Understanding the Photoionization Detector

By Brian Crimmins

With proper training, maintenance, and procedures, the photoionization detector (PID) can be a valuable tool for the fire service, but many firefighters do not understand its complexities. Without proper usage and upkeep, firefighters can endanger themselves and the public. Above all, it is important to remember that the PID alone is not an all-purpose gas detector, and it will not alert firefighters to certain explosive hazards.

The PID is a useful tool for various types of emergencies such as hazardous materials incidents, oil or gasoline spills, technical rescue, leaking cylinders or gas valves, nuisance complaints for an odor of gas, or even to test for evidence of accelerants during arson investigation.

Today’s five- or six-gas meters commonly include a PID sensor. Beyond the PID, the other sensors in a multigas meter typically monitor oxygen, carbon monoxide, explosive gases [as measured by the lower explosive limit (LEL)], and hydrogen sulfide.¹ When using a multigas meter, it is important to understand the capabilities and limitations of each sensor. For example, LEL sensors will respond reliably to explosive gases such as methane but poorly to volatile organic compounds (VOCs). By contrast, the PID sensor will respond accurately to VOCs and poorly to methane and other chemical compounds.

VOCs

The PID is most often used to detect and measure VOCs, which are organic materials such as gasoline, diesel, paint thinner, propane, nail polish remover, kerosene, and jet fuel. These chemicals can be flammable or toxic at very low concentrations. Liquid VOCs also vaporize readily at room temperature. In addition to VOCs, the PID is also capable of measuring certain semivolatile organic compounds or inorganic compounds.2

How the PID Works

Inside the PID is a powerful ultraviolet (UV) light. The light “zaps” VOCs pulled into the detector to remove an electron and create charged ions. The charged ions land on a collector electrode, and the detector measures electrical current in proportion to the concentration of VOCs present. The amount of energy required to ionize a gas (by removing an electron) is called the ionization potential (IP) and is measured in electron volts (eV). The power of the UV light is also measured in eV.3

The PID does not measure all VOCs. As a general rule, the PID only works when the light’s eV is greater than the IP of the chemical it is trying to measure. Otherwise, there is not enough energy to ionize and measure the chemical in question. Most PIDs available for fire department use have a UV light of 10.2 or 10.6 eV, which is effective at measuring VOCs without significant interference from other gases in the air such as carbon dioxide. Detectors with more powerful UV lights are available, but they require significantly more cost and maintenance, making them expensive and impractical for the fire service. Refer to your PID’s manufacturer specifications to determine the power of the UV light.4

(1) A five-gas meter with a PID sensor. (Photo by author.)

PID Benefits and Limitations

The benefits of the PID are its low cost, ease of operation, and near-instantaneous results. Another benefit is the PID’s ability to measure low levels of VOCs. As stated above, VOCs are typically toxic or flammable at low concentrations. Therefore, it is important to measure these gases and vapors at the parts-per-million (ppm) level.

A few years ago, I responded to a reported odor of gas at a residential building. We arrived on scene and confirmed the odor, but we could not identify the source. Ultimately, the PID was helpful in leading us to an attached garage, where a motorcycle mechanic had spilled a small amount of gasoline. Vapor from the gasoline had traveled through the walls from the garage and into the residential building. In our investigation, the closer we got to the source of the spill, the more the PID showed increasing levels of VOC vapor. After identifying the source, we mitigated the issue through ventilation and the use of absorbent material.

The PID’s major limitation is that it does not measure methane, the principal component in natural gas. This is because the IP of methane is 12.61 eV, well above the power of the 10.2 or 10.6 eV light inside most fire department PIDs. (3) Unless it is combined with other detection technology in the same device—such as a multigas meter—do not use the PID for natural gas emergencies. By contrast, other types of gas detectors such as the flame ionizing detector (FID) readily detect methane. It is therefore critical to use different types of gas detector technologies at the same time to account for different types of hazards and to confirm accuracy. As stated above, the PID alone is not an all-purpose gas detector.

Another consideration is that, like all gas detectors, the PID requires maintenance. The PID’s instruction manual will state how often to bump test and calibrate the detector. Bump testing is qualitative; it means exposing the PID to a test gas to ensure that sensors and alarms are functioning. Alternately, calibration is a quantitative test to ensure that the gas detector displays accurate readings. (1)

Bump testing does not replace the need to calibrate. PIDs that are not periodically bump tested and calibrated are likely to malfunction. Also, PIDs calibrated with expired calibration gas may not display accurate readings. It is important to document all bump tests and calibrations. If a PID fails calibration, place it out of service for repair.

An additional limitation of the PID is that it does not identify chemical compounds by name. Unless it is combined with other detection technology in the same device, the PID only displays the aggregate concentration of VOCs present. The results are based on the calibration gas. The PID’s readings will be inaccurate if the chemical present is different from the calibration gas or if multiple chemical compounds are present. In response to this limitation, firefighters should refer to the manufacturer’s instructions. PID manuals include response factor charts. When firefighters know what chemical compound is present, they may multiply the PID readings by the appropriate response factor to calculate more accurate readings. Firefighters should carry laminated response factor charts in their apparatus at all times.

The last major limitation of the PID is that certain factors can interfere with the detector’s readings. For example, high humidity levels or water vapor can fog the PID light and create inaccurate results. Significant changes in temperature can similarly impact the PID’s performance. Also, dirt or contamination can cloud the PID light. Dropping a PID on the ground can damage the detector or cause dirt to clog the intake probe. Interference from methane or other naturally occurring compounds may skew the detector’s results. Finally, strong electrical fields can likewise cause incorrect readings. In response to these limitations, firefighters should review manufacturer specifications and conduct calibration any time the reliability of the PID could potentially be compromised.

The PID is an important tool for firefighters, but its use requires training and maintenance. The PID gives real-time, near instantaneous results for detection of VOCs and other compounds. The detector even records the presence of chemicals at the ppm level. To ensure accurate readings, firefighters should always remember to do the following:

• Consult manufacturer specifications, manuals, and response factor charts.
• Conduct periodic calibration and bump tests.
• Conduct calibration any time the PID may be damaged or displaying inaccurate results.
• At emergencies, use different types of detectors at the same time to detect different hazards and to ensure accuracy of results.
• Practice proper gas sampling techniques—take fresh air samples, sample low and high areas, account for the response time of gas to travel through the probe to the sensor, and so on.
• Document all firefighter training and PID maintenance.

References

1. MSA—The Safety Company. (2015). Gas Detectors for the Fire Service. Retrieved from: http://s7d9.scene7.com/is/content/minesafetyappliances/0800-10-MC%20Gas%20Detection%20Instruments%20for%20the%20Fire%20Service%20-%20EN.

2. Henderson, R. E. (2011). Questions, myths and misconceptions about using photoionization detectors. Environmental Technology Online. Retrieved from: http://www.envirotech-online.com/articles/health-and-safety/10/robert_e._henderson/questions_myths_and_misconceptions_-_about_using_photoionization_detectors_-_robert_e._henderson/935.

3. RAE Systems by Honeywell. (n.d.). The PID handbook: Theory and applications of direct-reading photoionization detectors (3rd ed.). Retrieved from: http://www.raesystems.com/sites/default/files/content/resources/pid_handbook_1002-02.pdf.

4. United States Environmental Protection Agency. (1994). Photoionization Detector (PID) HNU (SOP #2114). Washington, DC. Retrieved from: http://www.dem.ri.gov/pubs/sops/wmsr2114.pdf.

BRIAN CRIMMINS is a battalion chief and tour commander with the Hoboken (NJ) Fire Department. He has a BA from Boston College and an MPA from John Jay College.

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