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particle counter working principle

日期:2019-11-14 00:04
浏览次数:7660
摘要:

Airborne particle counters, such as theCLJ-H603, are built to rapidly count and size contaminant particles in cleanrooms and other controlled environments.

Users tendtosee particle counters as simply a device which, whentheappropriate button is pressed, absolute results tumbleout.However, it is important to be aware of the technology withintheinstrument in order to understand the relevance of thedatagenerated and to put the operation of particle countersincontext.

Whenmeasuringvery small quantities of anything, it is important tounderstandthe operational technology, its benefits and itslimitations. Nomeasurement is absolute; all is relative to themeasuring techniqueemployed.

For example,ifparticles are measured by another technology, say, ascanningelectron microscope, we would not be surprised to getslightlydifferent results, we might even get surprisingly differentresultsat certain sizes because that particular technology isproducing aresponse which differs from that produced by theparticlecounter.

ParticlesTypes

Particlesexistin a tremendous range of sizes, shapes and compositions, forexamplefrom within the cleanroom we could possibly measure, flakesof skin,small pieces of silicon or metal or fungal spores. Thesources canbe very broad.

Particlesize

The sizeofparticles is measured in micrometers (i.e. microns), which isamillionth part of a metre (or a thousandth part ofamillimetre).

In "stateofthe art" semi-conductor industry facilities we are now measuringinnanometers, which is 10 -9 metres, (or a millionthpartof a millimetre). However, most people reading this articleareprobably more interested in the 0.5 and 5 micron particleswhichare so relevant to the pharmaceutical, health care,and medical device industries.

To putsizeinto perspective, visible particles are around 50 microns,(e.g. ahuman hair would be say 50 - 150 microns), non visibleparticlessuch as bacteria, are somewhere between sub 1 and 15microns, withplant spores and pollens falling between the visibleand nonvisible at around 10 to 100 microns.

Most reallifeparticles are non-uniform in structure, and this thereforeposesthe question of "how to qualify the size"? Ask people fromindustry"How would you size a particle"? Some would say it is basedon thelongest length, others the volume, and some an equivalenthole sizethrough which a particle could pass. There would be manydifferentanswers and it is probably fair to say that none of themwould bewrong, provided that they are qualified.

Particlecounterssize particles by matching a signal response generated bythecontaminant particle to an equivalent size of latexsphere.

Usersoftenlook at the size/number distribution reported by an instrumentandtreat this data as absolute without recognizing that there areanumber of variables operating i.e. physical properties,refractiveindex, orientation etc, which will play a part in thesizeindicated (and therefore the size channel in which the particleiscounted).

CalibrationReference Standard Using Latex Spheres

AerosolParticleCounters are calibratedforsize by sampling known size monodispersed (i.e. singlesize)Polystyrene Latex spheres (PSL"s) nebulised into the flow of HEPA/ULPA gradefilteredair. The instrument is adjusted for each test particle sizeusedand a calibration curve generated withintheinstrument.

Thesizingresponse from real life particles is therefore referenced bytheparticle counter as an equivalent to a perfectly sphericalLatexsphere and counted in one particular size range (or channel).Anymis-sizing could therefore affect not only the stated size butalsothe size channel into which the particle is allottedtherebyaffecting the number distribution.

How particlecounters work

All ofthecommonly used cleanroom airborne particle counters, regardlessoftheir manufacturer, work on a Light Scatteringprinciple.Essentially, this means the particle counters utilise avery brightlight source to illuminate the particles. Nowadays thissource is alaser diode, previously gas lasers and "white light"halogen bulbswere used.

Thisverybright light source shines through an optical block. Withintheoptical block are mirrors and either one or more photodetectors.Sampled air is drawn through the laser beam by a smallvacuum pump.As entrained particles in the air pass through thelaser beam, thelaser light interacts with the particles and isscattered.

Theterm"scattering' means that the light undergoes a directionalchange.This change occurs in all directions: forwards, backwardsandsideways. The red lines in diagram show this.

The clamshapesin the diagram are mirrors. They are silvered so that thereflectingsurface is inside. As the light scatters, it is pickedup by thesemirrors, which focus the scattered light on to one ormorephoto-detectors. The photo-detector converts the burst oflightenergy from each particle into a pulse of electrical energy.Bymeasuring the height of the signal and referencing it tothecalibration curve we can determine the size of the particle, andbycounting the number of pulses we can determine quantity. So itisrelatively straightforward from that point to allocateparticlenumbers into size channels.

Light"scattering"is a general term and is composed of various differentphysicalphenomena. Scattering is made up of:

1.  reflectedlight - when a light hits a particle and is angularlydeflected

2.  refractedlight - when a light goes through the particle and its direction oftravel is changed

3.  diffractedlight - where light comes close to the particle and is bent aroundit

There mayalsobe a degree of absorption when a percentage of the light energyisretained by the particle and in some instances effects suchasphosphorescence may occur from some particles types. Therefore,thewhole term 'scattering' is made up of different physicalpropertiesrelating to light, and the interaction of lightandparticles.

Theinteractionof light and particles therefore depends, essentially,upon theparticle composition, its refractive index and thedifferencebetween that particle and the background medium, (i.e.in the caseof a cleanroom particle counter, air).

Inoperation,the particle counter compares the response it is gettingfrom theparticle signal to the calibration curve generated withlatexspheres. What the particle counter is actually doing iscomparingthe response from the interaction of that particle and thelaserlight and then relating that, not to some irregular particleofunknown morphology, but to a latex sphere in a background ofair.So the instrument itself is not counting and sizing particles,itis counting and sizing flashes of light and relating them toasimilar response from latex in air. Therefore, users shouldbeaware that particles with different scattering responses willsizeeither smaller or larger relative to the latex standard.

For example,asilicon particle, because of its high reflectivity (relative tothelatex standard) is going to scatter a great deal oflight,therefore a particle of this material will size large totheparticle counter. A particle that absorbs light or does notscattermuch light, possibly a particle generated from a heatsource, isgoing to size small relative to the latex standard.Therefore, weare not looking at absolutes here. These sizingdifferences (fromthe latex standard) may possibly assign theparticles into largeror smaller size channels.

Theorientationthat the particle takes when passing through the laserbeam of theparticle counter will also have an effect on how it issized. In anextreme example, if we sampled a rod shaped particlewhich passedthrough the optical block so that the full length isexposed to thelaser beam, the light would strike the largestsurface area andscatter a relatively large amount of light. If itwent through thelaser beam end on, it would scatter from thesmallest crosssectional area so that would size as a smallparticle.

Conclusion

Whatparticlecounters do very well is to allow users to takeinstantaneoussamples and get a very good real time indication as tothe loadingof particles in a room or around a critical process.Alternativemethods, such as using a filter, pump and microscope,are timeconsuming, subjective and labour intensive.

This typeofparticle counter has been, and is, crucial and beneficial inthedevelopment, operation and advancement of cleanroomproductionenvironments. It is fast, well defined and nonsubjective, andmodern particle counters are now extremely stable,robust andsimple to use.

However,theease of operation should not cloud the users appreciation ofhowthe particle counters operate and the actual meaning of thedatathat is generated.
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