The problem with PIDs
PIDs – the information you get from them can be misleading.
PIDs or photoionization detectors are also known as volatile organic compound (VOC) detectors.
We love PIDs because they’re a great tool but people believe them without question way too often.
Think of a PID as a very powerful UV lamp. When the energy from that lamp comes into contact with a gas molecule, it temporarily splits (ionizes) the gas molecule into pieces (ions).
These ions migrate to positive (+ve), and negative (–ve) ions migrate to a detector which measures the current the ions generate, amplify and massage the current and produce a ppm or a ppb reading. There are lots of resources on how they really work. Just Google, “How a PID works”:
The top 5 things to watch for:
1. PIDs can lose accuracy:
Many things happen in the life of a PID sensor to make it lose accuracy. Once we have calibrated with isobutylene, how long is it good for?
The answer is that it depends on how important the decisions being made with the data are:
Dust, dirt and other contaminants accumulate on the lens of the PID over time. That’s why they all come with cleaning kits.
These contaminants can decrease output by obscuring the lens, reducing the number of photons available and reducing PID displayed output.
They can also be a pathway for electrons to escape, sometimes causing the display to read high.
Either way, too high or too low –if you don’t clean the lens, calibrate and record frequently, your results may not be useful or defensible.
Most PIDs include cleaning kits and directions can be found in the manual or on YouTube.
2. Humidity is a problem
Humidity is a huge problem with some PIDs. Most PIDs and their users are blind to RH effects:
Water vapour scatters and absorbs photons, reducing the PID output. This is also called ‘quenching.’
With high RH, you may find current leakage along the sensor walls, resulting in higher than actual readings. This is especially bad when there are contaminants on the sensor lens.
Since we calibrate on dry calibration gas, the actual reading in a humid environment or wet head space can be suspect.
Some manufacturers use algorithms or desiccant tubes to reduce RH affects, but both are fraught with problems and can introduce additional errors.
The newest PIDs are designed with fence electrodes to reduce electrical leakage and very small sensor cavities (1% of original size) to eliminate the RH quenching effect. Be sure to ask the question about RH affects from multiple vendors, because not all the answers will be the same.
Say you calibrate using 100 ppm isobutylene. Since most sensors are linear to 500 ppm, every number you see between 0-500 should be very accurate - but can you believe what you see?
You calibrated on isobutylene, so unless you are measuring only isobutylene, all you can really say is that you are reading in “isobutylene equivalents”.
You’re smart though, and you can read a Correction Factor Table (like this one you can download from RAE Systems, Tech Note 106)
That means you apply a 0.86 correction factor to a reading, so that 100 on your isobutylene calibrated instrument when you are measuring only MEK, means that there is only 86 ppm of MEK.
What happens when you have five different solvents present at the same time? Well, you could try the calculation at the bottom of the RAE Tech Note 106 (good luck!), or go back to reporting in isobutylene units, typically using the worst case TLV/TWA as your action level.
4. What does (or doesn't) a PID measure?
It doesn’t measure all gases, so you need to know what you might have and what you might not see.
In general a 10.6 eV lamp (MgF2 window, Kr gas) will measure:
- Hydrocarbons ending with –ane,-ene,-yne (except methane and ethane)
- Alcohols ending in –ol (except methanol)
- All aldehydes (except formaldehyde)
- Ketones ending in –one
- Esters ending in –ate
- All amines and sulfides - as long as none contain chloro, flouro or bromo in their name
PIDs do not measure these guys:
- Components of air (02, N2, CO2, H2O)
- Common toxics (CO, HCN, SO2)
- Natural gas (methane and ethane)
- Acid gases (HCl, HF, HN03, etc.)
- Non volatiles (PCBs, grease, etc.)
- With the use of a higher power 11.7 eV lamp (LiF2 window and Ar gas) we extend the number of detectable VOCs, to all of the following (but at the cost of a shorter lamp life):
- Methanol and formaldehyde
- 80% of compounds that contain chloro, flouro or bromo in their name
5. Quenching Effects:
Beside the quenching effect of RH on the detector output, many of the gases listed in the “not these guys “ list and halogenated compounds also cause quenching.
This occurs because the photon that is supposed to ionize the VOC hits something it can’t break apart (not enough energy in the photon). As a result, that photon becomes unavailable to ionize anything else. Kind of like a snowball hitting a brick wall - there is nothing useful left after the impact.
Net result: Quenching causes a reduction in detector output
Example: you can’t measure for aromatics reliably in a matrix containing %bv range methane.
PIDs are great indicators in safety:
As long as you understand their limitations, photoionization detectors are a great indicator in safety, industrial hygiene and environmental applications.