airborne counting fundamentals
TRANSCRIPT
AIRBORNE PARTICLE COUNTING
Theory and Practice
Sheesh GulatiMeasureTest Corporation, Mumbai
What is the definition of a particle?
An object of solid or liquid composition or both and generally between 0.001 and 1,000 µm (micrometer) in size
Federal Standard 209-E
A solid or liquid object which, for the purposes of classification of air cleanliness, falls within a cumulative distribution that is based upon a threshold (lower limit) size in the range from 0.1 µm to 5.0 µm
ISO 14644-1
What are “particles”?Viable counts
• Can be cultured• Will grow into “colonies”
– requires optimum growth medium– requires sufficient incubation
• Enumerated as– CFU = Colony-Forming Units– VU = Viable Units
• >> Note: A living but sterile organism would not be a “viable” entityNon-viable counts
• Sometimes referred to as “total count”• Includes all types of airborne material
– Solid particles– Fibres– Micro-organisms
Humans as Source of Particles
Dirt on a necklace Tip of a ball point writer A piece of nail polishon a 256 k DRAM chip
Hair with hair spray Red Blood cells Flakes of skinHair withhair spray
Blood cellsFlakes of skin
Humans as Sources of Particles
Bacteria on the head of a pin
A dust mite tucks into a tasty feast on a flake of human skin.
Humans shed over 5million skin cells per day.
How Big Is a Micron?
40 micronSmallest size visible to the human eye.
10 micron
0.5 micron
0.1 micron
1 micron
.
• Time to fall 1 metre in still air for a 10 micron particle is 33 seconds
• For a 1 micron particle it is 48 minutes
Instrumentation for measuring particles
• Gravimetric Method – Sampler, Filter + Precision Mass Balance
• Light Scattering Method (Cleanroomparticle counters use this method)
• Aerodynamic Particle Sizer• Electrical Mobility Analyzer• Condensation Nucleus Counter
Why count?
“If you can measure that of which you speak and express it by a number, you know something of the subject; but if you cannot measure it, your knowledge is meagre and unsatisfactory”
-- Lord Kelvin, English physicist
Even "clean" mountain air contains very roughly 106 particles per cubic foot
Particle Counter …what is it?
• Optical Instrument
• Must move air/fluid through sensor to work
• Can quantify particles from 5 nm to 1000 µm
• Cannot tell you chemical composition
• Results are immediate
Three types:
1. OPC (Optical Particle Counter): Light Blocking
(Extinction) type -- Liquid Particle Counters
2. OPC: Light Scattering type - airborne particle
counters (referred to as DPC –discrete particle
counter in ISO 14644)
3. CNC: Condensation Nucleus Counter (used in
semicon industry)
Detection Ranges
1 10 100 1000 10000 100000 1000000
CNC
Light Scattering
Light Blocking
1µm0.1µm 10µm 1000µm
nm
Light Scattering
Light can be thought of as a set of electromagnetic waves.
Light is scattered when these waves interact with other objects. The nature of the scattering depends on the object properties, especially the size of the object
The term scattering means the light undergoes a directional change. This change occurs in all directions -- forward, backward and sideways
THEORY OF OPERATION
All of you are familiar with the sight of dust particles “floating” in a ray of sunlight falling into a room. Four elements are necessary for this phenomenon:
1. Sunlight to illuminate the dust particles2. Dust particles to reflect the sunlight3. Air to carry the dust4. Your eyes to see the dust
Particle counters use the same basic principles in a “refined” manner
Light Scattering (cont’d)
A very bright light source (laser diode, previously halogen lamp) is used to illuminate the particles
Inside the sensor of the particle counter there are mirrors silvered on the inside, which pick up the scattered light and focus it on a photo-detector
The photo-detector converts the bursts of light energy from each particle into a pulse of electrical energy
The height of the signal is proportional to the size of the particle, and and the number of pulses is equal to the number of particles
Laser Light CollectionLaser Light Collection
Collection LensesCollection
Lenses
ReflectorReflector
Photo Detector (Diode)
Photo Detector (Diode)
Output Signal Output Signal
Background Scatter
Background Scatter
Light Trap
Principles: Light Scattering
Laser Diode
Detector
Mirror
Light Trap
Detector Output
Principles: Light Scattering
Laser Diode
Detector
Mirror
Particle
Light Trap
Particle SizingParticle Sizing
Disadvantages: Light Scattering
More complicated construction = higher costCharacteristics of particle surface (shiny, color)
affect response
Effect of colors and surfaces on Light Scattering
Dark Light Shiny
Effect of Flow Rate on SizingEffect of Flow Rate on Sizing
Sensor
Sample inlet
Sensor
Typical Counter Components
Sensor
Sample inlet
Flow Monitor Pump FilterSampleexhaust
Typical Counter Components
Sensor Pump
Control ElectronicsSerialDataOut
Sensor
Sample inlet
Flow Monitor Pump FilterSampleexhaust
Typical Counter Components
Sensor CounterPump
Typical Counter Components
Printer
0.3 1 4 1 0Display
Control ElectronicsSerialDataOut
Sensor
Sample inlet
Flow Monitor Pump FilterSampleexhaust
Sensor CounterPump Output
Particle Counters - Hardware
There are 3 general styles of airborne instruments:
• Hand-held
• Portable (stand-alone / trolley mounted)
• Remote (for continuous monitoring)
Handheld Counters
• Battery operated• Light-weight• Optical Particle
Counters • 0.3 or 0.5 micron• 0.1 cfm (2.83 l/min)• JIS• CE
Portable OPC
• Often put onto trolley / cart• Often used to take multiple samples at one
site for extended period• Usually has built-in printer• Memory storage for multiple samples • Until recently, AC only• Output air filtered to 0.2 or 0.1 µm
Portable Counters
Two categories• Standard Sensitivity (0.3 or 0.5 µm)• High Sensitivity (0.1 µm or lower)
Either 0.1 or 1.0 cfm flow rate
Used individually or with manifold (specific models)
0.1 cfm Counter
• 0.1 CFM• 0.3 microns (237B)• 2 - 6 channels• FS209E calcs• Built-in printer• 100, 115 or 230 VAC • AC/DC (battery operated)
1 CFM PORTABLE COUNTER
• 1 CFM or 50LPM• 0.3 microns (3413)• High speed printer• 6 channels• stainless steel body• ISO 14644-1 calculations• AC/DC (battery operated)• 90 to 260 VAC input• Long life laser
Cleanroom applications of Particle Counters
• Filter testing / verification• Certification of room• Process verification• Tracking sources of contamination• Verifying gowning procedures• Studying tool configuration and start-
up
FLOW RATES
Airborne particle counters are available with flow rates of 0.1cfm or 1.0 cfm or 50 lpm (1.77 cfm).
