A PID gas detector displays, for example, 50 ppm VOC. Does this mean that 50 ppm of toluene, benzene or hexane is actually present?
In many cases, the answer is:
No.
A photoionization detector, or PID, responds to many volatile organic compounds, but not to every substance and not with the same sensitivity to every detectable substance. Important factors include the energy of the UV lamp used, the ionization energy of the target substance and the sensor’s relative sensitivity to that substance.
Many PID instruments are calibrated with isobutylene. If, for example, toluene, benzene or n-hexane is then measured, a correction factor or response factor may be required to derive the concentration of the known target substance from the isobutylene-referenced reading.
At the same time, a correction factor must not be confused with selectivity. In a gas mixture, a PID normally cannot determine which of the detectable substances is causing the signal. If several VOCs are present at the same time, they produce a combined sensor response.
For safe instrument selection, it is therefore not sufficient simply to ask:
Does the PID measure VOCs?
.
At least the following must be checked:
- which substances may actually occur,
- their ionization energy or ionization potential,
- the energy of the PID lamp used,
- the manufacturer-specific correction factor,
- measuring range and required resolution,
- single substance or gas mixture,
- diffusion or pumped measurement,
- humidity, temperature and possible condensation,
- calibration and test gas concept,
- required alarm limits and occupational exposure limits.
A PID is a highly sensitive measurement method for many VOCs, but it is not a universal substance analyzer. Correct interpretation is only possible when lamp energy, target substance, correction factor and measurement task are compatible.
How Does a Photoionization Detector Work?
A PID uses ultraviolet light to temporarily ionize suitable gas molecules.
In simplified form, the process can be represented as:
M + hν → M+ + e−
Where:
Mis the molecule to be measured,hνis the energy of the UV photon,M+is the positively charged ion,e−is the released electron.
The generated ions and electrons are detected in an electric field. The resulting very small electrical current is amplified and evaluated as a concentration signal.
Unlike combustion-based sensors, the substance is not permanently consumed in the sensor. After measurement, the ion and electron can recombine.
Whether ionization takes place at all, however, depends on whether the energy of the UV light is sufficiently high.
What Does 10.6 eV Mean for a PID?
The specification:
10.6 eV
does not describe the measuring range of the instrument.
It describes the maximum photon energy of the UV lamp used.
eV
stands for:
electron volt
.
A 10.6 eV lamp is widely used in many portable VOC gas detectors because it provides a good compromise between a broad detection spectrum and practical industrial use.
Depending on the PID system, other lamp energies may also be available, for example:
- 10.0 eV,
- 10.6 eV,
- 11.7 eV.
As a general rule:
The higher the lamp energy, the greater the number of substances that can in principle be ionized.
However, higher lamp energy does not automatically mean better measurement. The more substances the sensor can detect, the lower its selectivity may be when several compounds are present at the same time.
Correctly Comparing Lamp Energy and Ionization Energy
Every substance requires a certain minimum amount of energy to remove an electron from its molecule.
This is referred to as:
ionization energy
or often:
ionization potential
.
The basic selection rule is:
The photon energy of the PID lamp must be sufficiently high to ionize the target substance.
Example
A substance has an ionization potential of:
9.24 eV
and is measured using a:
10.6 eV lamp
.
The lamp energy is above the required ionization energy. The substance can therefore, in principle, be ionized and detected by the PID.
If the ionization potential of a substance is higher than the lamp energy, the substance will normally not generate a usable PID signal.
This leads to an important principle:
A PID does not automatically detect every gas or every VOC.
Which Substances Can a 10.6 eV PID Measure?
A 10.6 eV lamp can detect many volatile organic compounds.
Typical substance groups include:
- aromatics,
- many solvents,
- ketones,
- esters,
- alcohols,
- aldehydes,
- certain hydrocarbons,
- fuel components,
- certain amines,
- various sulfur-containing compounds.
