviolet light, temporarily losing electrons and becoming positively charged ions. The gas ions, when detected at the detector‘s electrodes, quickly combine with the electrons to reconstitute the original gas and vapor molecules.

The PID is a non-destructive detector that does not change the gas molecules to be tested. The gas detected by the PID can still be collected for further determination.

UV lamp selection

The UV lamp (UV) that can be used on the PID sensor has three types: 9.8 eV, 10.6 eV, and 11.7 eV. Among them, the 11.7eV UV lamp has the highest ionization energy (IP) due to the emitted light, so the PID detection range is the widest.

However, all 11.7eV UV lamps use lithium fluoride as a high-energy UV output window. Lithium fluoride crystal materials, which are packaged on the tube glass, are quite difficult. When it is not used, the lithium fluoride crystal material in the air absorbs moisture, causing the window to rise and weaken the intensity of the ultraviolet rays passing through it.

Lithium fluoride crystal materials will also gradually age due to UV irradiation, resulting in damage to the entire instrument. These factors work together, resulting in a shortened lamp life of 11.7 eV. A 10.6eV UV lamp can last for 12-24 months, while a 11.7eV lamp can only last for 2-6 months. At the same time, the cost of the 11.7eV UV lamp is higher than 9.8eV and 10.6eV, which further reduces its practicability. The 11.7 eV UV lamp is generally used only when the ionization potential of the compound (dichloromethane, carbon tetrachloride) exceeds 10.6 eV. At the same time, 9.8 and 10.6eV have many features that are not available in 11.7eV UV lamps: 9.8 and 10.6eV PIDs have stronger characteristics: low ionization energy means less chemicals can be detected.

The 9.8 and 10.6eV PIDs are used for at least one year: their lifetime is comparable to that of a carbon monoxide sensor.

The 9.8 and 10.6 eV PIDs are more sensitive, and the 11.7 eV lamp is less sensitive, mainly because of its window material, lithium fluoride crystals, which hinder the 11.7 eV ultraviolet light. The decrease in the emitted light energy makes it difficult to sufficiently ionize the substance to be measured. Therefore, it is difficult to accurately detect the accurate data with a PID of 11.7 eV.

For the above reasons, a 10.6 eV UV lamp should be selected as the PID light source.