147 CHAPTER 16 Particle Sensor Construction All the manufacturers of drinking water particle counters provide particle sensors quite similar in operation and construction. The main variation to be found is in the size of the flow cell cross-section, and in the mounting of the sensor with regard to sample taps and access for cleaning. The former will mainly determine the sensitivity resolution and coincidence characteristics, while the latter impacts maintenance and potential for damage due to leaks. A. FLOW CELL Most of the manufacturers except for Met One provide a standard-type flow cell with synthetic sapphire windows and a stainless steel or aluminum flow cell. Met One has a sensor of this construction as well, but it is no longer supplied for drinking water applications unless specifically requested. 1. Met One The Met One drinking water particle sensors employ a replaceable quartz cell mounted in a plastic mechanical assembly. The quartz flow cell must be replaceable because the quartz is more susceptible to scratching than synthetic sapphire. There is no advantage to this type of design outside of lower cost of manufacture, which is important in that it helps to lower the cost of particle counting for the end user. This quartz/plastic flow cell does not have the same structural integrity of the metal/sapphire units, and is restricted to lower amounts of sample pressure. In this case, no more than 100 PSI is to be applied, and for less than 1 min. In most cases, this is many times in excess of what is required. The standard constant-head overflow weir provides an open system that limits the pressure to a few PSI. Closed systems should not be used because of the difficulty in maintaining constant pressure, and could potentially exceed the pressure limits, especially if severe water hammering L1306/frame/pt03 Page 147 Friday, June 23, 2000 2:15 PM © 2001 by CRC Press LLC 148 A PRACTICAL GUIDE TO PARTICLE COUNTING were to occur. Pressurized washing to clear clogs could exceed 100 PSI, so care should be exercised. It is probably better to remove the flow cell in these cases, to avoid ruining the whole assembly. Replacing the flow cell does leave the door open to potential problems. Care must be taken to follow the instructions carefully. If fingerprints are left on the flow cell, the optical integrity will be altered, affecting the sizing accuracy of the sensor. If the mechanical alignment is altered or loosened, the calibration will be affected, and leaks could occur. The directions for replacing the flow cell are not complicated, but if the person performing the task is not aware of the sensitivity of the instrument, carelessness could lead to problems. The sensor should be recalibrated when the flow cell has been replaced. See Figure 16.1. The flow path for the Met One sensors is 750 µm × 750 µm. This is the same size as the stainless steel/sapphire sensors provided with earlier Met One particle counters and still available as an option. All Met One sensors are volumetric. 2. Chemtrac The Chemtrac particle counters employ a nituff-coated aluminum flow cell. The nituff coating provides a tougher, wear-proof finish than standard black anodizing. The flow path dimensions are 1 mm × 1 mm (1 mm = 1000 µm) and the sensors are volumetric. Figure 16.1 Met One removable flow cell. (Courtesy of Pacific Scientific Instruments, Grants Pass, OR.) L1306/frame/pt03 Page 148 Friday, June 23, 2000 2:15 PM © 2001 by CRC Press LLC PARTICLE SENSOR CONSTRUCTION 149 3. ARTI ARTI offers two sensors for drinking water application. Each employs a stan- dard aluminum flow cell housing with a monolithic fused silica flow cell with an antireflective coating. The WPC 2000 is designed for 2 µm sensitivity, and has an 800 µm × 800 µm flow path. The WPC 1000, a 1 µm sensor, has a 600 µm × 600 µm flow path. 4. IBR The IBR sensor is available in two concentrations. Higher concentration limits are obtained by shrinking the flow path dimensions from 400 µm × 400 µm to 150 µm × 1000 µm. B. CELL WINDOWS The Chemtrac and IBR sensors come with synthetic sapphire windows, as well as the older Met One sensors. The Met One drinking water sensor employs a quartz flow cell as described above. This flow cell is a single bored piece of quartz, which is field replaceable. ARTI uses fused silica as opposed to synthetic sapphire. C. SAMPLE FITTINGS Sample fittings vary among the different manufacturers, with the simplest being the nylon barb fittings found on the Chemtrac particle counters. Since the Chemtrac flow connections are located outside the NEMA enclosure, the type of fitting employed is less critical, because leaks will not cause any damage or electrical hazards. The Chemtrac sensor uses standard NPT threads for mounting these fittings, so they can be easily replaced or changed to a different type if desired. Met One also uses barb fittings on the sensor, which is located inside the NEMA enclosure. Short lengths of tubing are then run from the sensor inlet and outlet to nylon quick-disconnect fittings mounted on the walls of the NEMA box. The external sample tubing is then connected to these fittings. IBR uses threaded stainless steel fittings designed for higher pressures. This sensor was designed for other applications requiring pressures higher than those found in drinking water plants. The same fittings are used on the older Met One sensors designed for the same purpose. ARTI employs nylon twist-lock fittings for smooth internal flow passages designed to prevent particle buildup. All of the sample fittings are designed for 1/4 inch outside diameter (or 6 mm metric) tubing. L1306/frame/pt03 Page 149 Friday, June 23, 2000 2:15 PM © 2001 by CRC Press LLC 150 A PRACTICAL GUIDE TO PARTICLE COUNTING D. LASER/OPTICAL ASSEMBLY All of the manufacturers employ a similar method of mounting the laser/optical assembly to the flow cell. Mechanical configurations vary, as well as the method of alignment during calibration. This assembly is not user serviceable, and the design features are not made public. L1306/frame/pt03 Page 150 Friday, June 23, 2000 2:15 PM © 2001 by CRC Press LLC . 150 A PRACTICAL GUIDE TO PARTICLE COUNTING D. LASER/OPTICAL ASSEMBLY All of the manufacturers employ a similar method of mounting the laser/optical assembly to the flow cell. Mechanical configurations. sensitivity resolution and coincidence characteristics, while the latter impacts maintenance and potential for damage due to leaks. A. FLOW CELL Most of the manufacturers except for Met One provide a standard-type. 147 CHAPTER 16 Particle Sensor Construction All the manufacturers of drinking water particle counters provide particle sensors quite similar in operation and construction. The main variation