Howto Select a Particle Counter forCleanroom
How to Select a Particle Counter for Cleanroom
Often, theselectionofa Airborne particle counter for use in a cleanroom isdonebasedupon the specifications of the instrument andthepurchaseprice.
Before gettingintothedetails of the specifications, it is important to look athowtheinstrument will be used, the environments in which it willbeusedand who will be using the instrument. Withoutthisinformationtaken into consideration, a less than optimal choiceofparticlecounter for the application could be made.
• In whattypeofenvironment will the particle counter be used? Will it beusedinan ISO Class 3 Cleanroom for routine particle counting orwillitbe used for verifying that a flow bench is operating priortoacritical process?
• What type ofdataisthe particle counter expected to collect? Will thisinformationberecorded as simple pass/fail data or will theinformation havetobe logged into a spreadsheet or database?
• Will theoperatorbecarrying the particle counter around and placing it onacriticalwork surface or will the instrument be cartmounted?
• Willthisparticlecounter be used to certify cleanrooms and travelfromlocation tolocation?
• Willtheparticlecounter be used to monitor the cleanroom on acontinuousbasis? Isthe particle counter intended to interface withaFacilityMonitoring System (FMS)?
How doesaparticlecounter work?
Anaerosolparticlecounter works on the principal of either lightscatteringor lightblocking. An aerosol stream is drawn through achamber witha lightsource (either Laser Based Light or WhiteLight). When aparticleis illuminated by this light beam, the lightis redirectedorabsorbed. Light scattered by a single particle inaspecificdirection in relation to the original direction hasauniquesignature which relates to the size of the particle.Thisallowsfor sizing and counting of individual particles.
A particlecounterismade up of 4 components:
1) Lightsource(GasBased Laser, Solid State Laser Diode, HighIntensityLight)
2)PhotoDetectionElectronics
3)SampleFlowSystem
4)CountingElectronics
Some ofthesecomponentsmay be located entirely within the particle counterorlocatedremotely. The light is collimated to illuminate thesamplevolume.As particles pass through this volume, they scatterlight.The photodetection system measures the amount of lightscattered.The sizesof the particles are determined by the intensityof thescatteredlight.
Specifications
Althoughallmanufacturers usethe same design principles, the details ofthedesign are whatdifferentiates one manufacturer from therest.Specifications likesample flow rate, sensitivity, sizerange,number of countingchannels, durability of the laser orlaserdiode, lifetime of thelight source, the ability toholdcalibration are all importantfactors to consider when choosingaparticle counter.
Sensitivity:thesmallest sizeparticle that can be detected.
Zero CountLevelorFalse Count Rate: the number of falsely reportedparticlesusingfiltered air at the optimum flow rate for a givenamount oftime.The standard reporting of this count is the numberofparticles per5 minute interval. A normally expected Zero Countrateis less than1 count per 5 minutes.
CountingEfficiency:theratio of the measured particle concentration to thetrueparticleconcentration. The true particle concentration ismeasuredwith amore sensitive instrument that has a countingefficiency of100% atthe minimum particle size of the instrumentunder test. Aproperlydesigned instrument should have a 50% countingefficiencyon itssmallest range.
Channels: thenumberof“bins” in which the particles are placed, based upontherespectivesize of each particle counted. Channels arerepresentedin microns.For example, you may have a particle counterwith 4channels. Thismeans that the particles can be counted andbinned in4 differentchannels. For example, a 4- channel countermightinclude thefollowing channel sizes: 0.3 μm, 0.5 μm, 1.0 μm,5.0μm.
Flow Rate: theamountofair that passes through the particle counter. Thisistypicallyrepresented in cubic feet per minute. Common flowratesare 1.0cfmand 0.1cfm. The greater the flow rate, the largerthepump to pullthe air and this usually translates into abiggerparticlecounter.
All toooften,minimumchannel size is chosen over the other criteria.Althoughthis is animportant consideration, other parameters shouldalsobeconsidered.
Typically,themoresensitive the instrument, the higher the initial investmentandthehigher the maintenance cost. If the instrument isusedinenvironments with extremely high concentrations ofparticles,itmay require frequent cleanings by servicetechnicians.
Byunderstandingtheintended use of the particle counter and thespecifications,thecustomer can make a more educated decision whenselectinganinstrument.
How to Select a Particle Counter forCleanroom