Photoionization Detectors (Pids) Theory, Uses and Applications

Photoionization Detectors (PIDs) Theory, Uses and Applications

Photoionization technology and operation PIDs eﬀectively detect and monitor for numerous hazardous substances, providing maximum benefit and safety to users. While many hazardous gas detection methods are available, photoionization detectors oﬀer the combination of speed of response, ease of use and maintenance, small size, and ability to detect low levels, including most volatile organic compounds (voCs).

PIDs rely upon ionization as the basis of detection. When sampled gas absorbs energy from a PID lamp, the gas becomes excited and its molecular content is altered. the compound loses an electron (e-) and becomes a positively charged ion. once this process occurs, the substance is considered to be ionized . Here we see an illustration of photoionization.

UV Light

PID Lamp Anode Cathode

AIR VOC

Because every life has a purpose... Most substances can be ionized, some suBstAnCes tHAt PIDs more easily than others. the ability of a CAnnot DeteCt substance to be ionized is measured as ionization potential (IP) using an PIDs cannot be used to measure the electron volt (ev) energy scale. this following common substances: scale generally runs from a value of 7 to a value of approximately 16. • oxygen substances with an ev rating of 7 are • nitrogen very easy to ionize; substances with an • Carbon dioxide ev rating of between 12 and 16 are • sulfur dioxide extremely difficult to ionize. IP ratings • Carbon monoxide of some common substances include: • Methane • Hydrogen fluoride SUBSTANCE IP • Hydrogen chloride • Propyl alcohol 10.22 ev Benzene 9.25 • Fluorine • Phosphine 9.87 ev • sulfur hexafluoride HeXAne 10.13 • vinyl chloride 9.99 ev • ozone • toluene 8.82 ev toluene 8.82 • Benzene 9.25 ev resPonse FACtors • styrene 8.47 ev stYrene 8.47 • vinyl acetate 9.19 ev the optimal method of calibrating a MetHYl etHYl 9.51 ketone (Mek) PID to different compounds is through suBstAnCe tYPes use of a standard of the gas of interest. XYlene 8.56 tHAt PIDs CAn DeteCt However, such is not always practical, PHosPHIne 9.87 as doing so requires that an assortment PIDs measure organic compounds such of sometimes hazardous gases be kept as benzene, toluene and xylene, and on hand for this purpose. to address When monitored chemicals are ionized also certain inorganics such as this issue, response factors are used. A using a PID instrument, current is ammonia and hydrogen sulfide. As a response factor is a measure of PID produced and compound general rule, if compounds measured sensitivity to a particular gas. using concentration displays as parts-per- or detected contain a carbon (C) atom, response factors, users can measure a million (ppm). PIDs use an ultraviolet a PID can be used. However, such is not large number of compounds via a (uv) lamp to ionize the compound to always the case, as methane (CH4) and single calibration gas – typically be monitored. the lamp, often the size carbon monoxide (Co) cannot be isobutylene . Isobutylene is used of a common flashlight bulb, emits detect with a PID. listed here are some because it is near the midpoint enough ultraviolet energy to ionize the common substances that a PID can ionization point of most voCs and is compound. A 10.6 ev lamp puts out detect and monitor: not flammable or toxic at low enough energy to ionize any concentrations used in calibration. compound with an ev rating of less • Benzene users simply multiply the instrument than 10.6 including everything that • toluene reading (calibrated for isobutylene) by can be ionized with a 9.8 ev lamp. • vinyl chloride the response factor to obtain the While there are a limited number of • Hexane corrected value for the compound of compounds that require an 11.7 ev • Isobutylene interest. lamp, there is an inherent instability of • Jet fuel available lamps that results in a very • styrene Instruction manuals for most PIDs list short operating life and many • Allyl alcohol response factors; some PIDs have customers seek alternative detection • Mercaptans response factors for common gases methods for these compounds. • trichloroethylene programmed into the instrument’s • Perchloroethylene • Propylene oxide • Phosphine software, enabling all response factor If the placard or MsDs reads that the chemicals; first responders can use calculations to be performed substance is vinyl chloride, set the PID PIDs to confidently determine if a automatically. If the compound at a response factor to vinyl chloride to chemical is present and, if so, to test site is known, the instrument can enable direct reading of actual vinyl accurately measure its concentration be set to indicate a direct reading for chloride level. using a reference factor. the target compound. InDustrIAl HYgIene PID ConFIneD sPACe tHresHolD lIMIt vAlues (tlv) AnD APPlICAtIons PerMIssIBle eXPosure lIMIts (Pel) Industrial activity produces many toxic PIDs are great for use as screening tools gases and vapors as components or Default low and high alarm values are in hazard assessments due to their byproducts. using a PID for assessment typically set for isobutylene. If users ability to detect multiple risks at very and continuous monitoring within must monitor a different gas, they low concentrations. While PIDs do not confined space allows for a more must determine tlvs for the gas of identify specific compounds, they are comprehensive evaluation and greater interest and change the instrument’s widely used to identify sources and protection than when used to alarm level accordingly. Instrument compound types. Potential chemical supplement standard 4-gas instrument manuals should be referenced to attacks may employ industrial configuration. ensure that correct instructions are followed. Chemical limit values can be Three Methods in which Response Factors are used with PIDs found by referencing ACgIH, nIosH, osHA, or local regulations. METHOD EXAMPLE InDICAtor versus AnAlYzer Method #1: Preprogrammed Response Factors the instrument is calibrated to read in typically, PID detectors are calibrated with 100 ppm isobutylene equivalents for a reading of A common misconception about PIDs isobutylene. other gases, for which there are hundreds, 100 ppm with 10.6 ev lamp. ethylbenzene is that they are analyzers . Many expect have corresponding correction values known as is the target gas, with response factor of that a PID will provide exactly the response factors. numerous corresponding response 0.51. select the pre-programmed response factors are preprogrammed into PID instruments. After factor; the instrument now reads vapor present at a spill site; such is not users select the desired gas to measure from the approximately 51 ppm when exposed to the case. While PIDs are extremely instrument menu, units will automatically calculate the the same gas, reading directly in sensitive and effective tools, they are corrected gas concentration reading for the gas of ethylbenzene concentration values. not analyzers and cannot determine if interest. the direct reading now measures the selected gas concentration. a spill is benzene or jet fuel, for 3 example. A PID can detect that a Method #2: Customized Response Factors tetrahydrofuran is the target gas. the substance is present and can alert you typically, PID detectors are calibrated with 100 ppm response factor for tetrahydrofuran is as to potentially hazardous situations, isobutylene. If users do not find a desired gas in the 1.6 using a 10.6 ev lamp. Program a custom but additional steps are necessary to preprogrammed instrument menu list, users can gas for tetrahydrofuran with rF 1.6 and program a custom gas and response factor. If users do select this rF for use. the instrument now properly identify the substance’s exact not know the corresponding response factor, they may reads directly in tetrahydrofuran composition and quantities present. call MsA and request that a customized response factor concentration values. listed here is a sample procedure to be calculated that is specific to their application. identify a substance’s concentration at a spill site: Method #3 Manually Calculated Response Factors the instrument is calibrated with typically, PID detectors are calibrated with 100 ppm isobutylene to isobutylene equivalents for a isobutylene. If users choose to read isobutylene’s direct reading of 10 ppm with 10.6 ev lamp. 1. set the PID to isobutylene. reading for a different gas and do not want to use the Cyclohexanone is the target gas, with a 2. Detect and record a reading. preprogrammed or customized response factors, users correction factor of 0.82. Multiply 10 by 3. Identify, via placard or MsDs, the may manually calculate the desired gas’ direct reading. 0.82 to produce an adjusted cyclohexanone specific substance. If users know the response factor of the desired gas, concentration of 8.2. they can manually multiply the isobutylene reading by the known response factor. the result of this equation can be recorded externally to the instrument.

