airborne counting fundamentals

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AIRBORNE PARTICLE COUNTING Theory and Practice Sheesh Gulati MeasureTest Corporation, Mumbai

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Page 1: Airborne Counting Fundamentals

AIRBORNE PARTICLE COUNTING

Theory and Practice

Sheesh GulatiMeasureTest Corporation, Mumbai

Page 2: Airborne Counting Fundamentals

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

Page 3: Airborne Counting Fundamentals

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

Page 4: Airborne Counting Fundamentals

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

Page 5: Airborne Counting Fundamentals

Hair with hair spray Red Blood cells Flakes of skinHair withhair spray

Blood cellsFlakes of skin

Humans as Sources of Particles

Page 6: Airborne Counting Fundamentals

Bacteria on the head of a pin

Page 7: Airborne Counting Fundamentals

A dust mite tucks into a tasty feast on a flake of human skin.

Humans shed over 5million skin cells per day.

Page 8: Airborne Counting Fundamentals

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

Page 9: Airborne Counting Fundamentals

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

Page 10: Airborne Counting Fundamentals

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

Page 11: Airborne Counting Fundamentals

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

Page 12: Airborne Counting Fundamentals

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)

Page 13: Airborne Counting Fundamentals

Detection Ranges

1 10 100 1000 10000 100000 1000000

CNC

Light Scattering

Light Blocking

1µm0.1µm 10µm 1000µm

nm

Page 14: Airborne Counting Fundamentals

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

Page 15: Airborne Counting Fundamentals

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

Page 16: Airborne Counting Fundamentals

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

Page 17: Airborne Counting Fundamentals

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

Page 18: Airborne Counting Fundamentals

Principles: Light Scattering

Laser Diode

Detector

Mirror

Light Trap

Detector Output

Page 19: Airborne Counting Fundamentals

Principles: Light Scattering

Laser Diode

Detector

Mirror

Particle

Light Trap

Page 20: Airborne Counting Fundamentals

Particle SizingParticle Sizing

Page 21: Airborne Counting Fundamentals

Disadvantages: Light Scattering

More complicated construction = higher costCharacteristics of particle surface (shiny, color)

affect response

Page 22: Airborne Counting Fundamentals

Effect of colors and surfaces on Light Scattering

Dark Light Shiny

Page 23: Airborne Counting Fundamentals

Effect of Flow Rate on SizingEffect of Flow Rate on Sizing

Page 24: Airborne Counting Fundamentals

Sensor

Sample inlet

Sensor

Typical Counter Components

Page 25: Airborne Counting Fundamentals

Sensor

Sample inlet

Flow Monitor Pump FilterSampleexhaust

Typical Counter Components

Sensor Pump

Page 26: Airborne Counting Fundamentals

Control ElectronicsSerialDataOut

Sensor

Sample inlet

Flow Monitor Pump FilterSampleexhaust

Typical Counter Components

Sensor CounterPump

Page 27: Airborne Counting Fundamentals

Typical Counter Components

Printer

0.3 1 4 1 0Display

Control ElectronicsSerialDataOut

Sensor

Sample inlet

Flow Monitor Pump FilterSampleexhaust

Sensor CounterPump Output

Page 28: Airborne Counting Fundamentals

Particle Counters - Hardware

There are 3 general styles of airborne instruments:

• Hand-held

• Portable (stand-alone / trolley mounted)

• Remote (for continuous monitoring)

Page 29: Airborne Counting Fundamentals

Handheld Counters

• Battery operated• Light-weight• Optical Particle

Counters • 0.3 or 0.5 micron• 0.1 cfm (2.83 l/min)• JIS• CE

Page 30: Airborne Counting Fundamentals

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

Page 31: Airborne Counting Fundamentals

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)

Page 32: Airborne Counting Fundamentals

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)

Page 33: Airborne Counting Fundamentals

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

Page 34: Airborne Counting Fundamentals

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

Page 35: Airborne Counting Fundamentals

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:

Page 36: Airborne Counting Fundamentals

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%

Page 37: Airborne Counting Fundamentals

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

Page 38: Airborne Counting Fundamentals

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.

Page 39: Airborne Counting Fundamentals

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

Page 40: Airborne Counting Fundamentals

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

Page 41: Airborne Counting Fundamentals

Counting ModesCounting Modes

Cumulative Differential

Size 3

Size 2

Size 1

Page 42: Airborne Counting Fundamentals

What Is the View Volume?

That area inside the sensor where the laser beam and sample path

are coincident

Laser Beam

Sample Air Flow

Page 43: Airborne Counting Fundamentals

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.

Page 44: Airborne Counting Fundamentals

Coincidence Loss Inside a SensorCoincidence Loss Inside a Sensor

=•Obscuration

=•Aggregation

•Single Particle

Page 45: Airborne Counting Fundamentals

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

Page 46: Airborne Counting Fundamentals

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

Page 47: Airborne Counting Fundamentals

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

Page 48: Airborne Counting Fundamentals

Sensor ResolutionSensor Resolution

Poor resolution Good resolution Perfect resolution

Assuming standard particle distribution

Page 49: Airborne Counting Fundamentals

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)

Page 50: Airborne Counting Fundamentals

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