73 PART II Understanding the Technology In this part of the book we turn our attention to a more in-depth look at particle counters, from the basic construction of the instrument to the features of the complete particle counting system. Particle counters are presented generically in this part, while in Part III the actual manufacturer’s product offerings are presented. The chapters for Parts II and III are designed to run in parallel, to allow for easy cross- reference. L1306/frame/pt02 Page 73 Friday, June 23, 2000 1:54 PM © 2001 by CRC Press LLC 75 CHAPTER 6 Specifications All instruments are designed for a specific task. Specifications are the measure of how well the instrument performs the various parts of that task. Some specifica- tions are common to all types of instruments (dimensions, weight, power require- ments, etc.) and are easily understood. Specialized instruments have specifications that are unique to the task for which they are designed. In many cases, common specification terms such as resolution and sensitivity must be understood in relation to the unique properties of the application. In this section, the primary specifications employed for evaluating particle counter performance are defined and discussed. Understanding of these few speci- fication parameters is essential to making informed decisions about particle counters. If these are not clear at first reading, readers should refer back to this section as they continue through the book. It will become evident that these specification parameters are interrelated, and will only be understood completely when the whole picture of particle counting is clear. A. SENSITIVITY Sensitivity is the smallest measurable amount that an instrument can detect. In the case of the particle counter, this refers to the smallest particle size that can be reliably detected. The general rule in the industry is that a particle can be “reliably” detected and measured if it produces a signal with a minimum 2 to 1 signal-to-noise ratio. B. SIGNAL-TO-NOISE RATIO All electronic instruments are subject to various forms of “noise,” or random, meaningless electrical signals. The term noise comes from the analogy to sound levels in a given space. Noise is meaningless sound, and every space where sound can exist will have some level of noise present. Our ability to discern specific sounds L1306/frame/pt02 Page 75 Friday, June 23, 2000 1:54 PM © 2001 by CRC Press LLC 76 A PRACTICAL GUIDE TO PARTICLE COUNTING depends upon those sounds being at a higher level than the noise present. The amount by which the specific sound exceeds the noise level is called the signal-to-noise ratio. The sound found between stations on a radio is a good example of electronic noise. Even the highest-fidelity audio system produces some of this type of noise, which becomes audible as the volume is increased. Every type of electronic device is affected by this phenomenon. In the case of the particle counter, noise is produced by the electronic detector circuit, the laser light source, the power supply, and is induced by other devices. The imperfections in the structure of the flow cell windows cause minute scattering and blurring of the laser light source. The com- bination of all these factors results in a noise level that can be observed and measured with an oscilloscope. (An oscilloscope displays electronic signals in a manner similar to a television.) See Figure 6.1 for a representation of noise as displayed on an oscilloscope. This noise level is measured with no particles in the water flowing through the particle counter sensor. (This is not exactly true. There are millions of particles well below the detection limit of the particle counter sensor. Some of these contribute to the noise level in a small way.) Once the noise level has been determined, particles of a known size can be sent through the particle counter sensor, and measured. The resulting amplitude produced by the particles is compared with the noise amplitude to determine the signal-to-noise ratio. See Figure 6.2. Figure 6.1 Noise on oscilloscope. L1306/frame/pt02 Page 76 Friday, June 23, 2000 1:54 PM © 2001 by CRC Press LLC SPECIFICATIONS 77 As mentioned is Section A, the smallest particle that can be reliably measured must produce a signal amplitude at least twice that of the noise, or a 2 to 1 signal- to-noise ratio. C. RESOLUTION This term refers to the degree to which an instrument can distinguish between differences in the object of measurement. In particle counting, resolution refers to the accuracy with which the sensor can distinguish and measure differences in particle size. This is stated in terms of percentage. A (+ or –) 10% resolution error would mean that particles ranging in size from 4.5 to 5.5 µm could be measured as 5 µm in size. Resolution measurement is a somewhat sticky subject in the particle counter industry. It will be discussed in further detail in relation to calibration and other issues. D. COINCIDENCE Coincidence was discussed in Chapter 5 in relation to sample dilution. Basically, it is the “coincidence” of two or more particles in the sensor view volume at the same time, which is a function of the concentration of particles in the sample. There are other factors that affect this, but they are beyond the scope of this book. Coin- Figure 6.2 Pulse signal-to-noise measurement. L1306/frame/pt02 Page 77 Friday, June 23, 2000 1:54 PM © 2001 by CRC Press LLC 78 A PRACTICAL GUIDE TO PARTICLE COUNTING cidence is usually presented as the number of particles for which the measured counts fall within 10% of the actual number of particles present. This number is the total number of particles per milliliter that are measurable by the sensor. E. SIZING RANGE This measure may appear under various names, but it is basically the range of measurable particles that can be sized and counted by the particle counter. It extends from the minimum size particle (usually 2 µm) up to 400 µm or so. The upper limit is generally not too important, as most particles of interest will fall well below 50 µm. F. SAMPLE FLOW RANGE All particle counter data must be collected for a known volume of sample, which usually means the flow rate must remain constant or be measured continuously. The specified sample flow range refers to the flow rates at which the particle counter sensor can measure particle size accurately. If the particles are flowing through the sensor at too high or too low a velocity, they will not be sized properly by the detector electronics. The reasons for this are covered in Chapter 7. Figure 6.3 Resolution error as a function of cell width. Cell Illumination L1306/frame/pt02 Page 78 Friday, June 23, 2000 1:54 PM © 2001 by CRC Press LLC SPECIFICATIONS 79 G. FLOW CELL DIMENSIONS The flow cell is the pathway through which the sample must flow. A larger flow cell will clog less often and be easier to maintain. On the other hand, a smaller flow cell will permit a higher concentration sample to be measured with less coincidence error. A trade-off is necessary. Resolution is also a function of flow cell size. The laser light source will not maintain an even intensity across the entire flow cell, and this error grows with the width of the flow cell. Particles passing through the edges of the beam will not produce the same amplitude pulse as particles of the same size passing through the center. See Figure 6.3. H. VOLUMETRIC The term volumetric refers to the coverage area of the laser light source. If the beam covers the entire width of the flow cell, the sensor is volumetric. If only a fraction of the width is covered, then it is non-volumetric, which is sometimes called in situ. An in situ arrangement allows for a much higher concentration of particles to be measured because it reduces the effective volume of the flow cell. The drawback is that resolution is diminished because particles may partially pass through the beam and thus be undersized. Figure 6.4 provides an illustration of this. Figure 6.4 Volumetric and in situ sensors. Volumetric In- Situ Cell Illumination L1306/frame/pt02 Page 79 Friday, June 23, 2000 1:54 PM © 2001 by CRC Press LLC . SPECIFICATIONS 77 As mentioned is Section A, the smallest particle that can be reliably measured must produce a signal amplitude at least twice that of the noise, or a 2 to 1 signal- to- noise ratio. . particle size that can be reliably detected. The general rule in the industry is that a particle can be “reliably” detected and measured if it produces a signal with a minimum 2 to 1 signal -to- noise. complete particle counting system. Particle counters are presented generically in this part, while in Part III the actual manufacturer’s product offerings are presented. The chapters for Parts II and