VOC Monitoring and PID Detectors

Question 1

Why will an LEL sensor not tell you about solvent vapor?

Accepted Answer

Because the concentration that matters for toxic exposure is far below the concentration an LEL sensor is built to resolve. An LEL sensor measures combustible gas as a percentage of the lower explosive limit, and it is designed around the fire hazard. The lower explosive limit for most solvents is in the thousands of parts per million. Take a solvent with an LEL around 1 percent by volume, which is 10,000 ppm. One percent of the LEL, the smallest increment a typical instrument displays, is 100 ppm. Many solvent exposure limits sit well below that. So an atmosphere that is comfortably over the toxic exposure limit can read zero, or one percent LEL, on a four gas instrument. The needle has not moved and the worker is overexposed. That gap is what a photoionization detector fills. A PID responds in parts per million, and often in tenths of a part per million, which is the resolution the health limit is written in.

Question 2

What does a PID actually measure?

Accepted Answer

A total number for everything in the sample that its lamp has enough energy to ionize. Not a compound, and not a named chemical. A photoionization detector shines ultraviolet light at the sample. A molecule whose ionization potential is below the lamp energy gets ionized, the resulting charge is measured, and the instrument reports a number. Everything ionizable in the sample contributes to that one number. The consequence is that a PID reading is a total volatile organic compound figure. It cannot distinguish benzene from toluene from a cleaning product somebody used in the next room. A reading of 5 ppm tells you that something is there and roughly how much, and nothing about what. The lamp energy sets the boundary of what is visible at all. A 10.6 eV lamp is the common choice and covers most aromatic and many aliphatic hydrocarbons. Compounds with ionization potentials above the lamp energy are invisible to it entirely, and methane is the one worth naming, because a PID will not see methane at any concentration. If methane is your hazard, that is an LEL or infrared measurement, not a PID measurement.

Question 3

Can a PID be used to measure benzene exposure?

Accepted Answer

Not on its own, and recording a total VOC reading as a benzene number is a documentation problem as well as a safety problem. Benzene has its own OSHA standard at 29 CFR 1910.1028, with its own permissible exposure limit, its own action level and its own monitoring and medical surveillance obligations attached to those numbers. Those obligations turn on the benzene concentration specifically, not on the total hydrocarbon concentration. A bare PID reports every ionizable compound in the sample together. In a crude oil, refinery or tank gauging atmosphere, the great majority of that reading is usually not benzene. Writing the total VOC figure into a record as benzene overstates exposure, and in the other direction a low total reading does not prove benzene is below the action level. Benzene specific field measurement uses a pre filter tube ahead of the PID, which strips the other compounds so that what reaches the lamp is benzene. That is a different consumable, a different procedure and a different measurement time from a straight PID reading, and it needs to be planned rather than improvised.

Question 4

Where does VOC monitoring belong?

Accepted Answer

Anywhere the atmosphere contains solvent or fuel vapor at concentrations that matter to health rather than to fire. Tank and vessel entry where the vessel has held product. A pre entry test on a tank that held solvent, run without a PID channel, can come back with acceptable oxygen, zero percent LEL and no toxic alarm, while the space is well over a solvent exposure limit. That is the case where a PID changes the decision. Refinery and petrochemical turnaround work. Environmental site work and remediation. Landfill and soil gas work. Paint, coating and fiberglass operations. Spill and leak response, where the first question is whether there is vapor and how much. For confined space entry the PID channel belongs on the pumped instrument used for the pre entry test, because that is the measurement that decides whether the entry happens. See confined space entry monitoring and the SP-MGTP .

Question 5

How should a PID reading be interpreted?

Accepted Answer

Carefully, and with the calibration in mind. A PID is calibrated against a reference compound, usually isobutylene, and the instrument reports everything as if it were that compound. Each real compound has a response factor relative to the reference, so the displayed number is not the true concentration of whatever is actually in the air unless you apply the factor for it. That requires knowing what is in the air, which is the thing the PID cannot tell you. The practical use is therefore comparative rather than absolute. A PID is excellent at telling you that this space is dirtier than that one, that the level is rising, that the concentration climbs as you lower the probe, and that something changed after the line was opened. Turning a PID number into a defensible exposure figure for a named substance is a job for a laboratory method, not for a direct reading instrument. Humidity and high concentrations of methane can both suppress a PID response, which is one more reason the LEL channel stays on the instrument rather than being replaced by the PID.

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