Typical substances detectable with a 10.6 eV PID include, for example:
- benzene,
- toluene,
- xylene,
- acetone,
- n-hexane,
- ethyl acetate,
- styrene,
- trichloroethylene.
Some inorganic substances can also produce a PID response. The term “VOC sensor” therefore describes its main field of application, but does not fully define every substance to which a PID can respond.
Which Substances Does a PID Not Detect?
A common mistake is to regard a PID as a universal gas sensor.
Many important gases have an ionization energy above 10.6 eV and are therefore not detected, or not detected sufficiently, by a conventional 10.6 eV PID.
Examples include:
- methane,
- ethane,
- hydrogen,
- oxygen,
- nitrogen,
- carbon dioxide.
Methane is particularly important in safety assessments.
A reading of:
0 ppm VOC
on a PID therefore does not mean:
no flammable gases present
.
For flammable atmospheres, an appropriate measuring principle must be used for the combustible gases that may be present.
Why Is Isobutylene Commonly Used for Calibration?
A PID responds differently to different substances. A defined reference substance is therefore required for calibration.
A very commonly used substance is:
isobutylene
.
During calibration, the instrument is exposed to a known isobutylene concentration and assigns the corresponding ppm value to the measured sensor response.
If another substance is subsequently measured, its sensor response at the same actual concentration may be stronger or weaker than that of isobutylene.
This is where the correction factor becomes important.
What Is a PID Correction Factor?
The correction factor, often also referred to as:
Correction Factor
or:
Response Factor
, describes the relative PID sensitivity to a substance compared with the calibration gas.
For a PID calibrated with isobutylene, the following simplified relationship applies according to the definition used by many manufacturers:
ctarget substance = PID reading × correction factor
.
For isobutylene itself:
correction factor = 1
.
Correction factor below 1
The PID responds more strongly to the substance than it does to isobutylene.
The actual concentration of the target substance is therefore lower than the uncorrected isobutylene-referenced reading.
Correction factor greater than 1
The PID responds less strongly to the substance than it does to isobutylene.
The actual concentration is therefore correspondingly higher than the uncorrected reading.
Always use the correction factor table for the specific manufacturer or sensor and for the lamp energy being used. Factors from different instruments must not be transferred without verification.
Correction Factors in Practical Examples
Depending on the sensor, a 10.6 eV PID may, for example, have the following values:
| Substance | Ionization potential | Example correction factor |
|---|---|---|
| Isobutylene | 9.24 eV | 1.0 |
| Benzene | 9.24 eV | 0.50 |
| Toluene | 8.82 eV | 0.56 |
| n-Hexane | 10.13 eV | 3.0 |
These figures illustrate why an uncorrected VOC reading can easily be misinterpreted.
Example: PID displays 50 ppm isobutylene equivalent
If only benzene were present and a factor of:
0.50
applies to the instrument being used, the result would be:
50 ppm × 0.50 = 25 ppm benzene
.
For toluene with:
CF = 0.56
the result would be approximately:
50 ppm × 0.56 = 28 ppm toluene
.
For n-hexane with:
CF = 3.0
the result would instead be:
50 ppm × 3.0 = 150 ppm n-hexane
.
The same raw reading can therefore represent very different actual concentrations depending on the substance.
The factors shown do not apply universally to every PID. The current table provided by the instrument or sensor manufacturer is decisive.
Taking the Correction Factor into Account for Alarm Limits
The correction factor becomes particularly important when the instrument is used for warning limits or occupational exposure limits.
Assume that a PID displays isobutylene equivalents and n-hexane is to be monitored.
For the system used, the following value applies, for example:
CF = 3.0
.
If the instrument is intended to respond already at:
50 ppm n-hexane
this would theoretically correspond to an isobutylene-referenced raw reading of:
50 ppm / 3.0 ≈ 16.7 ppm
.
If the alarm were instead set without correction to:
50 ppm
isobutylene equivalent, the actual n-hexane concentration could already be significantly higher before the alarm is triggered.
