Because every life has a purpose...

Photoionization Detectors (PIDs)Theory, Uses and Applications

Photoionization technology and operation

PIDs effectively detect and monitor for numerous hazardous substances, providing maximumbenefit and safety to users. While many hazardous gas detection methods are available,photoionization detectors offer the combination of speed of response, ease of use andmaintenance, small size, and ability to detect low levels, including most volatile organiccompounds (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.

+

AIR VOC

Anode CathodePID Lamp

UV Light

e-

Most substances can be ionized, somemore easily than others. the ability of asubstance to be ionized is measured asionization potential (IP) using anelectron volt (ev) energy scale. thisscale generally runs from a value of 7to a value of approximately 16.substances with an ev rating of 7 arevery easy to ionize; substances with anev rating of between 12 and 16 areextremely difficult to ionize. IP ratingsof some common substances include:

When monitored chemicals are ionizedusing a PID instrument, current isproduced and compoundconcentration displays as parts-per-million (ppm). PIDs use an ultraviolet(uv) lamp to ionize the compound tobe monitored. the lamp, often the sizeof a common flashlight bulb, emitsenough ultraviolet energy to ionize thecompound. A 10.6 ev lamp puts outenough energy to ionize anycompound with an ev rating of lessthan 10.6 including everything thatcan be ionized with a 9.8 ev lamp.While there are a limited number ofcompounds that require an 11.7 evlamp, there is an inherent instability ofavailable lamps that results in a veryshort operating life and manycustomers seek alternative detectionmethods for these compounds.

• Propyl alcohol 10.22 ev• Phosphine 9.87 ev• vinyl chloride 9.99 ev• toluene 8.82 ev• Benzene 9.25 ev• styrene 8.47 ev• vinyl acetate 9.19 ev

suBstAnCe tYPes tHAt PIDs CAn DeteCt

PIDs measure organic compounds suchas benzene, toluene and xylene, andalso certain inorganics such asammonia and hydrogen sulfide. As ageneral rule, if compounds measuredor detected contain a carbon (C) atom,a PID can be used. However, such is notalways the case, as methane (CH4) andcarbon monoxide (Co) cannot bedetect with a PID. listed here are somecommon substances that a PID candetect and monitor:

• Benzene• toluene• vinyl chloride• Hexane• Isobutylene• Jet fuel• styrene• Allyl alcohol• Mercaptans• trichloroethylene• Perchloroethylene• Propylene oxide• Phosphine

suBstAnCes tHAt PIDs CAnnot DeteCt

PIDs cannot be used to measure thefollowing common substances:

• oxygen• nitrogen• Carbon dioxide• sulfur dioxide• Carbon monoxide• Methane• Hydrogen fluoride• Hydrogen chloride• Fluorine• sulfur hexafluoride• ozone

resPonse FACtors

the optimal method of calibrating aPID to different compounds is throughuse of a standard of the gas of interest.However, such is not always practical,as doing so requires that an assortmentof sometimes hazardous gases be kepton hand for this purpose. to addressthis issue, response factors are used. Aresponse factor is a measure of PIDsensitivity to a particular gas. usingresponse factors, users can measure alarge number of compounds via asingle calibration gas – typicallyisobutylene. Isobutylene is usedbecause it is near the midpointionization point of most voCs and isnot flammable or toxic at lowconcentrations used in calibration.users simply multiply the instrumentreading (calibrated for isobutylene) bythe response factor to obtain thecorrected value for the compound ofinterest.

Instruction manuals for most PIDs listresponse factors; some PIDs haveresponse factors for common gasesprogrammed into the instrument’s

SUBSTANCE IP

Benzene 9.25

HeXAne 10.13

toluene 8.82

stYrene 8.47

MetHYl etHYl ketone (Mek)

9.51

XYlene 8.56

PHosPHIne 9.87

www.MSAsafety.com

3

METHOD EXAMPLE

Method #1: Preprogrammed Response Factorstypically, PID detectors are calibrated with 100 ppmisobutylene. other gases, for which there are hundreds,have corresponding correction values known asresponse factors. numerous corresponding responsefactors are preprogrammed into PID instruments. Afterusers select the desired gas to measure from theinstrument menu, units will automatically calculate thecorrected gas concentration reading for the gas ofinterest. the direct reading now measures the selectedgas concentration.

the instrument is calibrated to read inisobutylene equivalents for a reading of100 ppm with 10.6 ev lamp. ethylbenzeneis the target gas, with response factor of0.51. select the pre-programmed responsefactor; the instrument now readsapproximately 51 ppm when exposed tothe same gas, reading directly inethylbenzene concentration values.

Method #2: Customized Response Factorstypically, PID detectors are calibrated with 100 ppmisobutylene. If users do not find a desired gas in thepreprogrammed instrument menu list, users canprogram a custom gas and response factor. If users donot know the corresponding response factor, they maycall MsA and request that a customized response factorbe calculated that is specific to their application.

tetrahydrofuran is the target gas. theresponse factor for tetrahydrofuran is 1.6 using a 10.6 ev lamp. Program a customgas for tetrahydrofuran with rF 1.6 andselect this rF for use. the instrument nowreads directly in tetrahydrofuranconcentration values.

Method #3 Manually Calculated Response Factorstypically, PID detectors are calibrated with 100 ppmisobutylene. If users choose to read isobutylene’s directreading for a different gas and do not want to use thepreprogrammed or customized response factors, usersmay manually calculate the desired gas’ direct reading.If users know the response factor of the desired gas,they can manually multiply the isobutylene reading bythe known response factor. the result of this equationcan be recorded externally to the instrument.

the instrument is calibrated withisobutylene to isobutylene equivalents for areading of 10 ppm with 10.6 ev lamp.Cyclohexanone is the target gas, with acorrection factor of 0.82. Multiply 10 by0.82 to produce an adjusted cyclohexanoneconcentration of 8.2.

