Photoionization Detectors: Finding Combustible Gases
Often used along with wireless gas detectors, photoionization detectors, also called PIDs don’t find all gas. Instead with the PID you can detect Volatile Organic Compounds – or VOCs which indicate how likely an explosion is.
The first PID came out in 1973. Today RAE system’s continue to make a line of PIDs.
An Example of a PID is the AreaRAE series of gas detectors.
PIDs work by using a UV lamp on the front of the unit. When the VOCs come into contact with this lamp it causes the gas to ionize.
Through this process the gas picks up an electrical charge, either positive or negative.
The ions then move into the sensor which is able to detect the current going through them. From this current the gas detector is able to find the concentration of gas going in to the sensor in ppm or ppb.
What is a VOC
One of the downsides of the PID is that it only detects VOCs instead of specific gases. This means
What is a VOC then? What makes it different than other gases?
VOCs are things like amines and sulfides, as well as most aidehydes, many alcohols, and many hydrocarbons. Any substance that can become ionized by the photons coming out of the UV lamp is a VOC.
It is important to remember the PID will not detect many dangerous gases and chemicals if they are not VOCs. This includes some common elements like oxygen, as well as other toxic substances like methane, carbon monoxide, and hydrochloric acid.
Additionally having certain non-detectable gases present can lower the effectiveness of the detector, as they block the photons shot out by the UV light from reaching the right particles.
Calibrating your Photoionization Detector
PIDs are typically calibrated using 100 ppm isobutylene. Because of this gas measurements are in “isobutylene equivalents”.
Honeywell provides a guide to translating these isobutylene equivalents into other gases on their website.
Other issues can also throw off the accuracy of the numbers. High humidity, for example, since all device calibration happens in dry conditions, or an excess amount of dust over the sensor.
For this reason you should make sure the sensors on these units are kept clean using a kit.
If properly cared for, the photoionization detector is a powerful gas detection tool. Used well it can help keep workers safe.