What is a PID meter? In this article we answer that question and explain how a PID meter works. A photo ionisation detector can measure a wide range of volatile organic compounds (VOCs). Gases such as benzene, toluene, xylene and vinyl chloride are examples of VOCs. Fundamentally, VOCs can be recognised by the presence of carbon (C) and hydrogen (H) elements in their chemical structure.
The PID meter is, as it were, a detective searching for fingerprints with a magnifying glass. Fingerprints will be found, but whose fingerprint it is will not be immediately clear. A PID meter measures all VOCs present, but is not substance-specific and will not indicate which gases are involved. Good preparation and briefing are therefore important. If it is known in advance which gases can be expected, a PID meter will add a great deal of value.
How does a PID meter work?
As soon as the gas to be measured enters the PID meter (by diffusion or by means of a pump), it is exposed to high-energy photons. These photons are usually in the UV range. Through irradiation by these high-energy photons the molecules in the gas temporarily lose an electron, forming positively charged ions. The gas thereby becomes electrically charged and the ions generate an electric current. Using this electric current the PID meter can display a gas concentration. The more gas, the greater the electric current and the higher the reading on the display.
Where are PID meters used?
A PID meter can be used anywhere VOCs may occur. Think of refineries, paint factories, soil remediation contractors, water treatment plants, the fire service and CBRN. Depending on the application, different VOCs may be present. The meter can then be used for personal detection, space/tank clearance measurements, or continuous monitoring.
Ionisation potential and correction factors
To be able to detect a gas, the positively charged ions mentioned earlier have to be formed. This requires a certain amount of energy, expressed in electron volts (eV). Depending on the type of gas, a different minimum amount of energy is needed — after all, every gas is different. We call this value the ionisation potential (IP). The ionisation potential of a substance indicates how much power the PID meter needs in order to detect it.
In the world of gas detection there are three types of lamp with the following energy levels: 9.8 eV, 10.6 eV and 11.7 eV. The last of these (11.7 eV) is the strongest and can therefore ionise more different gases than the 10.6 eV or 9.8 eV lamps. The 10.6 eV PID lamp is, however, the most common, given its low total cost of ownership and the stability of the sensor. The 11.7 eV lamp is considerably less stable and has a shorter service life. The 9.8 eV lamp is the least powerful and is mainly used to measure gases such as benzene or butadiene. Because the lamp is less powerful, the PID measurement will detect fewer different gases, which in specific cases is precisely what is wanted.
In addition to an ionisation potential, a correction factor is often known for each gas to be measured. The correction factor can be used if the user of the PID meter is certain that only one gas can be expected. For example, in a production environment where only vinyl chloride is used or produced at a particular location. By applying the correction factor to the PID meter, the measured concentration can be tailored to this specifically chosen gas. Fundamentally, the PID meter takes a broadband measurement and is calibrated with isobutylene. To make this more specific (with the right prior knowledge), the correction factor can be applied.
Measuring VOCs with an LEL meter or a PID meter?
We are often asked whether VOCs can be measured with an LEL detector. Our answer depends on the application and the user's requirements. The LEL detector monitors the explosion risk of a range of flammable gases, which are often also volatile organic compounds. However, the LEL detector measures this at %LEL levels, and these are generally higher than the toxicity values. A PID meter has been developed precisely to monitor at ppm or ppb levels so that an early alarm can be raised on the toxic properties. Every gas has a different toxicity threshold; in the Netherlands these are laid down by the government as occupational exposure limits.
Which PID meter is right for my situation?
BaSystemen has a wide range of PID meters, from personal portable units to pumped clearance meters and area monitors. Fixed PID meters are also among the options. Our specialists will be happy to talk to you about the application and select the right measuring solution. Of course, you are also welcome to take a look at our website. Click the button to the left of this text to view various PID meters from BaSystemen.
