Sensing and Control

Background	Turbidity sensing provides a quick, practical indication of the relative amount of suspended solids in water or similar liquids. Turbidity is not a direct measure of suspended particles in water but instead, a measure of the scattering effect these particles have on light. The amount of light scattered by any particle depends on the particle's size, shape, composition and refractive index. The first real attempts to quantify turbidity date from about 1900 when scientists of the time, developed a standard suspension fluid using 1000 parts per million (ppm) diatomaceous earth in distilled water. Dilution of this reference suspension resulted in a series of standard suspensions used to derive a ppm silica scale for calibrating the contemporary turbidimeters. They also created a corresponding test method to compare samples. The test method consisted of a flat-bottomed glass tube and a special candle. Measurements were made by slowly pouring the sample into the tube until the visual image of the candle, when viewed from the open end of the tube, diffused to a uniform glow - this was called the extinction point. This was, of course, a very subjective measurement but the early workers achieved some remarkably consistent results. In conjunction with the silica scale, the apparatus shown in Figure 1 became known as the Jackson Candle Turbidimeter (named after the inventor) and was measured in Jackson Turbidity Units (JTU's). One JTU is equivalent to 1 ppm of silica. The Jackson Turbidimeter presented some serious practical limitations because it could not measure turbidity lower than about 25 JTU and was dependent on human judgment. In addition, the candle flame light source was in the yellow-red part of the spectrum which was not scattered effectively by small particles. Photoelectric detectors were later introduced to remove the human element and although sensitive to very small changes in light intensity, they still could not determine the extinction point on their own. Figure 1. The Jackson candle method for measuring turbidity At low concentrations, the change in transmitted light, viewed from straight on, is so slight that it's virtually undetectable by any means. At higher concentrations it becomes easier because multiple scattering interferes with direct transmission. The solution to the problems described above is to measure the light scattered at an angle to the incident light beam and then relate this off-axis scattered light to the sample's actual turbidity. Most instruments of this type measure the 90 degree scatter because this angle is considered to be the least sensitive to particle size. These types of instruments, illustrated in Figure 2, are referred to as Nephelometers. Figure 2. Basic Nephelometer operation .....

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