THE CARE AND HANDLING OF PHOTOIONIZATION DETECTORS
Kevin M. Mende
As civilization has grown, so has the use of products and practices that have the potential to release deadly toxins into our environment. We face threats ranging from poisonous gases in a confined space to the sudden and catastrophic release of chemicals in industrial and transportation accidents. Recently, the specter of death from chemicals used in war or by terrorists has added to the dangers. Our safety as responders depends on finding the hazardous atmospheres before they can harm us. We need something we can carry that will warn of impending danger and–perhaps even more important–tell us the extent of the danger present.
These needs are not new. The “liveliest” technology used to accomplish these goals was the fabled canary. The Industrial Revolution increased the need for ores and fuels dramatically, and mining became one of the world`s largest industries. Mining brought with it some very special hazards, among them miners` exposure to toxic or asphyxiating gases. To detect these gases, the men would take a canary into the mines. If the canary–as an “early warning device”–keeled over, the miners got out. The concept was simple: The canary, more sensitive to the poisonous or suffocating gases than the men, reacted more quickly to the gases` presence. The canary has given way to sophisticated equipment that quickly detects the presence of hazardous materials. Among this equipment is the photoionization detector (PID).
PHOTOIONIZATION DETECTORS
The PID can sense gases in concentrations as low as 0.1 part per million (ppm) and instantaneously displays what it is reading (real time response). It is easily carried and readily informs the user of the presence of certain toxic substances.
Theory of Operation
The PID works on the principle of electrochemistry–that is, molecules can be made to have a positive or a negative electrical charge that can be used to create an electrical current. This can be accomplished in several ways; the PID uses high-frequency ultraviolet light.
Ionization
The definition of ionization can be summarized as follows. All atoms are made up of particles called protons, electrons, and neutrons. These particles carry electrical charges. Protons carry a positive electrical charge, and electrons carry a negative electrical charge (neutrons have no charge, but we won`t go into that). A normally stable atom has the same number of protons as electrons so that the charges even out. Adding or removing electrons from it ionizes an atom or any molecule made up of atoms.
In the PID, as mentioned, this is done with high-frequency ultraviolet light. Keep in mind that light is energy and that as this energy is applied to an atom, the atom becomes “excited.” If enough energy is applied, the atom will become so excited that it will lose one of its electrons. The result would be an atom with one more positively charged proton particle than negatively charged electron particles. This gives the atom an overall positive electrical charge and is now said to be “ionized” or made into a positive ion. This ionization took place through the use of light, hence the name photo(light)ionization.
Ionization Potential
Different compounds require different amounts of energy to ionize. The amount of energy needed to ionize a particular compound is called its ionization potential (IP) and is measured in electron volts (eV). The amount of energy needed varies from compound to compound. For instance, benzene, a common solvent, needs 9.25 eV of photoionization energy to ionize, whereas methane requires around 12.98 eV.
Since this energy is in the form of light, it makes sense that the source of this energy should come from a lamp. The lamps in the PID are usually rated for varying amounts of energy output, also measured in electron volts. This output normally ranges from 9.6 eV to 11.8 eV, depending on the specific device. The important thing to remember is that the lamp will ionize only those gases and vapors with an ionization potential below the lamp`s eV energy output. A 10 eV lamp would be able to detect benzene because the energy needed to ionize it (9.25 eV) is within the lamp`s output energy range. Methane, on the other hand, would not be detected because the energy needed to ionize it (12.98 eV) is above the lamp`s output energy. Any substance with an ionization potential above the lamp`s energy output range would not be detected. It makes sense, then, that normal ambient air with an IP of more than 20 eV doesn`t cause any electrical current in the PID. To cause current, some gas or vapor that is ionizable by the lamp would have to be present.
Obviously, you must know a gas or vapor`s IP before you can determine whether the PID can detect it. Most PID manufacturers supply a list of the more common substances (with their IPs) detectable by their detector. However, that one uncommon substance you may happen to have in your jurisdiction may not be on that list. Sources for more comprehensive lists are given at the end of this article.1
Detection
A very simplified explanation of the PID`s basic theory of operation follows:
The PID senses substances by using the positively charged ion to create electrical current.
A “domino effect” order of events then occurs:
–the quantity of gas present determines how many molecules will ionize,
–the number of ions created determines the amount of electrical current, and
–this electrical current is translated into a ppm reading, which is displayed for the operator.
ADVANTAGES AND LIMITATIONS
Advantages
The PID`s main advantage over other gas detectors is its incredibly low sensing ability. As stated already, it can detect some gases and vapors in concentrations as low as 0.1 ppm, or 100 parts per billion (ppb). To show you just how sensitive this is, consider the following:
Fill a building 40 feet wide, 10 feet tall, and 100 feet long with white ping-pong balls. Now toss a single red ping-pong ball into the building, and stir well. Then reach into this building while blindfolded and pluck out the red ball. THAT is how sensitive the PID can be!
