PID / VOC Monitors: Questions, Myths and their Proper Use
Asphyxiates | Toxic Gases | Volatile Organic Compounds (VOCs) | Benzene | Explosive gases | Atmospheric pollution | Dust/Particulates | Noise & Vibration | Ionising Radiation | Calibration and Service | Hire
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.
“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.
“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.
An 11.7 eV lamp is capable of detecting more substances than a 10.6 eV lamp, however, the actual number of photons produced by the lamp, that is usually much less than that of the 10.6 eV lamp. So, the higher energy lamps tend to produce both a weaker ionisation current and have an increased tendency towards drift. Hence, the 10.6 eV lamp usually produces better resolution. Furthermore, high energy 11.7 eV lamps only last one or two months in normal operation. 10.6 eV lamps generally have last one to two years, or longer.
“PIDs can be used to replace traditional LEL sensors”
PID sensors are not good at measuring explosive levels of hydrocarbons. PIDs are designed to measure ppb and ppm concentrations of organic compounds and when exposed to over 1000ppm tend to give erratic reading as due to condensation of the VOC being detected and being outside the linear range of the calibration. As such, please ignore manufactures claims of reading in to the tens of thousands of ppm, this is largely nonsense; yes, you will read it once, but then your PID is so heavily contaminated it will stay reading high levels of VOCs until it is cleaned by a service centre.
“PIDs can be used in the presence of methane or to detect methane”
No, PIDs can not detect methane. It is outside the ionisation range of the PID.
Methane also absorbs UV light and does not ionise; hence it is a greenhouse gas. As such it quenches PID signal. At concentrations of 2.5% volume methane a reduction of approximately 50% of the PID signal occurs, by 4% by volume your PID is useless.
“PIDs do not work in high humidity”
Humidity can effect a PIDs performance. Water molecules can absorb UV light and at very high levels of humidity can also cause condensation of water on the lamp and electrodes. This condensed water can cause current leakage from the sensing electrode to the counter electrodes. The trick is to make sure that your PID is stored overnight before use in an indoor office location that is warmer than outdoors or leave it on charge and the charging process will keep the unit warmer than ambient. This stops the PID sensor becoming a condensing surface.
Ultimately, use a PID that is easily serviceable e.g. the MiniRae series. Avoid miniaturised PID technology, as this employs diffusion barriers to repel moisture, but at the same time reduces the sensitivity and response of the sensor at the low end of the range.
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