Many modern instruments allow the target substance to be selected directly and automatically apply the stored correction factor. In this case, the display is converted accordingly.
Before setting a safety-relevant alarm threshold, it must therefore be clearly established whether the instrument displays raw values in isobutylene equivalents or already corrected target-substance values.
Why Selecting a Target Substance Does Not Make the PID Selective
A common misunderstanding occurs with instruments that include an integrated substance library.
If, for example:
toluene
is selected in the menu, this normally only means that the instrument applies the stored correction factor for toluene to the PID signal.
The sensor itself does not suddenly detect only toluene.
If the atmosphere simultaneously contains, for example:
- toluene,
- acetone,
- xylene
all three substances can contribute to the PID signal.
Without additional selective measurement technology, the instrument cannot determine from this combined signal what proportion is caused by each individual substance.
A correction factor means conversion – not substance identification.
What Happens with VOC Mixtures?
In many industrial applications, a single VOC is not present on its own. Instead, a mixture is encountered.
Typical examples include:
- painting processes,
- cleaning agents,
- fuel vapors,
- adhesives,
- solvent mixtures,
- chemical production processes.
The PID responds simultaneously to all components that can be ionized by the lamp being used.
In simplified terms, with a linear sensor response, the total signal is made up of the individual contributions of the different substances.
If the composition and mixing ratio are known precisely and remain stable, an effective correction factor for the mixture can be determined for certain applications.
For an unknown or strongly varying mixture, however, such a conversion is not reliable.
The reading should then, for example, be interpreted as:
ppm isobutylene equivalent
or as a non-substance-specific VOC reading.
An unknown PID reading must not be assigned to an arbitrary hazardous individual substance without further analysis.
Correctly Accounting for Humidity and Condensation
Humidity is relevant to PID measurement for two reasons.
First, depending on sensor design and operating conditions, water vapor can influence the PID response. With some sensor designs, high relative humidity can reduce sensitivity.
Second:
high relative humidity
is not the same as:
condensation
.
Liquid water droplets on:
- filters,
- lamp windows,
- sensor electrodes
can significantly impair measurement.
Measurements are particularly critical when a warm, humid sample is transported through a hose into a significantly colder instrument.
Condensation may then form inside the hose.
For demanding measurements, the following should therefore be considered:
- permissible humidity range of the instrument,
- influence of humidity on sensor sensitivity,
- sample temperature,
- dew point,
- hose routing and hose temperature.
A specified operating range of, for example, 10 to 95% relative humidity does not automatically mean that the measurement error remains completely independent of humidity throughout this entire range.
Diffusion, Pump and Sampling Hose
Depending on the instrument design, a portable PID can either measure directly from the ambient atmosphere or actively draw in a sample.
Diffusion mode
In diffusion mode, the sensor is located directly in the atmosphere being monitored.
This is particularly suitable for:
- personal monitoring,
- workplace measurements,
- leak detection in the immediate vicinity.
Pumped mode
An internal or external pump allows measurements to be taken at a remote point.
Typical applications include:
- pre-entry testing,
- vessels,
- shafts,
- ducts,
- hard-to-access machine areas.
The sampling hose is part of the measuring system.
As hose length increases, the sample transport time also increases.
In addition, certain VOCs can:
- adsorb onto hose surfaces,
- condense,
- be released again with a delay.
This can delay or distort the measurement result.
Before measurement, sufficient time must therefore be allowed for:
hose filling time + sensor response time
.
Measuring Range, Resolution and Over-Range Conditions
In addition to lamp energy, the concentration measuring range must suit the application.
A PID with a range of:
0...1000 ppm
and:
1 ppm resolution
serves a different purpose from a highly sensitive VOC detector intended for very low concentrations.
Relevant selection criteria include:
- lowest concentration of interest,
- alarm threshold,
- expected normal exposure,
- maximum possible concentration,
- resolution,
- linearity range,
- behavior when the measuring range is exceeded.
