Confined Space Monitoring: Is a PID Sensor Really Necessary?

Knowing and understanding the potential hazards that can be present in a confined space is the critical part.

• By Matt Thiel
• Aug 01, 2005

A worker is about to enter a confined space. Prior to entry, he reaches down to his belt and pulls out his gas monitoring device. He points the device into the confined space and initiates a scan of the entire area. Beams of light emit out of the instrument in all directions and cover the entire interior of the confined space. When the instrument has completed the scan of the space, it starts to analyze exactly what is present. When the analysis of the entire space is complete, the hand-held unit displays a list of all known gases and airborne particulates, along with their concentrations. The instrument then informs the user that it is either safe to enter the confined space or there are high levels of a particular substance.

Detection and identification of the unknown are things everyone in the safety industry wants, but are they really possible? With today's sensing technologies, the scenario previously mentioned is not yet viable. As sensor technologies continue to emerge and mature, this may become a reality in the near future--but for now, workers must rely on the sensing technologies available today for confined space entry. These include electrochemical sensors, semiconductor sensors, catalytic bead sensors, and a newcomer to the confined space market: the photo-ionization detector (PID). Each sensor type has its strengths and weaknesses. They should be evaluated to see whether what they are detecting is actually needed for entry into a confined space.

What Do You Monitor For?
The first step in assessing a confined space is to know and understand the confined space. This includes identifying the confined spaces that may be entered and knowing what potential hazards to monitor for in that area.

The number one cause of deaths in confined space accidents has been asphyxiation due to lack of oxygen. OSHA requires that the oxygen content is monitored prior to and continuously during entry in a confined space. The typical oxygen content is around 20.9 percent by volume; If levels are depleted below 19.5 percent by volume, the instrument alerts the worker. Continuing to work in oxygen levels below 19.5 percent can cause increased/labored breathing, lightheadedness, or if low enough, death. If the oxygen content enriches to levels higher than 22.5 percent by volume, the excess levels of oxygen can enhance combustion, and so OSHA has made 22.5 percent the upper limit for acceptable levels of oxygen.

Besides oxygen, combustible gas monitoring is also extremely important for confined space entry. If a combustible gas were present and were to ignite, the explosion could cause immediate danger to the entrant. And depending on the severity of the explosion, damage could be caused to surrounding structures and individuals in the area. For an explosion to occur, 100 percent of the Lower Explosive Limit (LEL) must be present. This is why OSHA has set the action limit for combustible gases at 10 percent LEL.

Outside of combustible gases and oxygen content, the confined space should be monitored for any specific toxic gas that is related to the process or operations of the organization. Numerous gases can be present as a result of industrial process; this is why it is so important to know the potential hazards that could be encountered when entering confined spaces.

The two most common toxic gases monitored by general industry are hydrogen sulfide (H₂S) and carbon monoxide (CO). Carbon monoxide is probably the most common of the toxic gases because it is a byproduct of combustion. The action level for CO is 35 parts per million. Hydrogen sulfide, a byproduct of decomposition, is commonly found in water/wastewater applications and in the petrochemical industry. The action level set for hydrogen sulfide is 10 ppm.

Where Do PIDs Fit into Confined Space Entry?
To understand where a photo-ionization detector fits into confined space entry, one must first understand what a PID measures and how it works. A PID is a general, non-specific detector of volatile organic compounds, commonly referred to as VOCs.

Organic compounds are chemical compounds that contain at least one atom of carbon or a carbon chain. Because carbon is the basis of all compounds, organic compounds are found in all living things. Volatile organic compounds are organic compounds that can easily become vapors or gases from the solid or liquid state. Along with carbon, volatile organic compounds can also contain oxygen, hydrogen, chlorine, and sulfur atoms, to name a few.

VOCs can be found in almost everything, from solvents to air pollution. Some common VOCs in the petrochemical industry are gasoline, benzene, toluene, and xylene. However, VOCs exist not only in industry, but also in everyday products including paint thinners and solvents, glues, degreasers, aerosol sprays, paints, household cleaning supplies, and even underarm deodorant. On hot summer days, the high temperatures react with air pollution to produce smog, which contains VOCs.

How Does a PID Detect VOCs?
A PID sensor works differently than other sensors. The PID contains a lamp that is rated to a specific ionization potential measured in electron volts (eV). Some common lamps available are 9.8 eV, 10.6 eV, and 11.7 eV. When the lamp ignites and a gas molecule passes through the light emitted from it, the molecule is ionized (if the ionization potential of the molecule is less than the ionization potential of the lamp) or nothing happens (if the molecule's ionization potential is above that of the lamp). Once ionized, positive and negative ions are collected on electrodes, which produce a signal that is directly proportional to the amount of ions present at the electrodes. The signal is then displayed in parts per million on the instrument display.

Limitations of PIDs
Because a PID ionizes any molecule with an ionization potential less than the ionization potential of its lamp, the detector is not specific to any gas. The detector itself measures the amount of positive and negative ions detected on the electrodes. These ions can come from any compound that was ionized. Unless a specific VOC is known to be the only VOC present in a certain area or to be a byproduct of a specific process, the PID will be able only to accurately inform the user or entrant to a confined space that a compound has been ionized. It will not be able to distinguish what the compound actually is.

Another limitation of a PID is that many of them respond to humidity. If a high-humidity sample is taken, the water vapor could cause false positive readings. This has been a major limitation to PIDs over the years.

Why Use PIDs for Confined Space Entry?
Currently, there is no clear "yes" or "no" answer on whether a PID should be used for confined space entry. The one benefit of a PID sensor is that it can detect some potentially explosive gases in the ppm range instead of % LEL. With the OSHA standards for combustible gases being set to 10 percent LEL, it is really not necessary to use a PID to detect these combustible hazards.

Because a PID sensor displays an output that is directly proportional to the amount of ions present on the electrodes, the instrument itself is not specific to any one compound. The PID will respond to any compound that has an ionization potential less than the ionization potential of the lamp; these compounds could be anything from solvents, to air pollution, to even a worker's cologne or deodorant. As a result of the PID's limitations of specificity, using a PID to assess a confined space could cause a great deal of end user/entrant confusion when the sensor responds to a compound. The non-specificity of the sensor does not inform the end user what is present and causing the PID sensor to respond, just that there is a VOC present.

Knowing and understanding the potential hazards that can be present in a confined space is the critical part in determining whether a PID sensor is necessary for confined space entry. The non-specificity of the sensor can cause confusion if the potential hazards are not known or identified. A PID cannot assess the area to determine which compounds or gases are present in the area; it can tell you only that some volatile organic compound has been ionized by the PID and that the amount of ions produced has caused a reading. This is the major limitation of PIDs for confined space entry.

Today's available sensing technologies still limit the amount of information and specificity of the gas monitoring instrument. As sensing technologies continue to advance and improve over time, detection and identification of the "unknown" may be a reality someday.

This article appeared in the August 2005 issue of Occupational Health & Safety.

This article originally appeared in the August 2005 issue of Occupational Health & Safety.