Fundamentals of Particle Counting

Transcription

1 Fundamentals of Particle Counting 1

2 Particle Counting: Remains the most significant technique for determining the cleanliness level of a fluid Useful as a tool for qualification and monitoring cleanroom operations The preferred means of classification testing for cleanrooms (ISO 14644) Required for continuous monitoring in GMP for aseptic operations (US, EU and International GMPs)

3 Semiconductor Flat Panel Aerospace Pharmaceutical High Technology Industries Needing Contamination Control Data Storage

4 Automotive Food/Beverage Defense/Homeland Security Medical High Technology Industries Needing Contamination Control Air Quality

5 Optical Solar Coatings Hospital High Technology Industries Needing Contamination Control Electronics

6 What is a Particle Counter? A Device Used to Count and Size Discrete Particles in a Fluid. A Fluid is: A substance, as a liquid or gas, that is capable of flowing and that changes its shape at a steady rate when acted upon by a force tending to change its shape. Air Liquid Gas Utilizing Either: Light Obscuration (Light Blocking) Light Scattering

7 Components of a Particle Counter Sample Input User Interface: Touch Screen Display Printer

8 Components of a Particle Counter Sample Input HEPA Filter Exhaust

9 Components of a Particle Counter Sample Input The Particle Detector Light Stop Photo Detector Counter Electronics Laser Diode Vacuum Pump Flow Control

10 Particle Counter Terms Particle: Solid Material Ranging in Size from 5nm to 100µm Particle Counter: Device Used to Count and Size Individual Particles Sample Time: Duration of Sample Sample Flow Rate: Volume of Fluid Passing Through the Particle Counter per Unit Time Cubic foot / minute Milliliter / minute View Volume (Percentage): Percentage of Sample Measured Sample Volume: Sample Flow Rate X View Volume X Sample Time Counting Efficiency: Comparison of Particle Counter Under test to More Sensitive Instrument Refractive Index: The ratio of the velocity of light in vacuum to the velocity of light in a medium

11 Particle Counter Terms Sensitivity: Minimum Detectable Particle Size Zero Count Level (False Count): Number of False Counts When Inlet Fluid Stream is Particle Free Concentration Limits: Maximum Particle Concentration Permitted for Particle Counter Operation allowing no more then a 10% Coincidence Error Coincidence Error: Counting Error from More Then One Particle Entering View Volume at the Same Time Channels: Particle Sizing Bins Cumulative: Mode of Particle Counter Operation Indicating Particle Counts for the Particle Channel Size of Interest and all Particles Larger (for classification purposes, this is the mode to use) Differential: Mode of Particle Counter Operation Indicating Particle Counts for the Particle Channel Size of Interest up Until The Next Larger Particle Size Channel

12 Important Terminology Flow Rate Amount of air that is measured over a period of time in a particle counter. Expressed in volume / time. (Cubic feet / minute, Liters / minute) 0.1 CFM or 1/10 th of a Cubic Foot of Air / Minute (2.83 Liters / Minute) 1.0 CFM or 1 Cubic Foot of Air / Minute (28.3 Liters / Minute) 50 LPM or 1.77 CFM (20 Minute Sample time for 1 Cubic Meter) 2.0 CFM or 56.6 LPM (17.6 Minute Sample time for 1 Cubic Meter) 75 LPM or CFM (13.3 Minute Sample time for 1 Cubic Meter) 100 LPM or 3.53 CFM (10 Minute Sample time for 1 Cubic Meter)

13 Important Terminology Cubic 1 ft Cubic Meter Foot 1 Meter 3 = 35.3 Feet 3 1 M 1 ft 1 ft 1 M 35 Minute Sample

14 Important Terminology Class Maximum Allowable Concentration of Particles Specified in a maximum number of particles per volume of air (either particles / cubic meter or cubic foot). Grade Air Cleanliness Levels specified for the manufacture of sterile medicinal products There are 4 grades A The local zone for high risk operations B For aseptic preparation and filling, this is the background environment for the grade A Zone C & D Clean areas for carrying out less critical stages in the manufacture of sterile products

15 Particle Size Comparison Chart

16 Relative Size Micron Scale Andenovirus Micron Herpes Virus 0.1 Micron Human Blood Cell 7 Micron E-Coli 0.5 Micron Human Hair Micron 35 Micron Visible Particles