www.MSA safety .com leAk DeteCtIon reMeDIAtIon Arson InvestIgAtIon often, leak concentration is too low to HazMat spills can contaminate bodies PIDs are often used to detect be smelled by humans. PIDs are often of water or soil, potentially posing accelerants at post-fire scenes. When a used to detect low-level leaks in order long-term environmental concerns. PID reading is detected, a sample from to detect compounds at levels of less PIDs are extremely useful in taking that specific area can be taken to a than 1 ppm. samples from soil to determine if laboratory for analysis. For this remediation is necessary in application, it is recommended that PIDs can be used for leak detection to conjunction with applicable PIDs be set to the isobutylene response detect leak sources. Higher environmental regulations. factor for general purpose indications. concentrations of gases are found at or near the source of a leak. When a DIesel Fuel tlv MonItorIng substance is detected, users wearing adequate personal protective Marine chemists follow diesel fuel tlv equipment should move in the limits that are determined by the direction of higher concentrations American Conference of governmental when trying to identify the leak source. Industrial Hygienists (ACgIH). the presence of diesel fuel in the workplace PerIMeter MonItorIng and associated exhaust has long been connected with carcinogens, and At HazMat sites, perimeters are set to particulate pollution associated with contain hazardous areas. PIDs can be lung disease. Diesel vapor tlv is 15 used to set and, if necessary due to ppm; sampling for diesel fuel vapors changing environmental conditions, and recording results are significant change perimeter lines. For example, aspects of inspections. surveys are toluene concentration reads 5 ppm at conducted in fuel tanks, cargo spaces Perimeter line A at 10:50 a.m. At 11:05 and engine rooms. a.m., the line A reading rises to 10 ppm due to wind direction, telling HazMat ConClusIon workers that the perimeter line may need to be extended. PIDs are extremely valuable tools for industrial, homeland security, law sPIll DelIneAtIon enforcement, fire service, and HazMat applications. PID sensitivity, low level As water and foam are often used at detection and ability to detect many HazMat sites, a variety of liquids may different compounds enable PIDs to be present on the ground in addition make accomplishing these difficult to any material inadvertently spilled. A jobs easier and more efficient. PID is effective in locating hazardous substances while ignoring foam and water, as PIDs will not respond to foam or water.

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