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Introduction


This article defines turbidity, how it is measured, and the effects that suspended particle size, shape, distribution, and stray light have on the turbidity measurement. Information is also provided on calibration standards and the different optical configurations that are available.

figure 1Definition and Measuring Principle
Turbidity is defined as an "expression of the optical property that causes light to be scattered and absorbed rather than transmitted in straight lines through the sample."1 Simply stated, turbidity is the measure of relative sample clarity. It is not color. Figure 1 shows the interaction of a light beam and undissolved, finely distributed particles known as suspended solids. When the light beam passes through the sample of fluid, the suspended solids scatter the light in all directions (360°spherically). Reduction in the intensity of the light beam is primarily caused by the suspended solids scattering the light. However, absorption (color) from dissolved substances can also reduce the intensity and should be taken into consideration by manually or automatically subtracting its effect. This can be accomplished optically if a turbidimeter is used (refer to the "Modulated Four-Beam Method" later in this article for details) or electronically if a separate absorption photometer is used in combination with a turbidimeter and their outputs are subtracted. Low quantities of suspended solids are normally monitored by measuring the scattered light effect rather than the absorption effect because with scattered light, the photocell detects small changes in light intensity with respect to a dark background. The disadvantage occurs at higher suspended solids levels where multiple scattering limits the amount of side-scattered light received by the photocell. This condition results in lower-than-actual turbidity readings. At suspended solids concentrations above 2000 ppm, alternate measurement methods such as absorption must be used in place of the turbidity measurement. Turbidity measurements provide a reading of the amount of scattered light and cannot be directly related to a gravimetric equivalent unless a working curve for the specific sample is created. The intensity of scattered light is affected by many variables including wavelength, particle size, color, and shape.

figure 2

Particle Effects
There is no absolute difference between dissolved and undissolved matter. The water treatment authority considers all particles of less than 0.45 microns in diameter as being dissolved. It is important to note that particles smaller than 0.45 microns will also scatter light.

The amount of scattered light is not the same in all directions and the spacial distribution pattern varies with particle size. Figure 2 illustrates this fact in which large and small particles show different lines of equal light intensity.

Scattering distribution patterns show that when particles are equal to or larger than the wavelength of the incident light beam (1 micron), there is a higher amount of forward scattered light. As the particle size becomes smaller, the pattern becomes somewhat peanut-shaped (see Figure 2). However, particles smaller than 0.05 microns in diameter (colloids) scatter light equally in all directions.

Other Factors Which Influence Light Scattering are:

1. Particle color-This determines the ability to absorb or reflect the incident light beam. For example, two different types of filter beds are typically used in water treatment: carbon and sand. The sand is light in color which reflects the incident beam very well. Conversely, the black carbon has a tendency to absorb the incident beam. Therefore, with all else being constant (particle size, shape, etc.), the fines from carbon filters have lower scattered light intensity.

2. Particle shape-This determines the ability of the suspended solids to provide a constant spatial distribution pattern. A smooth, spherical-shaped particle will provide predictable results, whereas an irregularly-shaped particle can produce widely varying responses depending on the side that the incident light beam strikes.

3. A difference between the refractive indexes of the particle and the sample fluid-This allows light scattering to occur. The intensity of the scattered light increases as the difference increases.

Optical Design Elements
The properties of suspended solids are not the only factors affecting the value of scattered light. Other important factors include:

- the angle of detection
- light beam aperture
- incident beam wavelength
- color sensitivity of the photocell

The influence of the angle of measurement can be seen in Figure 2. As the suspended particle size changes, the intensity of light which is scattered in all directions changes. An instrument which measures at 90° will be different than an instrument which measures in a forward direction. Additionally, as the wavelength of detection changes, the scatter pattern will again be altered. The type of light source, photocell, and filter that are used in a turbidimeter determine its color sensitivity spectrum. The closer to a single wavelength the color sensitivity spectrum, the more consistent the scatter pattern which will be produced.

figure 3Optical Design Limits
There is a relationship between the total suspended solids in the liquid and the light intensity due to particle scatter. This relationship can be determined through the development of a working curve for each specific sample (see Figure 3). This relationship exists up to a transition point where the rate of scatter intensity no longer increases with more undissolved particles. This point is the maximum limit of the optical design of a turbidimeter.

Light which is detected by the photocell that is not caused by the scattering of light by suspended particles is called "stray light". The lower limit of the optical design of a turbidimeter is dependent on the amount of stray light. Causes of stray light include reflections, scattering by dust, scratches or fingerprints on the sample cell, or imperfections of the glass.

figure 4Optical Measuring Configurations Single-Beam Method
This basic instrument design uses a single light source and a single photodetector located at 90 degrees to the transmitted light as shown in Figure 4. This design, which has been in existence for many years, has some inherent problems. Immediately after the light source is turned on, it begins to slowly burn out which reduces its intensity. Since instrument calibration is based upon the light intensity, this reduction necessitates frequent calibration to re-establish a new value of the scattered intensity.

