Standards for Airborne Particle Counter Performance and Operation
Two particle counters can produce different data from sampled air due to differences in design, performance and operation. This article examines some of the standards and specifications imposed for discrete particle counters used in cleanrooms.
By Alvin Lieberman
Correlation between data batches from different airborne particle counters–or even from the same counter at different times –is always looked for when defining cleanroom performance, testing filters, or pinpointing contamination sources in a cleanroom. If counters are calibrated correctly and operate without error, reasonable correlation can be expected. This article examines why two counters may not produce the same data from a single particle suspension in terms of differences in counter design, performance, and operation.
In addition, documented specifications and standards are summarized. Counter performance standards, which define sizing and counting accuracy, sizing resolution, and operating limits, are also discussed, as well as standards that illustrate operating requirements such as calibration methods, sample acquisition needs, suspension concentration limits, and environmental effects, etc.
Used in essentially all cleanrooms, airborne particle counters are single particle counting instruments that use light-scattering phenomena to measure particles in the sampled air of a cleanroom. These devices are commonly referred to as discrete particle counters (DPCs). They size and count the particles in terms of the amount of scattered light from each particle. These light-scattering devices are capable of counting and sizing individual particles as small as 0.05 µm in diameter. When it is necessary to count particles as small as 0.01 µm, a condensation nucleus counter is used. It observes the scattered light pulse generated when the particle serves as a nucleus to produce a liquid droplet by condensation of supersaturated vapor on each particle larger than an initial threshold size. This device counts, but does not size, particles larger than its threshold size. All DPCs share one operating factor: the reported particle size depends on more than one parameter in addition to the actual particle size. Considering the applications for DPCs, it is necessary that DPCs meet and be operated in accordance with valid standards and specifications.
Application Areas and Uses
The DPC reports the particulate content of the air in the cleanroom and the cleanroom air supply. DPCs have two major uses in cleanrooms. One is the measurement of cleanroom air cleanliness and cleanroom component-generated contamination released into the air. The second involves assurance of the integrity of the cleanroom`s air filtration system. This definition may be determined when the cleanroom is first classified. The definition may occur during normal operation by moving a single DPC about the room or by using several DPCs at various locations in the cleanroom.
The reported airborne particle concentration verifies that the cleanroom meets the performance specification or that conditions in the cleanroom have not changed. If the particle content is above a specified level, costly modifications may be required for that room. The airborne particle content may be measured at or near a suspected contamination generation source. DPC data can justify changes in component selection or layout; and it can be used to enforce personnel activity restrictions. DPCs can also be used to verify the particulate removal efficiency of the HEPA or ULPA filters used in the cleanroom or in a clean air device. In either situation, DPC counting and sizing accuracy is paramount in assuring that the cleanroom, the air processing system, cleanroom components, tools in the cleanroom, and its personnel are all performing as required.
Counting and Sizing Data Generation
The operation of light-scattering DPCs will be presented here, in brief, to aid in understanding how the data is generated and why different DPCs may generate different data from the same air sample. A detailed description of the physics of light-scattering, the operation of the DPC internal air flow control systems, and the function of electronic control and measurement systems is beyond the scope of this discussion.
The DPC draws an air sample through a sensing volume where a defined light level is present. Each particle in the air sample produces a scattered light pulse of a level dependent on particle size, refractive index, and the illumination wavelength. Figure 1 shows the scattering pattern from a particle larger than the wavelength of illumination. The scattered light amplitude level varies with the scattering angle.
The DPC collects scattered light pulses over a solid angle specific to that DPC. The DPC`s electronic system then counts and sizes the number of light pulses over a solid angle specific to that DPC. The electronic system of the DPC counts and sizes the number of light pulses, controls component operations, and reduces and reports the data as having been derived from particles of sizes based upon the calibration of the DPC.
Present day DPCs operate by collecting scattered light over one of a limited number of classes of solid collection angles, as shown in Figure 2.1 It has been found that DPCs with different optical systems will produce different data from the same aerosol sample, even when both have been recently calibrated.2 The reason for this difference is shown in Figure 3, which shows relative scattering for DPCs with different collection optics for particles with a range of refractive index values. Note that the relative scattering for a DPC operating over a small scattering angle varies much more with particle size than the relative scattering for a DPC operating over a larger scattering angle. DPCs are normally calibrated with standard calibration latex particles of refractive index 1.6 – 0i. If the particles in the cleanroom were the same as these particles, different DPCs would produce the same data for cleanroom particles. However, in the real world, particles have refractive indexes different from that of latex. DPCs with different optical systems will then produce different data due to the differences between the calibration particles and the contaminant particles.
In addition to DPC design and performance effects, another reason for data differences is in the operation of the DPC. Remember that the instrument observes individual particles and sums the results to produce its data. If cleanroom particle concentration is excessive, more than one particle will be present in the sensitive volume at any time. If this occurs, the DPC will report the signal from a group of particles as resulting from a single larger particle. The DPC should not be used in areas where the concentration is greater than the recommended maximum. If particle concentration is very low, random variations in particle concentration, with time, will cause the data to have very poor statistical validity. If the DPC has been operated for an excessive time period without calibration or maintenance, sample flow rate, illumination intensity, and electronic system performance may not be correct. These conditions can result in generation of erroneous data both in terms of counting and sizing accuracy.
