PID Response Factors: How Tiger Measures a Specific Compound When Calibrated with Isobutylene

A photoionisation detector (PID) responds to hundreds of volatile organic compounds. Yet it is almost always calibrated with a single gas: isobutylene. Hence the natural question from site: "The instrument reads 50 ppm, but our workshop uses toluene. How much toluene is there really?"

The answer is the response factor (RF). Below: what it is, how to use it with Tiger and other Ion Science PIDs, and where conversion stops working.

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Why PIDs are calibrated with isobutylene

Isobutylene is used as the reference for practical reasons: it is non-toxic at calibration concentrations, stable in a cylinder, ionised by a 10.6 eV lamp and gives a mid-range response. The calibration mixture is usually 100 ppm isobutylene in air.

After calibration the instrument reads in isobutylene equivalent: "this many ppm of isobutylene would give the same signal". For any other substance the signal per 1 ppm will be different, higher or lower.

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What a response factor is

A response factor is a multiplier that converts a reading in isobutylene equivalent into the concentration of a specific substance:

C(substance) = Reading × RF

• RF < 1: the substance ionises more easily than isobutylene, so the instrument "sees" it more strongly. The reading is high and the correction reduces it.
• RF > 1: the substance ionises less easily, the reading is low and the correction increases it.
• RF undefined: the substance's ionisation potential is above the lamp energy and the PID does not see it.

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Indicative response factors for a 10.6 eV lamp

Substance	Ionisation potential, eV	RF (indicative)	What it means
Benzene	9.24	~0.5	Instrument reads about twice the actual value
Toluene	8.82	~0.5	Similar to benzene
Xylenes	~8.5	~0.4–0.6	Strong response
Styrene	8.43	~0.4	Strong response
Acetone	9.70	~1.2	Close to isobutylene
Methyl ethyl ketone	9.52	~0.8	Close to isobutylene
n-Hexane	10.13	~4	Weak response, reading much too low
Ethanol	10.47	~10	Very weak response
Ammonia	10.07	~9–10	Very weak response
Methane	12.6	—	Not detected

Important: the values in the table are indicative. Response factors differ between manufacturers and even between sensor designs. For actual work, use the Ion Science table built into the instrument: on a Tiger you simply select the substance from the library and it converts the reading itself.

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How to use it: an example

A paint shop uses toluene. A Tiger calibrated with isobutylene reads 40 ppm.

Without correction: 40 ppm. With correction: 40 × 0.5 = 20 ppm of toluene.

Now the opposite case, a hexane store. The instrument reads 25 ppm.

With correction: 25 × 4 = 100 ppm of hexane, four times what is on the display. Without correction the hazard would have been underestimated.

Conclusion: for substances with an RF above 1, working without correction systematically underestimates risk. That is why it makes sense to select the target substance in the Tiger library when it is known.

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How to handle a mixture

If the air contains a mixture of known composition, the mixture response factor is calculated as:

RF(mix) = 1 / (x₁/RF₁ + x₂/RF₂ + … + xₙ/RFₙ)

where xᵢ is the mole (volume) fraction of the component in the vapour phase.

Example: solvent vapour of 50% toluene (RF 0.5) and 50% acetone (RF 1.2).

RF(mix) = 1 / (0.5/0.5 + 0.5/1.2) = 1 / (1 + 0.417) ≈ 0.71

With a reading of 30 ppm, the mixture concentration ≈ 30 × 0.71 ≈ 21 ppm.

Limitation: the formula works only if the vapour composition is known and stable. Liquid composition is not vapour composition; lighter components evaporate faster.

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When conversion does not work

1. Unknown composition. Over gasoline, crude oil or condensate at a refinery there are dozens of compounds with different RFs. The PID reading is an indicator that "organics are present here", not a quantitative concentration of a specific substance.
2. Monitoring a specific toxic component in a mixture. You cannot convert TVOC using the benzene factor: benzene is only part of the mixture. This needs a selective instrument, Tiger Select or the fixed Titan 2. More in the article on benzene monitoring.
3. The substance is not ionised by the lamp. Methane, ethane, propane: the PID does not see them with any factor.
4. Very high concentrations. PID response is linear within the working range; at the edge of the range and in high humidity deviations are possible.

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Choosing a lamp

The lamp energy determines which substances the PID sees:

Lamp	What it additionally sees	Notes
10.0 eV	A narrow set of easily ionised substances	More selective, fewer interfering components
10.6 eV	Most VOCs: aromatics, ketones, most solvents	Standard, long service life
11.7 eV	Additionally some chlorinated compounds, methanol, formaldehyde	Short lamp life, needs careful storage

For most oil, gas and chemical tasks a 10.6 eV lamp is sufficient. An 11.7 eV lamp is chosen for specific substances that a 10.6 eV lamp cannot see.

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Practical recommendations

• Know what you are measuring. Before a walkdown, find out which substances are present in the area and select the target substance in the instrument library.
• Record the mode. State in the report whether the reading was in isobutylene equivalent or converted to a substance.
• Compare like with like. Trends at a point only make sense with identical instrument settings.
• Calibrate regularly. A response factor is useless if the instrument itself is wrong. The routine is in the article on PID calibration and maintenance.

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Summary

A response factor turns a PID reading into the concentration of a specific substance, but only when the substance is known and the composition is stable. For substances with a high RF (hexane, alcohols, ammonia), working without correction dangerously understates concentration. Mixtures of unknown composition and monitoring of a specific toxic component need a selective instrument.

KEG TRK supplies Ion Science PID detectors in Kazakhstan and helps configure instruments for the substances at your plant. Model selection is covered in the article on choosing a PID detector.