Photoionization Detectors: Kings of Concentration


CarolinaFireJournal - By Glenn Clapp,
By Glenn Clapp, CHMM, CFPS
01/10/2015 -

In the realm of hazardous materials response, one of the primary pieces of information we need to know is the concentration of any harmful vapors or gases we are dealing with. One of the primary tools we utilize in our “hazmat toolbox” — especially for substances for which we do not have a dedicated, direct-reading sensor — is the photoionization detector (PID). Just as with any other piece of hazmat equipment that we utilize, we should have a thorough understanding of the operating principles, idiosyncrasies, and proper situations in which to use the PID. We will discuss those points in the discussion that follows.

One of the major “cardinal rules” of PID operational theory is that you must have a lamp of adequate power to ionize the substance being monitored.

PIDs are very valuable components in our array of air monitoring equipment, and as such their relevant operating principles should not be a mystery to hazmat technicians. PIDs utilize an ultraviolet (UV) lamp to ionize gasses or vapors that are pulled into the meter through the use of a pump, a process that consists of the UV energy from the lamp “breaking apart” the molecules of the substance we are monitoring for, in the form of dislodging electrons from the molecules and thereby creating positive ions. A positive or bias electrode is located on one side of the vapor/gas flow and a negative or collector electrode is located on the opposing side. Following ionization, the negatively charged electrons that are released as part of the ionization process are attracted in the direction of the positive electrode and the positively charged ions are attracted in the direction of the negative electrode. The current resulting from the above process is then collected at the negative electrode and routed to an amplifier, with the resulting output then sent to a digital display screen.

The current produced is proportional to the concentration of the substance being monitored — although the initial displayed value may not be the actual concentration, as will be discussed shortly. An additional point regarding PIDs is that following the ionization process, the detached electrons of the substance being monitored rejoin the positively charged ions and the substance exits the PID virtually unchanged from when it entered the intake of the device.

One of the major “cardinal rules” of PID operational theory is that you must have a lamp of adequate power to ionize the substance being monitored. If the lamp is not of greater output than the energy required to ionize the particular gas or vapor, ionization will simply not take place and the output of the meter will be invalid. How can we obtain the energy required to ionize the substance we are monitoring — known as ionization potential (IP)? Two major sources for such information exist, namely the NIOSH Pocket Guide — in hard copy or electronic versions — and the Wireless Information System for Emergency Responders (WISER), which is available for free download to your computer or smart phone.

The ionization potential of substances is measured in electron volts (eV) and the energy output of PID lamps is measured in the same units to allow a direct comparison. For example, one major manufacturer of PID equipment produces lamps of 9.8 eV, 10.6 eV, and 11.7 eV energy output. As a point of reference, benzene has a relatively low IP of 9.24 eV and can be ionized by even a 9.8 eV lamp, while carbon monoxide has an IP of 14.01 eV and therefore cannot be ionized even by an 11.7 eV lamp. Hazmat teams may be tempted to purchase the most powerful lamp available so that the widest range of substances can be accurately monitored; however we also have to remember that as the energy output level of PID lamps increases, the service life of the lamp normally decreases and the cost normally increases. Many hazmat teams will opt for a “middle of the road” solution that meets their air monitoring needs in terms of the products encountered in their territory.

PIDs are also most often associated with the monitoring of volatile organic compounds (VOCs). As the name implies, VOCs are simply substances that are volatile (they emit vapors) and are organic (contain carbon molecules). PIDs, however, are not only limited to the monitoring of VOCs. Substances that are not VOCs but do have an IP of less than the energy output of the PID lamp can also be accurately monitored. In addition, PIDs themselves are manufactured in many different setups, ranging from stand-alone single-sensor PIDs to a PID sensor that can be a component of a five-gas meter that combines the standard oxygen, carbon monoxide, hydrogen sulfide, and flammability (LEL) sensors with a PID.