Most users select the DPC to be procured on the basis that it will count and size particles correctly in almost any situation. The correct DPC for any application should be based on the specific needs of that application. “Particle Counter Operating Requirements” lists the DPC performance parameters of concern in providing acceptable data.3 Maximum levels of performance are not always required for good operation. For example, signal-to-noise ratio, which allows an insignificant noise count, is acceptable. Sensitivity to the smallest particles possible is not needed; detection of particles present in sufficient quantity to classify the area is adequate. Since particle spatial distribution is random, repetitive measurements will vary sufficiently so that counting errors of 1 – 2 percent are not significant, especially when individual particle counts in the range of several thousand can be acquired.
A DPC with high volumetric flow rate will increase purchase costs; this should be considered and compared with possible savings in measurement time costs. The effects of improvements in DPC performance on the other factors in this table should be considered on the same cost and time basis. Verification of DPC performance for these factors can be obtained by use of the correct test standard.
Cleanroom classification is determined by particle count data more than by particle size data. Counting and sizing accuracy are closely related. Most DPCs are specified by the vendor to size the sampled particles with an error of less than 5 percent. When measuring cleanroom aerosol particles, the typical particle size distribution has been observed to follow a logarithmic response. The concentration varies inversely with approximately the third power of particle size, as shown in Figure 4. When a sizing error of 5 percent is accepted, the concentration error for a distribution based on an inverse third power law becomes nearly 20 percent. Smaller sizing errors are required to keep counting errors at low levels.
Standards and Specifications
In order to reduce the differences in single sample count data from both different and similar DPCs, it is necessary to impose standards and specifications on both DPC performance and DPC operation in cleanrooms. Experience shows that using standards will aid in generating valid and useful data. Implementation of the correct standard(s) will ensure that the DPC is operating correctly and that the reported particle data is valid for the cleanroom area or operation being measured. Differences between data sets from different DPCs will be minimized and reasonable inter-DPC correlations will be obtained.
DPC performance standards discuss the design limitations and performance requirements of the instruments. These standards cover such instrument parameters as sizing and counting accuracy, flow rate stability, sizing resolution, environmental ranges, and calibration intervals. DPC operation standards discuss the procedures for operation within the cleanroom to minimize errors in sampling, counting, and sizing. These include calibration procedures that can be carried out by the vendor or by the user`s metrology facility.
A listing of current DPC standards is provided in this article. The standards are identified in terms of DPC performance and/or operational requirements. The listed standards include documents which were developed both in the United States and in other countries. Further standards applicable to DPC performance and operation are in the process of development in the International Standards Organization`s Technical Committee 209 on Cleanrooms and Associated Clean Environments. It is anticipated that the DPC performance and operation document will be issued in 1996. The standards from this committee will probably replace many of the current national standards.
DPC Performance Standards
Army Missile Command (1980): Calibration System Requirements, MIL-STD-45662, DODSSP June 10, 1980–This standard provides for the establishment and maintenance of a calibration system to control the accuracy of measuring and testing equipment used to assure that supplies and services provided to the government are in conformity with prescribed technical requirements. Although this standard does not refer specifically to DPCs, it is invaluable for ensuring correct calibration of any measurement system.
ASTM (1989): Practice for Determining Counting and Sizing Accuracy of an Airborne Particle Counter Using Near-Monodisperse Spherical Particulate Materials. ASTM F-328-80–Sizing and counting accuracy determination procedures are given for verifying operation of an optical airborne particle counter. Monosized latex particles are used for size definition; counting data for suspension of particles are obtained for comparison by a referee method to indicate counting accuracy.
ASTM (1992): Practice for Secondary Calibration of Airborne Particle Counter Using Comparison Procedures. ASTM F-649-80, 1992– Procedures are given for fine-tuning response of an airborne particle counter to match that of a standard particle counter for defining atmospheric aerosol, following calibration with monodisperse latex particles. The procedure is helpful in correlating several instruments.
JIS (1989): JIS B 9921-1989, Japanese Industrial Standard: Light Scattering Automatic Particle Counter. Japanese Standards Association, Tokyo, Japan, 1989–This JIS specifies the light-scattering automatic particle counter to obtain particle concentration in air by drawing air continuously and by measuring the particle sizes and number of airborne particles. The standard specifies performance for sensitivity 0.1 µm, flow rate accuracy, maximum false-count level, cross-channel sensitivity, counting efficiency, count loss, signal-to-noise ratio limit, and resolution. Test methods are also described.
DPC Operation Standards
ASTM (1992): Continuous Sizing and Counting of Airborne Particles in Dust-Controlled Areas and Cleanrooms Using Instruments Capable of Detecting Single Sub-Micrometer and Larger Particles. ASTM F-50-92, 1992–Particle concentration by number and size distribution of airborne particles in cleanrooms are determined for particles in the size range of approximately 0.01 µm to 5 µm concentrations up to 3,500 per liter. Sample acquisition and instrument performance requirements are stated. This standard includes requirements for both DPC performance and operation; the standard is not restricted to definition of any specific instrument type.