Particularly with correction factors significantly greater than 1, it must be checked whether the resulting target-substance measuring range remains sufficient.
Example
A PID has a maximum raw measuring range of:
1000 ppm isobutylene equivalent
.
For a target substance with:
CF = 3
the corresponding target-substance scale can differ considerably in mathematical terms.
The exact permissible conversion and the over-range behavior must be taken from the instrument manual.
Calibration, Bump Test and Zero Adjustment
A PID must be checked regularly.
Different procedures must be distinguished.
Zero adjustment
The sensor is exposed to a suitable VOC-free reference atmosphere and the zero reading is checked or adjusted.
Functional test or bump test
The instrument is exposed to a test gas to verify whether:
- the sensor responds,
- the gas supply path works,
- alarms are triggered,
- the instrument is fundamentally ready for use.
A successfully completed bump test does not automatically replace a full calibration.
Calibration
During calibration, the reading is adjusted against a defined and known test gas concentration.
Isobutylene is commonly used for PIDs.
The test gas concentration, gas flow, exposure time and calibration interval must comply with the instrument manufacturer’s specifications.
Contaminated PID Lamp and Declining Sensitivity
The UV lamp and sensor chamber are critical to the sensitivity of a PID.
Certain substances can cause deposits on the lamp window or inside the measuring chamber.
As a result, less UV energy reaches the gas sample.
Typical indications of contamination may include:
- readings that are too low,
- slower response,
- calibration errors,
- poor repeatability,
- failed bump tests.
Depending on the instrument, the following may be required:
- filter replacement,
- cleaning of the PID lamp,
- cleaning or replacement of the electrode stack,
- replacement of the lamp.
The components should only be cleaned in accordance with the manufacturer’s specified procedure.
Why a PID Does Not Replace an LEL Sensor
PIDs and combustible gas sensors perform different measurement tasks.
A PID can detect many solvents and VOCs already in the:
ppm range
.
This is particularly important because, for many toxic solvents, concentrations relevant to health may already be reached long before a flammable atmosphere approaches the lower explosive limit.
Conversely, a typical 10.6 eV PID does not detect important combustible gases such as methane.
For work in potentially hazardous atmospheres, several measuring principles may therefore be required, for example:
- oxygen sensor,
- combustible gas sensor or %LEL sensor,
- electrochemical sensors for specific toxic gases,
- PID for VOCs.
A PID complements gas monitoring – it does not automatically replace the other required sensors.
Using a PID for Workplace Monitoring and Pre-Entry Testing
Typical applications of a PID include:
- monitoring solvent vapors,
- leak detection,
- pre-entry testing of tanks and vessels,
- work in confined spaces,
- cleaning and maintenance work,
- painting and coating processes,
- chemical and plastics industries,
- tank farms and fuel areas.
For pre-entry testing, however, it is not sufficient simply to place a PID into the vessel.
The risk assessment must first establish:
- which substances may occur,
- which of them the PID can detect,
- which additional sensors are required,
- at which locations or heights measurements must be taken,
- how long the sample transport time is,
- which alarm and assessment limits apply.
If the composition of the atmosphere is unknown, a PID provides a valuable total or screening value, but not an unambiguous substance identification.
Practical Example: Solvent Vapors in a Vessel
A maintenance team needs to check whether solvent vapors are still present in a process vessel before entry.
It is known from the process that mainly n-hexane was used.
A portable gas detector with a 10.6 eV PID is available for the measurement.
Step 1: Check ionization energy
n-Hexane has an ionization energy below 10.6 eV and can therefore, in principle, be detected with the available PID lamp.
Step 2: Check the correction factor
The manufacturer’s table for the PID being used specifies, for example:
CF = 3.0
.
Step 3: Check the instrument configuration
It is checked whether the instrument displays:
isobutylene equivalent
or already:
corrected n-hexane
.