17 Relative Size Gas Molecules (AMC) 2-50 Å ( µm) 0.3 µm particle 0.1 µm particle

18 The Problem: Contamination is all Around Us. 20µm Common Dust Mite

19 Particle Counting Basics Two Particle Detection Methods 1) Light Extinction (Light Blocking) Attenuation of light signal. Measurement of particles >1.0 micron 2) Light Scattering Redirected light energy (Scattering) Measurement of particles >0.05 micron

20 Light Interaction with a Particle Reflection Diffraction Absorption Refraction Internal Scattering When a particle is illuminated by a light beam, it is redirected or absorbed Reflection redirects light through surface interaction Refraction redirects light from the light entering the particle

21 Particle Counter Operation Laser Source Photo Detector

22 Types of Light Scattering: Mie Examples of Mie Scattering Clouds Fog I d2 λ 4 I = intensity of light scattered d = diameter of the Particle λ = wavelength of light Direction of Incident Light

23 Types of Light Scattering: Rayleigh Example of Raleigh blue sky I d6 λ 4 I = intensity of light scattered d = diameter of the Particle λ = wavelength of light Direction of Incident Light

24 Sizing Light scattered by a single particle in a specific direction in relation to the original direction has a unique signature which relates to the size, shape and material of the particle. As size is dominant the light intensity collected by the receiving optics of an instrument can be calibrated to the diameter of a sphere of a reference material, typically Polystyrene Latex. There is a dependency of the scattered signal on particle shape and material. However, it is a common practice to report particle size information in equivalent Polystyrene Latex sphere diameters.

25 Sizing Assumptions in Particle Counting: 1. All Particles are Spherical in Nature 2. All Particles have the Same Refractive Index of Polystyrene Latex

26 MCA Graph C O U N T S Peak to Valley Signal / Noise Millivolts Ideally Signal to Noise Ratio should be > 2:1

27 Counting Efficiency Test Question: If you have 100 particles coming through your particle counter and the instruments counting efficiency is 50% how many particles would you expect to count?

28 50% Counting Efficiency Example of Counting Efficiency at 0.5 Micron Particles C o u n t s Nominal Size 50% 70% 30% 50% point of in a monodisperse distribution Smaller than nominal size Larger than nominal size Particle size Definition: In a monodisperse distribution, the sensor detects all particles at the nominal size and larger. ISO requires that the counting efficiency of the smallest detectable size of a sensor be between 30 and 70%.

29 Noise Cumulative 1.0um Cumulative 0.7um Cumulative 0.5um Diff 0.3um Cumulative 0.3um Diff 0.5um Diff 0.7um Diff 1.0um 0.3um 0.5um 0.7um 1.0um 50% counting efficiency

30 Counting Efficiency Measurement Test Set-up Mono-Dispersed Aerosol Reference Particle Counter With 100% Counting Efficiency (for Particle Size of Interest) FLOW CONTROL P.S.L. Particles DISPERSION CHAMBER Tubing Lengths Precisely the Same Length Flow Rates on Both Counter Must be Equal AEROSOL GENERATOR Mono-Dispersed Aerosol Particle Counter Under Test FLOW CONTROL Components Needed: Particle Source (Polystyrene Latex Spheres) Aerosol Generator Dispersion Chamber Airborne Particle Counter (Test Unit) Airborne Particle Counter (Reference Instrument) External vacuum source when required Vacuum Pump If required for units without Built in Pump

31 Scope of ISO Instruments that conform to this part of ISO are used for the classification of air cleanliness in cleanrooms and associated controlled environments in accordance with ISO , as well as the measurement of number and size distribution of particles in various environments. The following are within the scope of this part of ISO 21501: 1. Size calibration 2. Verification of size setting 3. Counting efficiency 4. Size resolution 5. False count rate 6. Maximum particle number concentration 7. Sampling flow rate 8. Sampling time 9. Sampling volume 10. Calibration interval 11. Test report

32 Coincidence Error / Concentration Limits If particle counter is used in environments where the concentrations of particles are too high, more than one particle at a time can enter the view volume. This results in coincidence errors. Sensor View Volume = Normal This example shows one 0.5µm particle in the view volume and its output pulse. Sensor View Volume = High Concentration Exceeding Concentration Limit This example shows two 0.5µm particles in the view volume at the same time. The counter would report a size larger than 0.5µ and only one count. Per ISO 21501: Concentration limits are represented with no more then a 10% coincidence error.

