PID / VOC Monitors: Questions, Myths and their Proper Use

Introduction

Solvents and fuel vapours are commonplace in our workplace environment. However, increasing awareness of their toxicity has led to lower exposure limits and increased requirements for direct measurement of these substances at their Occupational Exposure Limit concentrations.

Photoionization detectors (PIDs) are increasingly viewed as the best method for the measurement of VOCs at exposure limit concentrations. However, the capabilities and limitations of these instruments need to be understood so that test results and subsequent decisions based on this important atmospheric monitoring technology are reliable.

I will outline some common misconceptions about PIDs and what their true capabilities are:

1st Misconception: Correction Factors

“Changing the PID correction factor (CF) or choosing a chemical from the on-board library makes the instrument readings specific for that substance.”

Most PID instruments include a built-in library of correction factors. The same principles apply. Choosing a chemical correction factor e.g. ethylbenzene, does not make the PID substance-specific detector for ethylbenzene. The PID will continue to respond to other detectable VOCs which may be concurrently present. PIDs are usually calibrated using isobutylene. Hence it is most often used as the measurement scale. It is vital to understand that no matter how comprehensive the list of correction factors, choosing the CF for any particular chemical never makes the readings exclusive or substance-specific for that contaminant. Some PIDs like the UltraRae 3000 can have benzene or butadiene specific tubes attached to them to make the reading specific, but even then, some other VOCs are able to cross and give cross interference.

2nd Misconception

“I can’t use PID because I never know which VOC is producing the reading”

Dealing with single-component VOC mixtures is simple. Once you know which contaminant you are dealing with, assign the correct CF, and set the alarms to the appropriate take action.

Varying mixtures can be more challenging. Here you need to identify which chemical is the “controlling” compound. This is the most toxic and / or hardest to detect, hence “controls” the alarm setpoint that should be used for the entire mixture. Determine a hazardous condition threshold alarm that will ensure that the Occupational Exposure Limit (OEL) for any contaminant potentially present is never exceeded.

3rd Misconception

“If a 10.6 eV lamp is good, an 11.7 eV lamp must be better”

No, is the simple answer.

The energy output of a UV lamp determines whether a specific chemical is detectable. The energy must be higher than the ionisation potential of the contaminant for it to be detected. Manufacturers tend to allow for the use of several lamps in the same detector.

So, the lower the energy of UV light produced by the lamp, the fewer number of chemicals the PID will be able to detect. The higher the energy of the light produced, the wider the range of detectable chemicals.