81 CHAPTER 7 Particle Sensor Construction A. FLOW CELL The flow cell is a foundation piece of the particle counter sensor. It is generally made out of aluminum or stainless steel or even plastic. Stainless steel is more expensive primarily because it is much harder than aluminum and thus more difficult to machine. Aluminum must be coated or specially anodized to prevent pitting when exposed to water. The sample flows up through a channel formed by the flow cell windows and the slit through the flow cell. The sample tube fittings are mounted into the flow cell on both ends of the slit. In most cases, the laser/optics assembly and the detector circuit are bolted to either side of the flow cell. See Figure 7.1. The flow cell must be rigid enough to hold all these pieces together without allowing any alteration in alignment, which would result in the particle counter sensor losing calibration. The cell windows must fit tightly and be sealed to prevent sample leakage. The materials of construction are not important, as long as the functional integrity is maintained. The ability to withstand repeated cleanings and clog removal is a necessity. B. CELL WINDOWS The cell windows employed are usually round disks of synthetic sapphire with flat, polished surfaces. These windows are seated on either side of flow cell slit and sealed with O-rings to create the flow cell path. Quartz can be used for this type of window as well, or can be drilled and polished to produce a single four-sided piece that contains the flow cell path as well. The laser light beam is sent through the center of the window through the flow cell path and then out the other side, where it strikes the detector. L1306/frame/pt02 Page 81 Friday, June 23, 2000 1:54 PM © 2001 by CRC Press LLC 82 A PRACTICAL GUIDE TO PARTICLE COUNTING The cell window materials vary in optical quality. Small imperfections in the material diffuse the light and are a source of resolution and sizing error. Quartz may have fewer imperfections as far as optical quality is concerned. It is a softer material than sapphire, and thus can be bored and polished to create the single window piece. This softness accounts for the fact that it can be scratched from cleaning, or from contact with particles over a long time period. Different grades of sapphire are available. The entrance window (the side where the laser enters before passing through the flow cell) must be of a higher grade than the exit window. The beam must be tightly focused where it contacts the particles. Since the detector is measuring total light energy, it is not greatly affected by some diffusion in the exit window. The entrance window is covered with an antireflective coating to keep the light energy from reflecting back into the laser. Lasers produce light by bouncing light energy back and forth at a high frequency between mirrored surfaces, which amplifies the light intensity to produce the desired output beam. If light is reflected back into the laser, it can disrupt this process, and cause variations in the beam intensity. This is another reason that aluminum flow cells are anodized a dark color, as the shiny metallic surface can increase the problems with reflections. Stainless steel flow cells cannot be coated in this way, and thus require finer alignment to minimize reflections. C. SAMPLE FITTINGS A variety of sample fittings are available, and are usually of concern only as a matter of convenience or preference. Ease of accessibility is important. Plastic or nylon fittings should be easily replaceable, as they will break periodically. Some older sensors still in use have pressure fittings that can alter the optical alignment if tightened too snugly. Figure 7.1 Particle sensor. (Courtesy of L & H Environmental, Inc., Roseburg, OR.) Power Regulator Sensor Cover Flow Cell Channel Flow Cell Housing Laser Diode Holder Pin Photodiode Collimation and Focus Optics Sapphire Windows 780 nm Laser Diode Fitting Optic Holder X,Y,Z Adjust Spacer Laser Driver Signal Amplification Board L1306/frame/pt02 Page 82 Friday, June 23, 2000 1:54 PM © 2001 by CRC Press LLC PARTICLE SENSOR CONSTRUCTION 83 D. LASER/OPTICAL ASSEMBLY The laser light source is passed through a series of lenses to produce a thin, even beam across the flow cell. These lenses must be rigidly mounted to maintain the proper alignment. This is usually accomplished by mounting the laser and other optical lenses on a single block or in a tube. This “subassembly” is then bolted to the flow cell. The detector electronics assembly is mounted to the opposite side of the flow cell. Once the entire assembly is in place, final adjustments are made to produce the desired alignment. L1306/frame/pt02 Page 83 Friday, June 23, 2000 1:54 PM © 2001 by CRC Press LLC . materials vary in optical quality. Small imperfections in the material diffuse the light and are a source of resolution and sizing error. Quartz may have fewer imperfections as far as optical. coated in this way, and thus require finer alignment to minimize reflections. C. SAMPLE FITTINGS A variety of sample fittings are available, and are usually of concern only as a matter of convenience. optical quality is concerned. It is a softer material than sapphire, and thus can be bored and polished to create the single window piece. This softness accounts for the fact that it can be scratched