How to Select a Particle Counter for Cleanroom
byMorgan Polen, VP ofApplication Technology, Lighthouse WorldwideSolutions
Often, the selectionofa Airborne particle counter for use in a cleanroom is donebasedupon the specifications of the instrument and thepurchaseprice.
Before getting intothedetails of the specifications, it is important to look at howtheinstrument will be used, the environments in which it will beusedand who will be using the instrument. Without thisinformationtaken into consideration, a less than optimal choice ofparticlecounter for the application could be made.
• In what typeofenvironment will the particle counter be used? Will it be usedinan ISO Class 3 Cleanroom for routine particle counting or willitbe used for verifying that a flow bench is operating prior toacritical process?
• What type of dataisthe particle counter expected to collect? Will this informationberecorded as simple pass/fail data or will the information havetobe logged into a spreadsheet or database?
• Will the operatorbecarrying the particle counter around and placing it on acriticalwork surface or will the instrument be cart mounted?
• Will thisparticlecounter be used to certify cleanrooms and travel fromlocation tolocation?
• Will theparticlecounter be used to monitor the cleanroom on a continuousbasis? Isthe particle counter intended to interface with aFacilityMonitoring System (FMS)?
How does aparticlecounter work?
An aerosolparticlecounter works on the principal of either light scatteringor lightblocking. An aerosol stream is drawn through a chamber witha lightsource (either Laser Based Light or White Light). When aparticleis illuminated by this light beam, the light is redirectedorabsorbed. Light scattered by a single particle in aspecificdirection in relation to the original direction has auniquesignature which relates to the size of the particle. Thisallowsfor sizing and counting of individual particles.
A particle counterismade up of 4 components:
1) Light source(GasBased Laser, Solid State Laser Diode, High IntensityLight)
2) PhotoDetectionElectronics
3) SampleFlowSystem
4)CountingElectronics
Some of thesecomponentsmay be located entirely within the particle counter orlocatedremotely. The light is collimated to illuminate the samplevolume.As particles pass through this volume, they scatter light.The photodetection system measures the amount of light scattered.The sizesof the particles are determined by the intensity of thescatteredlight.
Specifications
Althoughallmanufacturers use the same design principles, the details ofthedesign are what differentiates one manufacturer from therest.Specifications like sample flow rate, sensitivity, sizerange,number of counting channels, durability of the laser orlaserdiode, lifetime of the light source, the ability toholdcalibration are all important factors to consider when choosingaparticle counter.
Sensitivity:thesmallest size particle that can be detected.
Zero Count LevelorFalse Count Rate: the number of falsely reported particlesusingfiltered air at the optimum flow rate for a given amount oftime.The standard reporting of this count is the number ofparticles per5 minute interval. A normally expected Zero Count rateis less than1 count per 5 minutes.
Counting Efficiency:theratio of the measured particle concentration to the trueparticleconcentration. The true particle concentration is measuredwith amore sensitive instrument that has a counting efficiency of100% atthe minimum particle size of the instrument under test. Aproperlydesigned instrument should have a 50% counting efficiencyon itssmallest range.
Channels: the numberof“bins” in which the particles are placed, based upon therespectivesize of each particle counted. Channels are representedin microns.For example, you may have a particle counter with 4channels. Thismeans that the particles can be counted and binned in4 differentchannels. For example, a 4- channel counter mightinclude thefollowing channel sizes: 0.3 μm, 0.5 μm, 1.0 μm, 5.0μm.
Flow Rate: the amountofair that passes through the particle counter. This istypicallyrepresented in cubic feet per minute. Common flow ratesare 1.0cfmand 0.1cfm. The greater the flow rate, the larger thepump to pullthe air and this usually translates into a biggerparticlecounter.
All too often,minimumchannel size is chosen over the other criteria. Althoughthis is animportant consideration, other parameters should alsobeconsidered.
Typically, themoresensitive the instrument, the higher the initial investment andthehigher the maintenance cost. If the instrument is usedinenvironments with extremely high concentrations of particles,itmay require frequent cleanings by service technicians.
By understandingtheintended use of the particle counter and the specifications,thecustomer can make a more educated decision when selectinganinstrument.