Photo-ionisation detection (PID) technology is generally considered the technology of choice for monitoring exposure to toxic levels of VOCs. The sensors include a lamp as a source of high-energy ultraviolet (UV) light. The lamp encases a noble gas, most commonly krypton, and electrodes. The UV light’s energy excites the neutrally charged VOC (Volatile Organic Compounds) molecules, by removing an electron.
The amount of energy needed to remove an electron from a VOC molecule is called the ionization potential (IP). The larger the molecule, or the more double or triple bonds the molecule contains, the lower the IP. Thus, in general, the larger or more fragile the molecule, the easier it is to detect.
This technology does not require use of a sinter, which might prevent the gas reaching the sensor. It is also not susceptible to poisoning by chemicals in cleaning products, or silicone, although some cleaning agents containing large fragile molecules can cause positive readings.
Benefits of PID Technology
A high number of solvent species are sensed by this technology. Books have been written detailing the PID cross calibration responses to more than 750 solvent and gas types at ppm concentrations. It does not need air to function, it does not suffer from poisons and gives minor variation for moderate changes in temperature.
PID is extremely sensitive and will respond to many different VOCs. The magnitude of the response is directly proportional to the concentration of the gas. However, 50ppm of one gas will give a different reading to 50ppm of a different gas. To cope with this, detectors are usually calibrated to isobutylene and then a correction factor is employed to get accurate readings for a target gas. Each gas has a different correction factor. Therefore, the gas must be known for the right correction factor to be applied.
Consequently, pellistor sensors and photo-ionization detectors can be considered complementary technologies for many applications. Pellistors are excellent at monitoring for methane, propane, and other common combustible gases at %LEL (Lower Explosive Limit) levels. On the other hand, PID detects large VOC and hydrocarbon molecules that may be virtually undetectable by pellistor sensors, certainly in the parts-per-million range required to alert to toxic levels. Thus, the best approach in many environments is a multi-sensor instrument equipped with both technologies.
PID sensor technology is very versatile and can be used, for example, for clearance measurements during shutdowns in the chemical and petrochemical industries, monitoring operations in shafts and enclosed spaces, detecting leaks and many other applications.
Factors that affect PID Technology and their problems
Lack of voltage to the sensor affects the function of a PID sensor, also extremely high humidity, or particle densities. Also, the lamps last 2 years, but they will not last for 3 so the output must be monitored to check it has not gone into a fault condition.
The problems with this sensor are limited to age related issues.
- Lamps age, voltage stacks work less well when they get dusty
- Some common gas types have zero response, e.g., methane and propane. The risk assessment needs to show the gas types expected have a response. If this information is not known for a gas type, then our website or customer support personnel can help.
- PID sensors are the highest cost sensors we use in our products. They are good, but with the quality comes the cost.
How do I know when the technology is failing?
The results drop from the pedestal value sensed by out PID bearing products, causing our instrumentation to go into fault.
Our portable and fixed products are fitted with PID sensors that will detect large VOC and hydrocarbon molecules that may be virtually undetectable by pellistor sensors, certainly in the parts-per-million range required to alert to toxic levels.
To explore more, visit our technical page for more information.