IES (1995): Recommended Practice for Calibration of Particle Counters. IES-RP-CC-014, 1995–This practice establishes definitions and procedures for calibrating single particle counting devices used in cleanrooms and discusses instrument specifications. The document covers procedures for determining sizing and counting accuracy, DPC sampling flow rate, and sizing resolution.
JIS (1989): JIS B 9920-1989, Japanese Industrial Standard: Measuring Methods for Airborne Particles in Cleanroom and Evaluating Methods For Air Cleanliness of Cleanroom. Japanese Standards Association, Tokyo, Japan, 1989–The JIS specifies the measuring methods for concentrations of airborne particles and the evaluating methods for the air cleanliness of cleanrooms. The method is similar to USA Federal Standard 209E, but different class levels are stated and details of measurement methodology is given in more detail.
Anon, SAA (1989): Cleanrooms, Workstations, and Safety Cabinets–Methods of Test–Particle Counting in Work Zone By Automatic Particle Counter. AS 1807.8-1989, Standards Association of Australia, 1989–This standard sets out a series of methods, including specifications for all test equipment. Test methods for testing airborne particle concentrations by optical particle counters are included in this document.
Anon, VDI. (1989): Particulate Matter Measurement: Methods for Characterization and Monitoring of Test Aerosols: Optical Particle Counter. VDI 3489, Part 3, 4000 Dusseldorf 1, Germany: December, 1989–This document shows procedures for operating and verifying performance of optical particle counters in measuring airborne particles.
Standards Source Information
ASTM–American Society for Testing and Materials, 1916 Race St., Philadelphia, PA 19103-1187, fax (215) 977-9879.
DODSSP–Department of Defense Standardization Documents Order Desk, Bldg. 4D, 700 Robbins Ave., Philadelphia, PA 19111-5094, fax (215) 697-2978.
IES–Institute of Environmental Sciences, 940 E. Northwest Highway, Mt. Prospect, IL 60056, fax (708) 255-1699.
JIS–Japanese Industrial Standards Committee, c/o Stds. Dept. Agency-MITI, 1-3-1, Kasumigaseki, Chiyoda-ku, Tokyo 100, Japan, fax 81-3-3580-1418.
SAA–Standards Association of Australia, P.O. Box 458, North Sydney, NSW 2059, Australia, fax 61-2-959-3896.
VDI–Verein Deutscher Ingenieure, VDI-TGA, Graf-Recke-Strasse 84, D-4000 Dusseldorf, Germany, fax 49-211-6214-575. n
References
1. Hodkinson, J.R. and Greenfield, J.R., 1965. Response Calculations for Light Scattering Aerosol Counters. Applied Optics. 41 (1): 1463-1474.
2. Lieberman, A. 1980. Laboratory Comparison of Forward and Wide Scattering Angle Optical Particle Counters. Optical Engineering. 19(6):870-872.
3. Lieberman, A. 1989. Performance Definitions Effects on Submicrometer Particle Measurements. Proceedings of the 1989 Fine Particle Society Meeting, Boston, MA.
Alvin Lieberman received an MS in chemical engineering from Illinois Institute of Technology in 1949. He is an engineering consultant for Particle Measuring Systems, Inc., where he had been their technical specialist from 1983 to 1985 and from 1987 to 1992. He had been chief scientist at Hiac/Royco Instruments from 1968 to 1983 and from 1985 to 1987. His primary responsibility at both organizations was in product development, planning, and specifications.
He has written over 100 technical publications dealing with particulate systems and contamination measurement and control, as well as a 1992 book on contamination control. He has received the Whitfield award from IES, the Hausner award from FPS, and the Hall of Fame award from CleanRooms.
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Figure 1 demonstrates the scattering pattern from a particle larger than the wavelength of illumination. The scattered light amplitude level varies with the scattering angle.
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Figure 2. Today`s DPCs operate by collecting scattered light over one of a limited number of classes of solid collection angles as shown.
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Figure 3. DPCs with different optical systems will produce different data from the same aerosol sample even when both have been calibrated recently. The reason for this difference is shown–the relative scattering for DPCs with different collection optics for particles with a range of refractive index values.
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Figure 4. Most DPCs are specified by the vendor to size the sampled particles with an error of less than 5 percent. When measuring cleanroom aerosol particles, the typical particle size distribution has been observed to follow a logarithmic response. The concentration is seen to vary inversely with approximately the third power of particle size, as shown.
Particle Counter Operating Requirements
Good signal-to-noise ratio
Acceptable particle-sizing sensitivity
Maximum particle-sizing accuracy
Acceptable particle-sizing resolution
Adequate particle-counting accuracy
Maximum sample volumetric flow rate
Maximum particle concentration capability
Minimum data processing time
Correlation with other instruments
Adequate particle size definition range
Compatibility with fluids and environment
Documented calibration/maintenance records
Reliable operation and ease of maintenance