Benzene Detection Guidance: Comprehensive PID Buyer’s Guide

Properties, exposure and going forward with Benzene detection.
Click here to download the free guide from Ion Science. 

A useful guide from Ion Science about fixed, portable or personal Benzene monitoring.
Click here to download the free guide.

Photo-Ionisation Detectors (PIDs) will detect and measure around 400 Volatile Organic Compounds (VOCs). A PID will also detect and measure some inorganic gases.

Today PIDs are used across many industries including petrochemical, manufacturing, marine, pharmaceutical and printing. They are also used by organisations responsible for land clean-up, fire investigation and HAZMAT teams within civilian and the military services.

Over the years PIDs have become more attractive through the reduction of the size and weight of instruments, making them more portable and easier to handle. Portable PIDs have been used for over 40 years and now personal wearable devices are also available to provide individual protection for workers in situations where there is a possibility of over exposure to toxic volatile compounds.

PIDs are very sensitive monitoring devices which allow us to detect hundreds of different compounds at very low levels. A PID uses an ultra-violet lamp to ionise chemicals which are subsequently detected over electrodes, thereby measuring their concentrations in parts-per-million (ppm) or parts-per-billion (ppb).

Straightforward to use with a variety of applications, a PID is best used to detect low levels (0-3000 ppm) of broad band toxic chemicals or VOCs. Breakthroughs in lamp and sensor technology allow a PID to be small, rugged (resistant to contamination and humidity) and affordable.

Main advantages of a PID sensor:

- Instantaneous display, updated every second, for real time monitoring of toxic chemicals and VOCs.

- STEL, TWA and Peak values, updated every minute, accessible to the user at the end of the work shift.

- Threshold monitoring - visual and audio alarms in real time for STEL, TWA and Peak. Alarm signals vary for each condition.

- Data log for compliance and work shift trend analysis.

Gas carrying VOCs enters the PID and passes over the face of the ultra-violet lamp (with a standard output of 10.6eV), which subsequently ionises the VOC sample if it has an ionisation potential less than the output of the lamp. 

The charged ions flow to charged plates in the sensor and current is produced. This current is measured and a gas concentration is shown on the display. Only a fraction of the sample is destroyed during this process allowing for sample collection and more detailed laboratory analysis.

Ionization potential is typically specified in electron volts (eV) and refers to the energy required to remove a single electron from a single atom or molecule to produce an ion. The I.P. of an individual element or compound (found in instrument manufacturers’ charts) may be used as an indicator of its detectability using a PID.

A PID lamp generates a specific voltage, the most common being 10.6eV. If the I.P. of the compound is less than the lamp voltage then that compound will be ‘seen’ by the PID. Most volatile organic compounds have an I.P. of less than 10.6 eV, so will be measured.

For those compounds with an I.P. greater than 10.6eV, a higher voltage 11.7eV lamp is available. This is frequently used to detect and measure compounds that will not be seen by the standard lamp. Any compound with an I.P. greater than 11.7eV is not detectable by PID

Unfortunately the operating costs for this higher voltage lamp are significantly greater, and the lamp lifetime much shorter. This lamp is therefore only used when absolutely necessary.

A lower voltage, 10/9.8eV lamp is also available. This can be used as an alternative to the standard lamp in order to discriminate between compounds. If two compounds are known to be present and the 10.6eV lamp sees them both it is sometimes possible to substitute the low voltage lamp in order to only see one of the compounds.

This lower voltage lamp may also be used to effectively ‘screen out’ a large number of compounds that are not of interest. This is used, for example, in Benzene specific instruments widely used in the petrochemical industry.

Anyone involved with chemicals, gases and petrochemical products. Examples of applications include:

- Environmental: Land clean-up

- Industrial Hygiene: Personal Monitoring

- Safety: Confined space entry

- Hazardous Material Response: Major incidents

- Maintenance/operations: Leak detection, emissions

A PID will measure the following detectable compounds:

There are some compounds with an I.P. greater than 11.7eV. These compounds cannot be measured using a PID:

- Radiation

- Air - nitrogen, oxygen, water, carbon dioxide etc

- Toxics - carbon monoxide, hydrogen cyanide, sulphur dioxide

- Natural gas - methane, ethane

- Acids - hydrogen chloride, hydrogen fluoride, nitric acid

- Others – Freons, ozone. There is alternative technology available to measure these substances. If you need further clarification for any compound, contact Shawcity and we will be happy to advise you.