Step 4: Perform a functional test
Before the actual measurement, the instrument’s functionality is checked in accordance with operational requirements and the manufacturer’s instructions.
Step 5: Draw in the sample
When measuring through a hose, the transport time to the instrument is taken into account. The measuring points required by the risk assessment are checked.
Step 6: Interpret the reading correctly
If a PID calibrated with isobutylene and without automatic correction displays, for example:
20 ppm
then with:
CF = 3.0
the approximate result is:
20 ppm × 3.0 = 60 ppm n-hexane
.
Step 7: Measure other hazards separately
The PID provides no information on whether, for example:
- sufficient oxygen is present,
- methane is present,
- the atmosphere is potentially explosive,
- another toxic substance not detected by the PID is present.
The remaining hazards must therefore be assessed separately using suitable measuring methods.
Planning and Selection Checklist
- Identify all gases and vapors that may be expected based on the process and risk assessment.
- Determine the ionization energy of the relevant substances.
- Check whether the PID lamp energy is suitable for the substance list.
- Define the required concentration range and resolution.
- Check manufacturer-specific correction factors for the target substances.
- Determine whether a single substance or a varying gas mixture is present.
- Specify whether isobutylene equivalents or corrected target-substance values are to be displayed.
- Calculate or set alarm thresholds according to the instrument configuration.
- Select diffusion or pumped operation according to the measurement task.
- For hose measurements, take transport time and possible substance losses into account.
- Check temperature, humidity and condensation risk.
- Define the calibration and bump-test concept.
- Take maintenance of the filter, lamp and sensor chamber into account.
- Check which additional sensors are required for oxygen, combustible gases or specific toxic gases.
- For unknown mixtures, do not make an unjustified substance-specific interpretation of the PID reading.
Common Mistakes
- Interpreting 10.6 eV as the measuring range: The value describes the photon energy of the lamp, not the maximum measurable concentration.
- Assuming every VOC can be measured: If the ionization energy is above the lamp energy, the substance cannot be reliably ionized by the PID.
- Ignoring the correction factor: The displayed isobutylene-equivalent value is incorrectly interpreted as the actual target-substance concentration.
- Using a factor from an arbitrary table: Response factors can depend on the sensor, lamp type and measuring system.
- Confusing target-substance selection in the instrument menu with selectivity: The PID continues to respond to other ionizable substances.
- Evaluating a gas mixture as a single substance: Several VOCs can contribute to the measurement signal simultaneously.
- Using the PID as an explosion-protection sensor: Important combustible gases such as methane cannot be detected with a 10.6 eV lamp.
- Confusing the permitted humidity range with humidity independence: High humidity and especially condensation can influence the measurement.
- Ignoring hose length: The sample requires time to reach the sensor and certain substances may be delayed by the hose.
- Equating a bump test with calibration: A functional test confirms sensor response but does not necessarily replace a complete calibration.
- Overlooking a contaminated lamp: Deposits can gradually reduce sensitivity.
- Interpreting 0 ppm as “the atmosphere is safe”: Hazardous substances that the PID cannot detect may still be present.
Suitable PID Gas Detector
For mobile VOC measurements, the Crowcon Gas-Pro PID is one suitable option.
The portable multi-gas instrument can be equipped with a 10.6 eV PID for detecting a wide range of volatile organic compounds.
Features particularly relevant to this application include:
- 10.6 eV PID technology for numerous VOCs,
- PID measuring range up to 1000 ppm,
- 1 ppm resolution of the PID channel,
- multi-gas configuration for up to five gases,
- optional internal pump for sampling and pre-entry measurements,
- audible, visual and vibration alarms,
- rugged design for industrial applications.
A multi-gas instrument is particularly useful when oxygen, toxic gases or combustible atmospheres must be monitored in addition to VOCs.
Further instruments for portable and fixed gas detection applications can be found under Gas Detectors / Gas Warning Instruments at ICS Schneider.