33 Sample Delivery A vacuum pump is used to pull the sample air through the sensor. The flow is measured in cubic feet per minute (cfm). The use of some type of flow control is required to maintain the correct flow. 1.0 cfm pump 28.3 LPM 0.1 cfm pump 2.83 LPM

34 Critical Orifice In remote counters a critical orifice is used to regulate the flow through the sensor. A critical orifice is a precision opening that will only allow a certain amount of air flow once the critical vacuum pressure is applied. In the case of Lighthouse counters the critical pressure is 18 Hg. Regulated flow (1.0cfm or 0.1cfm Depending on the sensor) Critical orifice 18 Hg vacuum pressure

35 Counting Electronics Threshold Circuit This circuit is duplicated from one to 8 Channels depending on the model of particle counter. Analog Signal in +V +V CH3 CH2 Digital signals to counting circuitry +V CH. 3 CH. 2 CH. 1 CH1

36 Counting Electronics cont d Threshold Circuit This circuit is duplicated from one to 8 Channels depending on the model of particle counter. Analog Signal in +V +V CH3 CH2 Digital signals to counting circuitry +V CH. 3 CH. 2 CH. 1 CH1

37 Counting Modes (Cumulative & Differential) Cumulative Differential

38 Spec Sheet

39 Laminar Air Flow And Isokinetic Sampling LAMINAR AIR FLOW: UNIDIRECTIONAL, NON-TURBULENT. AIR FLOW. STREAM LINES DO NOT CROSS. A KEY ELEMENT TO FUNCTION OF CLASS 100 (ISO 5) OR BETTER ISOKINETIC SAMPLING: INLET VELOCITY AND DIRECTION TO SAMPLER ARE THE SAME AS THE FREE STREAM VELOCITY AND DIRECTION SUB-ISOKINETIC ERRORS SUPER-ISOKINETIC ERRORS WHAT ABOUT TURBULENT AIR FLOW SAMPLING?

40 Isokinetic Sampling HEPA Filter Laminar Flow Stream Ft/Min ( M/Sec) Air Stream Lines ISOKINETIC Probes ISOKINETIC SAMPLING: Inlet velocity and direction to the sample probe are the same as the air stream velocity and direction

41 Sub-Isokinetic Sampling HEPA Filter Laminar Flow Stream Ft/Min ( M/Sec) Air Stream Lines SUB-ISOKINETIC SAMPLING ERROR ISOKINETIC Probes SUB-ISOKINETIC SAMPLING ERROR: Underestimate the population of small particles because the probes flow rate is too low.

42 Super-Isokinetic Sampling HEPA Filter Laminar Flow Stream Ft/Min ( M/Sec) Air Stream Lines SUPER-ISOKINETIC SAMPLING ERROR ISOKINETIC Probes SUPER-ISOKINETIC SAMPLING ERROR: Overestimate the population of small particles because the probes flow rate is too high.

43 Turbulent Flow Cleanrooms HEPA Filter HEPA Filter ISOKINETIC Probes Non Laminar (Uni-Directional) Cleanrooms are TURBULENT flow Cleanrooms. Isokinetic Sampling is NOT possible in this type of cleanroom. Recommended proceedures for particle counting remain as if sampling in a Laminar (Uni-Directional Cleanroom)

44 Summary Counting Efficiency: Comparison of Particle Counter Under test to More Sensitive Instrument Sensitivity: Minimum Detectable Particle Size Zero Count Level (False Count): Number of False Counts When Inlet Fluid Stream is Particle Free Concentration Limits: Maximum Particle Concentration Permitted for Particle Counter Operation allowing no more then a 10% Coincidence Error Cumulative: Mode of Particle Counter Operation Indicating Particle Counts for the Particle Channel Size of Interest and all Particles Larger (for classification purposes, this is the mode to use) Differential: Mode of Particle Counter Operation Indicating Particle Counts for the Particle Channel Size of Interest up Until The Next Larger Particle Size Channel

45 Summary ISO Calibration Standard Yearly Calibration Annex 1: Sampling a Cubic Meter of Air

46 Thank You 46