Three Methods in which Response Factors are used with PIDs

software, enabling all response factorcalculations to be performedautomatically. If the compound at atest site is known, the instrument canbe set to indicate a direct reading forthe target compound.

tHresHolD lIMIt vAlues (tlv) AnDPerMIssIBle eXPosure lIMIts (Pel)

Default low and high alarm values aretypically set for isobutylene. If usersmust monitor a different gas, theymust determine tlvs for the gas ofinterest and change the instrument’salarm level accordingly. Instrumentmanuals should be referenced toensure that correct instructions arefollowed. Chemical limit values can befound by referencing ACgIH, nIosH,osHA, or local regulations.

InDICAtor versus AnAlYzer

A common misconception about PIDsis that they are analyzers. Many expectthat a PID will provide exactly thevapor present at a spill site; such is notthe case. While PIDs are extremelysensitive and effective tools, they arenot analyzers and cannot determine ifa spill is benzene or jet fuel, forexample. A PID can detect that asubstance is present and can alert youas to potentially hazardous situations,but additional steps are necessary toproperly identify the substance’s exactcomposition and quantities present.listed here is a sample procedure toidentify a substance’s concentration ata spill site:

1. set the PID to isobutylene.2. Detect and record a reading.3. Identify, via placard or MsDs, the specific substance.

If the placard or MsDs reads that thesubstance is vinyl chloride, set the PIDresponse factor to vinyl chloride toenable direct reading of actual vinylchloride level.

InDustrIAl HYgIene PIDAPPlICAtIons

PIDs are great for use as screening toolsin hazard assessments due to theirability to detect multiple risks at verylow concentrations. While PIDs do notidentify specific compounds, they arewidely used to identify sources andcompound types. Potential chemicalattacks may employ industrial

chemicals; first responders can usePIDs to confidently determine if achemical is present and, if so, toaccurately measure its concentrationusing a reference factor.

ConFIneD sPACe

Industrial activity produces many toxicgases and vapors as components orbyproducts. using a PID for assessmentand continuous monitoring withinconfined space allows for a morecomprehensive evaluation and greaterprotection than when used tosupplement standard 4-gas instrumentconfiguration.

www.MSAsafety.com

note: this bulletin contains only a generaldescription of the products shown. While usesand performance capabilities are described,under no circumstances shall the products beused by untrained or unqualified individualsand not until the product instructionsincluding any warnings or cautions providedhave been thoroughly read and understood.only they contain the complete and detailedinformation concerning proper use and care of these products.

ID 0803-11-MC / June 2015© MSA 2015 Printed in U.S.A.

MSA — The Safety Compnay1000 Cranberry Woods DriveCranberry township, PA 16066 usAPhone 724-776-8600www.MSAFire.com

U.S. Customer Service CenterPhone 1-877-MsA-FIreFax 1-877-672-3930

MSA CanadaPhone 1-877-MsA-FIreFax 1-800-967-0398

MSA Mexico Phone 1-877-672-3473Fax 011 52 55 5359 4330

MSA International Phone 724-776-8626toll Free 1-800-672-7777Fax 724-741-1553email msa.international@msasafety.com

leAk DeteCtIon

often, leak concentration is too low tobe smelled by humans. PIDs are oftenused to detect low-level leaks in orderto detect compounds at levels of lessthan 1 ppm.

PIDs can be used for leak detection todetect leak sources. Higherconcentrations of gases are found at ornear the source of a leak. When asubstance is detected, users wearingadequate personal protectiveequipment should move in thedirection of higher concentrationswhen trying to identify the leak source.

PerIMeter MonItorIng

At HazMat sites, perimeters are set tocontain hazardous areas. PIDs can beused to set and, if necessary due tochanging environmental conditions,change perimeter lines. For example,toluene concentration reads 5 ppm atPerimeter line A at 10:50 a.m. At 11:05a.m., the line A reading rises to 10 ppmdue to wind direction, telling HazMatworkers that the perimeter line mayneed to be extended.

sPIll DelIneAtIon

As water and foam are often used atHazMat sites, a variety of liquids maybe present on the ground in additionto any material inadvertently spilled. APID is effective in locating hazardoussubstances while ignoring foam andwater, as PIDs will not respond to foamor water.

reMeDIAtIon

HazMat spills can contaminate bodiesof water or soil, potentially posinglong-term environmental concerns.PIDs are extremely useful in takingsamples from soil to determine ifremediation is necessary inconjunction with applicableenvironmental regulations.

Arson InvestIgAtIon

PIDs are often used to detectaccelerants at post-fire scenes. When aPID reading is detected, a sample fromthat specific area can be taken to alaboratory for analysis. For thisapplication, it is recommended thatPIDs be set to the isobutylene responsefactor for general purpose indications.

DIesel Fuel tlv MonItorIng

Marine chemists follow diesel fuel tlvlimits that are determined by theAmerican Conference of governmentalIndustrial Hygienists (ACgIH). thepresence of diesel fuel in the workplaceand associated exhaust has long beenconnected with carcinogens, andparticulate pollution associated withlung disease. Diesel vapor tlv is 15ppm; sampling for diesel fuel vaporsand recording results are significantaspects of inspections. surveys areconducted in fuel tanks, cargo spacesand engine rooms.

ConClusIon

PIDs are extremely valuable tools forindustrial, homeland security, lawenforcement, fire service, and HazMatapplications. PID sensitivity, low leveldetection and ability to detect manydifferent compounds enable PIDs tomake accomplishing these difficultjobs easier and more efficient.