For further industrial measurement and process applications, solutions are also available under Siemens Process Instrumentation.
Conclusion
A PID is a powerful measuring principle for the rapid detection of many volatile organic compounds. However, the displayed concentration can only be interpreted correctly if the properties of the target substance and the instrument configuration are known.
The lamp energy first determines which substances can in principle be ionized. With a 10.6 eV lamp, the ionization energy of the target substance must be sufficiently low. However, this alone does not indicate the strength of the sensor response.
Differences in sensitivity are taken into account using a correction factor or response factor. With isobutylene calibration, the same uncorrected PID reading can therefore correspond to significantly different actual concentrations depending on the target substance.
Particular caution is required with gas mixtures. Selecting a target substance or correction factor does not make the PID selective. Several ionizable compounds can contribute to the signal at the same time.
Sampling, humidity, condensation, lamp contamination, calibration and the required concentration range are equally important.
The decisive question when selecting an instrument is therefore not simply “Does the device have a PID?”, but rather: “Can the lamp used ionize the relevant substances, which correction factor applies, is the gas composition known, and can the displayed value be interpreted unambiguously for the specific safety or measurement task?”
FAQ: PID Gas Detectors for VOCs
What does 10.6 eV mean for a PID?
10.6 eV describes the photon energy of the UV lamp. Substances with a sufficiently low ionization energy can be ionized by this UV radiation and therefore detected by the PID.
Can a 10.6 eV PID measure all VOCs?
No. Whether a substance can be detected depends particularly on its ionization energy. If this is higher than the available lamp energy, a usable PID signal will normally not be generated.
Why is a PID calibrated with isobutylene?
Isobutylene is commonly used as a standardized reference gas. This allows the different sensor responses to other substances to be described using manufacturer-specific response or correction factors.
How is the PID correction factor applied?
With the commonly used definition, the isobutylene-referenced reading is multiplied by the correction factor of the target substance. Modern instruments can sometimes perform this conversion automatically. The instrument manufacturer’s definition is always decisive.
What does a correction factor greater than 1 mean?
The PID responds less strongly to this substance than it does to isobutylene. At the same sensor signal, the actual target-substance concentration is therefore correspondingly higher than the isobutylene-referenced raw value.
What does a correction factor below 1 mean?
The PID responds more strongly to the target substance than it does to isobutylene. The actual target-substance concentration is therefore lower than the uncorrected isobutylene reading.
Does a PID become selective if toluene is selected in the instrument?
No. Selecting toluene normally only causes a correction factor to be applied to the sensor response. Other substances detectable by the PID can still contribute to the measurement signal.
What does a PID display when a VOC mixture is present?
All ionizable components can contribute to the overall signal. Without a known composition, the reading therefore normally cannot be assigned unambiguously to a single compound.
Can a PID detect methane?
A conventional PID with a 10.6 eV lamp does not reliably detect methane because methane’s ionization energy is above the lamp energy. A different suitable measuring principle is required for methane or for assessing combustible atmospheres.
Does a PID replace a combustible gas sensor?
No. PID and LEL sensors perform different tasks. A PID can detect many toxicologically relevant VOCs in the ppm range, while a combustible gas sensor is used to assess explosion hazards. Depending on the risk, both measuring principles may be required.
Can high humidity influence a PID?
Yes. Depending on the sensor design, high humidity can influence the measurement response. Condensation on the filter, lamp or sensor is particularly critical. The specifications of the relevant instrument manufacturer must be observed.
How often does a PID need to be calibrated?
The required interval depends on the instrument, operating conditions, company procedures and manufacturer recommendations. Frequent exposure to heavily contaminating VOCs, difficult environmental conditions or safety-critical applications may require shorter inspection intervals.
Can a PID identify unknown solvent vapors?
No. A PID can indicate the presence of ionizable vapors, but it does not automatically identify their chemical composition. A more selective analytical method may be required for unambiguous substance identification.