Many PID users will identify VOCs and may not be aware of the compound or compounds that are being measured. If you have no idea regarding the specific compounds, the measurement is still important but further investigation is needed. Without being able to qualify the exact compound, the measurement should be presented as an equivalent reading with reference to the calibration gas used. The calibration gas commonly used is isobutylene, so the gas value is ‘isobutylene equivalent’.

If you know there are several compounds in the sample it is virtually impossible to accurately identify the health risk. This is an issue if one of the compounds is highly toxic and the others present are mildly toxic. Under these circumstances a PID should be considered a simple screening device, and further consideration given to more detailed analysis.

For further help and advice, please contact us and we will be happy to discuss your specific circumstances. 

Most manufacturers recommend calibration of their PIDs with Isobutylene. 

Isobutylene is an excellent surrogate calibration gas because the response of most VOCs is reasonably close to, and consistent with, this gas.

The Correction Factor (CF) is the measure of the sensitivity of a PID to a specific gas. The CF will only be used if you have an instrument calibrated with one compound when you are sampling another. The relationship between the calibration gas and the alternative compound determines the sensitivity of the PID to that gas, and gives you the Correction Factor.

For example with an Isobutylene-calibrated instrument, a measurement of toluene will read high. This is because the response of the PID to toluene is different compared to Isobutylene. This difference is the response/ correction factor.

Correction Factor Example:
For toluene the correction factor is 0.5. Therefore if the instrument reads 100ppm and you know it is toluene, the correct value is 100ppm x 0.5 = 50ppm

Most manufacturers publish a list of response/correction factors for several hundred different compounds. These factors are now usually built in to the PID to allow users to correct the readings.

Example correction factors:
Acetone- 0.7
Ammonia - 8.5
Benzene - 0.5
Butadiene - 0.85
Diesel Fuel - 0.75
Ethanol - 8.7
Gasoline - 1.1
n-Hexane - 3.3
Styrene - 0.45
Toluene - 0.51
Vinyl Chloride - 2.2
Xylene - 0.43 

A PID is similar to other continuous, real time gas monitors. It indicates a gas concentration. It also will provide an audible, visual (flashing light) and vibrating alarm when a high concentration is present. PIDs usually have two high alarms to provide an early first warning and a second higher action needed alarm. 

Also, 15 minute STEL (short term exposure level) and eight hour TWA (time weighted average) alarms can be provided. The exact capabilities and functions of any particular instrument will vary from model to model.

Because a PID measures numerous different compounds it is important to gather as much information regards the probable gases that are to be measured. Only then can you set the alarm levels with any degree of confidence. 

If you know the VOC that is to be measured you can simply configure the STEL and TWA alarm levels in accordance with the HSE’s document, EH40. Because EH40 does not provide instantaneous high alarm limits the setting of these alarms are at the discretion of the user.

Annual Service and Calibration

Each year our manufacturer approved engineers at Shawcity will inspect and test the performance of your PID. Depending on the model, this service may include:

Lamp cleaning, sensor cleaning, lamp performance test, probe replacement, battery operation time test, display test, external water/dust filter replacement, internal inline pump filter replacement, firmware upgrades, PC communication test, calibration against an NPL traceable source, exterior clean and issue of a service report and calibration certificate.

Bump Test / Calibration
As well as returning your PID to Shawcity for an annual calibration and service, we recommend you yourself regularly check the performance. Switch on the PID and apply calibration gas. The PID should read between 90-110ppm with a known volume of 100ppm +/- 5%. Depending on the sensitivity you require your instrument to work at it may require calibration.

Moisture and Cold Temperatures
If the weather is cold, instruments stored overnight in vehicles or underheated buildings will take a little longer to warm up. Moisture may condense inside units, giving erratic, unstable or false zero readings.

To avoid problems switch on the instrument and allow it to run for 10-15 minutes before use. Routinely clean the lamp and probe, replace filters regularly and keep your PID on charge when not in use.

Lamp Cleaning
Routine cleaning of the PID lamp/sensor will ensure stable readings. It will also help ensure that ‘false positive’ readings are avoided when the gas is present.

Replace Filters Routinely
It is not possible to give an accurate recommendation as to the frequency, as type and concentration of compound will affect the performance. However, we recommend you change filters monthly or sooner.

Extension Tubing
If you want to connect extension tubing to a PID only use Teflon tubing. Tygon, rubber and most other types of tubing act like a sponge and absorb most VOCs. Isobutylene, however, is an exception, so you can use Tygon tubing for calibration.