Other common detecting devices such as combustible gas indicators (CGI), electrochemical sensors, and oxygen sensors will detect at best 1 ppm, though usually not even that low.
The PID can detect virtually hundreds of chemicals–although not every gas that`s out there.
The device is easy to handle, gives a real time reading (as previously noted), and is easy to read. In addition, most brands of PIDs have some type of data recording mechanism as an integral part of the unit. This added function allows you to review what is going on in the atmosphere at the incident and provides a hard copy of the information for your files, which is especially useful if legal actions result from the incident. With a reliable battery supply, PIDs can be used for an extensive period of time at an incident.
LIMITATIONS
As noted, the PID will not detect anything below the lamp`s energy output; some ways to rectify this limitation are given below.
The PID will be 100 percent accurate only for the gas with which it is calibrated. For a gas other than the calibration gas, you will have to apply a factor to the reading. The PID`s manufacturer can supply a list of factors. Unfortunately, this is usually a very short list. Some actions that may help to overcome this obstacle are presented below.
Detection Problems
If a product`s IP is above the lamp`s output, there won`t be any reading.
If an extremely high concentration of an ionizable product is in the chamber, not all the product will ionize because some of the molecules will be “hidden” behind others and will not receive the light energy.
If a gas with an IP higher than the lamp`s is present and another gas with a detectable IP is also present, the gas with the higher IP will “absorb” some of the energy from the light and prevent some of the ionizable gas present from being detected, giving a faulty reading.
If more than one ionizable gas is present, the PID reading will represent the ionization of two gases.
Calibration Is Needed
The PID must be calibrated–a fact of life in haz mat. You will need someone who knows how to calibrate the unit, the time to perform the calibration, and a supply of calibrating gas. The manufacturer usually recommends that the unit be calibrated every time it is used. If this can be done, it is a nice way to ensure your calibration, but if that means waiting for properly trained personnel and the equipment needed to perform the calibration, it may not be feasible. We have found that if the unit is calibrated on a quarterly basis, it holds the calibration quite well. This means more paperwork, which in itself can be a twofold blessing: (1) You maintain a calibrated unit ready to be used, and (2) the action of recalibrating the unit keeps personnel trained in the use of the PID. Calibration is further discussed on page 47.
HAZARD/RISK ANALYSIS
To determine whether you need a low-concentration detector such as a PID, conduct a hazard/risk analysis following these suggested steps:
Determine the chemicals in your jurisdiction. Obtain the inventories of chemicals of all the facilities in your area. The Environmental Protection Agency (through SARA Title III) requires that they have this information on file. You may encounter a problem with facilities that fail to report.
Try to determine which chemicals pass through your jurisdiction by any transportation routes (i.e., road, rail, water, air, or pipeline). Contact railroad companies, freight companies, and air freight handlers in your area. Even with their help, compiling this information will be a tough job.
Compile a list of the chemicals` threshhold limit values (TLVs). This information can usually be found on material safety data sheets or in common haz-mat references.
Determine if any of these chemicals are not detectable by your present equipment. When doing this, look for two categories of chemicals: those that cannot be detected by your present equipment in any concentrations and those that can be detected but have TLVs below the detection range of your present equipment.
Determine if any of these chemicals are detectable by the PID. To do this, you will have to find the IP for each of these chemicals. After this list is compiled, you can look to see which of them have IPs that fall within the output range of the PID`s lamps.
If you have gone through this process and have a list of chemicals still sitting before you, seriously consider purchasing a photoionization detector.
PURCHASING CONSIDERATIONS
Consider cost, training, and maintenance when contemplating the purchase of a PID.
Cost
Prices vary from manufacturer to manufacturer and from design to design (generally from around $3,000 to $10,000), so shop around to get the best price. Don`t be afraid to let the manufacturers know that you are looking at other PIDs. It may help to get you a better deal. Ask the dealer/manufacturer some questions about the unit:
Does it come with an easily rechargeable battery? (This is almost a must for response units.)
How is the gas sample taken–by air pump or diffusion? (An air pump is preferred.)
Is the unit easy to operate?
Can it be used with gloved hands?
How easy is it to carry?
Does it come with straps?
How can it be decontaminated?
Is it intrinsically safe?
Will it maintain a record of operations? If so, how is the record retrieved? (PC, printer, or detector screen?)
Is the detector`s display easy to read and interpret?
How readable is it at night?
How readable is it in direct sunlight?
Is a list of ionization potentials provided?
How is it calibrated? Is it easy to calibrate?
Is it easy to maintain?
Training
Some PID manufacturers and distributors will send a representative to train personnel in the use and maintenance of the PID. Others offer a video that goes over the basic aspects of the unit. It generally is good policy to have one or two technically minded individuals in your organization read through the manufacturer manuals and learn how to operate the PIDs themselves.2 Then they can train the other members of the response unit.
Maintenance
Maintenance costs primarily involve the recurring costs for lamps and calibration.
