Optical Particle Counter Operating Procedures, Calibration and Correlation Methods
Data from any OPC are derived on the basis of a number of OPC design and operating parameters. Most operators assume that the data produced are valid for any material and under any circumstances. However, it must be realized that the OPC is not a primary measurement instrument. Even though it reports only the size for each particle and the number of particles observed, it responds to several parameters such as particle size, the ratio of particle to fluid refractive indexes, and particle shape. OPC response is affected by the fluid flow field from which the sample is produced. Response also varies with OPC optical and electronic design. The particle size data produced depend on several system parameters. These effects occur even when the OPC is operating at its best and has been recently calibrated. Therefore, the OPC is calibrated to a known standard material using an accepted and standard method. Further, when more than one OPC is used in a large cleanroom area, it may be necessary to control operation of the several instruments to assure that each one provides similar data in similar environments. Minor differences in response to particle properties can result in a large change in indicated particle concentration. It is advisable to carry out effective correlation procedures whenever more than one OPC is to be used in a single installation in order to minimize this problem. These procedures may include selection of OPC designs so that similar systems are used for this purpose, as well as verifying calibration of each OPC.
KeywordsLatex Particle Particle Counter Polystyrene Latex American National Standard Institute Sensor Resolution
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