Earlier in our discussion, we alluded to the point that the digitally displayed reading on a PID may not actually be the concentration of the substance that you are monitoring for — most PIDs have a resolution of parts per million (ppm), however some are extremely sensitive with a resolution of parts per billion (ppb). This concept is known as relative response, and occurs when the substance being monitored is not the same substance as the meter is calibrated to.

For example, one popular manufacturer of PIDs suggests that their products be calibrated utilizing isobutylene. If you are then monitoring for the concentration of isobutylene in an environment, “what you see is what you get” in that the digital output corresponds to the isobutylene concentration in ppm. If you are monitoring for any other gas or vapor than isobutylene, the digital display is dimensionless and must be multiplied by a relative response correction factor (CF) to obtain the actual concentration in ppm. Reputable manufacturers of PIDs publish a list of relative response CFs for their products, and many have the data also available in electronic format.

As a point of conversation, let us say that you have a PID calibrated to isobutylene and are monitoring for the concentration of propyl alcohol (propanol) with a PID containing a 10.6 eV lamp. The IP for propanol is 10.22 eV, which is below the 10.6 eV rating of the lamp so we are good to go in that area. We then obtain a reading from the PID of 20 meter units. As the manufacturer states that the relative response CF is five, we multiply the meter units by the CF to obtain a concentration of 100 ppm. Many manufacturers also incorporate a gas selection feature in their meters so that the user selects the gas or vapor being monitored and the conversion from meter units to ppm — or sometimes ppb — is computed internally so that the output is direct reading in units of concentration.

Although we have briefly alluded to the calibration of PID’s above, we will now take a look at the calibration of PIDs in more detail. Many manufacturers stipulate that their PID products be calibrated prior to the initial use and on at least a monthly basis thereafter. Calibration simply means that we flow a known concentration of a gas — such as isobutylene — through the PID sensor, and the instrument then changes the output reading to match the concentration of the calibration gas. During the calibration process, we should also perform a fresh air calibration in which the PID sensor is “zeroed out” in a fresh air environment. The process as a whole is known as a “two point” calibration. We should also “bump test” the PID prior to each use.

Bump testing consists of simply flowing the same known concentration of calibration gas that we utilize during the calibration process through the PID sensor and then comparing the reading in ppm (or ppb) with the known concentration of the calibration gas to determine if the reading is within the parameters stipulated by the manufacturer. If it is not possible for personnel to possess calibration gas in the field to perform a bump test prior to each use of the PID, a fresh air calibration should be performed at a minimum.

PIDs indeed have many applications ranging from determining the concentration of a gas or vapor, to monitoring the flammability of a gas or vapor by converting ppm to percent concentration and then percent of the lower explosive limit (LEL), to even using a PID to detect the presence of accelerants in fire investigations. In terms of the operational limitations of PIDs, one additional item stands out — the fact that dust and other contaminants can accumulate on the PIDs UV lamp. If such contamination occurs and is noted due to any warnings, erroneous readings, or a failure of the PID to properly calibrate, the PID lamp should be cleaned. One prominent PID manufacturer stipulates that their PID lamp windows and sensor plates should be cleaned with isopropanol or methanol lamp cleaner in those aforementioned situations. As PIDs and their associated maintenance procedures vary from one manufacturer to another, please be sure to follow the manufacturer’s recommendations for your specific equipment.

In summation, PIDs are very useful components of our air monitoring arsenal. Among other things, we can use PIDs to determine one of the most important items on hazmat incident scenes — namely the concentration of any harmful gases or vapors that we are encountering. As competent hazmat technicians, we should understand the operation, maintenance needs, and limitations of the PID equipment we utilize in the field so that we can use such instruments at appropriate times and in the correct manner. As always, stay safe out there and be sure to visit the North Carolina Association of Hazardous Materials Responders website at www.nchazmat.com.

Glenn Clapp is Past President of the North Carolina Association of Hazardous Materials Responders and is a Fire Training Commander (Special Operations) for the High Point Fire Department. He is a Technician-Level Hazmat Instructor, a Law Enforcement Hazmat Instructor, and is a Certified Hazardous Materials Manager and Certified Fire Protection Specialist.
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