F.S.209E /ISO 14644 does not specify using a particular flow rate.
However, the following would be the problems using a 0.1 cfm counter while validating pharma clean rooms:
Flow rates (contd.)
• We would have to multiply the 1 minute measurement result by 10. This is a valid approach provided that the measured value has statistical significance i.e. it is 23 counts per minute or greater to provide a reliable basis. If the measured value was only 3 this would convert into 30 or if it is changed to 4 it would jump to 40.
• In other words, the uncertainty of a spread of 10 in a total multiplied count result of 40 would amount to 25%
FLOW RATES (Contd.)
• The other method would be to let the 0.1cfm counter run for 10 minutes to get a sample volume of 1 cubic foot. The draw back of this approach is that, if any short term particle burst occurs during the relatively long 10 minutes count cycle, this burst would be integrated and would not show up as significant.
• The best way to count large particles is to sample a large quantity of air, thus maximising the chances of capturing any 5 micron particle that may be present. Therefore 1 cfm flow rate is better than 0.1 cfm as you get 10 times the sample quantity
FLOW RATES (Contd.)
• In fact EU GMP Annex 1 recommends a sample volume of 1 cu.metre = 35 cft which can be tested in 35 minutes by a 1 cfm instrument, whereas the 0.1 cfmcounter would take 350 minutes ( 6 hours) to test such a volume.
• Based on the above, it is recommended that a 0.1 cfmcounter would not be suitable to certify a class 100 clean room.
Zero Count
• Tests noise immunity• Tests for internal contamination
JIS: After 30 minute purge, measure counts for 5 minutes
Met One example: • less than 3 counts in 5 minutes
Counting Modes
Cumulative count - the total count for the selected size range and all larger sizes
Differential count - the number of particles in the selected size channel
Concentration = the number of particles per cubic foot. Concentration mode gives a quick estimation (in seconds) of the particle concentration
Beep - gives one beep for each particle counted.Useful for locating leaks in filter banks
Counting ModesCounting Modes
Cumulative Differential
Size 3
Size 2
Size 1
What Is the View Volume?
That area inside the sensor where the laser beam and sample path
are coincident
Laser Beam
Sample Air Flow
Coincidence Loss
The error introduced when particle density becomes so great that two or more particles are in the sensor view volume at the same time, causing two particles to be detected as one.
Coincidence Loss Inside a SensorCoincidence Loss Inside a Sensor
=•Obscuration
=•Aggregation
•Single Particle
SensitivityThe smallest size particle a counter can detect at a
specified counting efficiency ex.0.3 micron at 50% counting efficiency
Very high sensitivities ex 0.1 micron can only be used in very clean rooms (Class 1 or Class 10)
Very high sensitivity counters are expensive (helium neon laser instead of laser diode) and delicate
For pharma industry, 0.3 or 0.5 micron is adequate
Counting Efficiency
Counting efficiency is an expression of the probability that a Particle Counter will sense and count a particle passing through its sample volume. This probability is a function of size up to a certain critical size above which all particles are normally sensed and counted. ISO/CD 21501-1 describes it as the ratio of measured result of a particle counter to that of a reference instrument, using the same sampleThe counting efficiency is 50% at the minimum reported size range and 100% at the next size range ex. 50% at 0.3 microns and 100% at 0.5 microns
Counting EfficiencyCounting Efficiency
Particle size
Counts
NoiseLevel
50% Efficiency,
At least 2:1signal to noise ratio
0% Efficiency
100% Efficiency,
Below noise level
90%Efficiency,
Not 2:1Signal to
NoiseRatio
Greater than nominal sizeLess than nominal size
Sensor ResolutionSensor Resolution
Poor resolution Good resolution Perfect resolution
Assuming standard particle distribution
Isokinetic probes
• Minimize turbulence• Matching air flow with particle counter flow rate• Different size openings for counters with different flow rates
80 -120 fpm(0.4 - 0.6 m/sec)
0.1 to 3 cfm(2.83 to 80 lpm)
Isokinetic probes
80 -120 fpm(0.4 - 0.6 m/sec)
0.1 to 3 cfm(2.83 to 80 lpm)
Flowrate Size (inches) Size (mm)
0.1 cfm(2.83 lpm)
0.5 12
1.0 cfm(28.3 lpm)
1.25 33
2.0 – 3.0 cfm(56 – 84 lpm)
2 50 - 55