Lamps. The lamp, the heart of the unit, has a life span of between two and four years, depending on use and storage. The cost of a lamp ranges from $200 to $500. Since the lamp works on “light” energy, anything that blocks the lamp`s output affects its reliability. Just think of it as any other lamp. Dust or a condensation buildup will dim the lamp`s light and result in a faulty PID reading. Acid gases, such as hydrogen chloride and hydrogen fluoride, will attack the chamber or the lamp, also giving an inaccurate reading.
To replace the lamp, the unit usually has to be dismantled. The manufacturer can replace the lamp, which will add to the cost and also necessitate taking the equipment out of service, or you can do it yourself.
Calibration. Calibrating the unit yourself is not very expensive and usually is not too hard to do. The calibration gases run from $100 to $300 for a 17-liter cylinder, depending on the gas and the manufacturer. A cylinder`s expected lifetime depends on the extent of its use. The manufacturer recommends calibrating the unit before each use. As already mentioned, a quarterly (or monthly) calibration works fairly well in keeping the unit in acceptable calibration ranges. Information on the types of gases, the cylinders, and costs will have to be obtained from the manufacturer, who can also help you if you need to calibrate for specific gases.
Following are some tips for calibrating the PID:
Use a calibration gas that has an IP in the middle of the unit`s detection range. This way, if you are trying to detect a gas other than the one to which the unit is calibrated, you won`t have to apply too large a factor. Some units offer a big advantage over others here. They can calibrate the unit to more than one gas by storing the calibration information in the unit`s computer. You can use this data in two ways: (1) If your total list of gases to be detected is short enough, you can calibrate the unit to each of the gases and store the calibrations in the unit to be called up when needed, or (2) You can find a range of gases, from the lowest detectable IP to the highest detectable IP, and store their calibrations in the unit. Then, should you have to detect a gas or vapor for which the unit is not calibrated, use the calibration with the closest IP. In this way, you can use a very small factor to get the correct reading.
SUGGESTIONS FOR USE
The PID can basically be used in the same manner as any other gas-sensing unit. However, it offers some unique capabilities.
It is an excellent monitor for determining your perimeter zone. You can do this by approaching the release from upwind until a specified reading (usually the TLV/TWA) is reached. Mark this point with a flag, chalk, or a rock. Don`t use spray paint to mark the perimeter, since the PID is sensitive enough to pick up those fumes. Next, retreat about 10 feet, move around the release in one direction, and then approach again until the reading is reached again. Mark this point. Proceed in this manner all the way around the release. Personnel doing this work must don the appropriate personal protective equipment.
The PID, because of its sensitivity, is very effective for detecting leaks. It can be used to scan along pipes, drums, tankers, and so on, to locate small leaks that may be releasing just enough product to be harmful but not enough to be detected by other means. Ethyl mercaptan is a good example here. We know its odor detection level is around five ppb. Because of this, it is used as an odorant for odorless fuel gases such as methane and propane. If the odorized product is leaking, most modern detectors will sense it. However, if the mercaptan itself is leaking, many types of detectors probably will not sense it. You can use the PID to scan the outside of the tank or pipeline to sense the source of the leak.
The PID is also a good second source for verifying other detectors. As an example, using the PID in conjunction with a combustible gas indicator (CGI) can be helpful in three ways. Let`s take as examples benzene and methane, which were discussed previously. If both these gases are on your list of gases that may be encountered in your area, you can use two detectors to help you identify the gas. If both the CGI and the PID display the same concentrations, the gas released was probably the benzene. On the other hand, if only the CGI displays a concentration, the gas released was probably methane. If the PID displays a concentration higher than the CGI, it could be that another unidentified gas is present.
If you have two units, you can equip each with a different energy output lamp. If used together, they may help to narrow the search by determining the IP range of an unknown material. For example, if a product`s IP is 10.8 eV and you have the units with a 10.4 eV lamp and an 11.2 eV lamp, only the unit with the lamp with the higher output will detect the product. You now know that the product has an IP between 10.4 eV and 11.2 eV. This may eliminate some of the materials from your “possibles” list.
Endnotes
1. A more comprehensive list of IPs (more than 1,000 chemicals) is contained in “Ionization Potential and Appearance Potential Measurements 1971-1981,” published by the National Bureau of Standards and compiled by Rhoda D. Levin and Sharon G. Lias (Government Publication Number PB83-137364). The book is packed with information but may be difficult to understand.
Another good source for IPs is the NIOSH Pocket Guide to Hazardous Materials, one of the mainstays of any haz-mat response team. It does not contain every chemical you will encounter, however. If you have access to a university or a manufacturer who works with IPs, you may be able to obtain the IPs of specific chemicals not listed.
Some material safety data sheets (MSDSs) also list IPs.
2. A very good reference aid is Air Monitoring Instrumentation by Carol J. and Steven P. Maslansky (Van Nostrand Reinhold, 1993). This book discusses several types of monitoring equipment including the photoionization detector.
KEVIN M. MENDE is a lieutenant in the El Paso (TX) Fire Department, where he has served for more than 12 years. He has been a member of the haz mat response team for two years and a haz mat instructor for four years.