Turbidity Measurement Modulated Four-Beam Method Optical DiagramCalibration Standards
Every turbidimeter, regardless of its optical configuration, must be calibrated with a known standard. This is the reference to which the measured values are compared. The most widely used measurement unit for turbidity is the FTU (Formazin Turbidity Unit). This unit can be used for all turbidimeters which employ formazin as the calibration standard. The USEPA uses formazin but states its measuring units as NTU (Nephelometric Turbidity Units). ISO refers to its units as FNU (Formazin Nephelometric Units).

Formazin suspension is the polymerization of hexamethylenetetramine and hydrazine sulfate under strictly controlled conditions. Although formazin offers advantages over sand or diatomaceous earth, it still lacks the long term stability and reproducibility necessary in a universally accepted primary standard.

Formazin is produced in a 4000 FTU (Formazin Turbidity Unit) suspension. Reproducibility of this suspension using the same hexamethylenetetramine and hydrazine sulfate is ± 1%. (Suspensions using different brands, lots, etc. of hexamethylenetetramine and hydrazine sulfate have a ± 15% reproducibility.) The subsequent dilutions of this suspension to lower turbidity values become more unstable with greater dilution. It is recommended that diluted calibration suspensions be used immediately and properly disposed.

Most modern day turbidimeters are equipped with a secondary calibration standard. These standards are used to standardize the instrument or check calibration to determine when calibration with formazin is necessary. Secondary standards made of liquids or gels are still unstable and must be replaced each year. The only absolute stable standard is turbid glass. A calibration glass standard consists of small particles uniformly suspended in a specially formulated glass cube. The turbidity value of this glass cube lasts indefinitely. It cannot be affected by time, temperature or, most importantly, by a change in personnel performing the calibration.

Standard Specifications
There are two standard specifications for turbidity measurement which are generally in use worldwide. These are the international standard ISO 7027 (1984 edition) and the USEPA 180.1. The specification for the ISO standard is more stringent and requires the use of a monochromatic light source. This specification allows for greater reproducibility of the measured values and greater agreement between other measuring instruments. These two standards are compared in Figure 8.

figure 8

Conclusion
Turbidity is a very complex analytical measurement which can be affected by many factors. To choose the best measuring system for a specific application, all factors must be carefully considered before selecting the appropriate turbidimeter. An important consideration is the comparison of readings between different instruments calibrated with the same standard. For reasons described in this bulletin, it is unreasonable to expect different instruments to indicate the same value for the same sample. On-line process instruments should be used for trending information rather than for absolute values.

Another problem is the measurement of colored liquids. The color will absorb some of the light, thereby reducing the light intensity. As a result, the detector senses less scattered light, providing a false, lower-than-actual turbidity reading. The single-beam method also lacks the ability to provide a stable measurement reading at higher levels of turbidity.

figure 5Ratio Method
The ratio method expands upon the single-beam concept. In addition to the one photodetector at 90° to the transmitted light, additional detectors can be added at other angles as shown in Figure 5. Using a ratio of the multiple detector system increases the stability of the measured turbidity values. This design also cancels the effects of light intensity reduction when measuring colored liquids, making the turbidimeter color-compensated. Although a multiple detector design using a ratio measurement method is an improvement to the single-beam detector design, the problem with light source decay and frequent calibration still exists.

Dual-Beam Method
To minimize the effect of light source decay, a dual-beam method was developed, as shown in Figure 6. This method uses a single light source which is split by an oscillating mirror into two beams: a measuring beam and a reference beam. The measurement is made differentially with a single photo-detector detecting the different light intensities of both beams. This method reduces the need for frequent calibration and, when used with a monochromatic light source, totally eliminates the need for calibration. Although the dual-beam method minimizes or eliminates the need for frequent calibration, it does not address the problem of unstable readings at higher turbidity values.Figure 5 - Ratio Method Turbidimeter.

figure 7Design Modulated Four-Beam Method
The modulated four-beam method uses two light sources and two photodetectors. As Figure 7 illustrates, these components are spaced at 90° intervals around a circular sample chamber. Every half second, the sensor accomplishes two measurement phases, and a microprocessor calculates a turbidity reading. In the first phase, Light Source 1 momentarily pulses a light beam directly into Photodetector 2. Simultaneously, Photodetector 1 measures the light scattered at 90°.

In the second phase, Light Source 2 momentarily pulses a light beam directly into Photodetector 1. Simultaneously, Photodetector 2 measures the scattered light at 90°. Every time a light source is illuminated, it provides both an active signal and a reference signal. The two light sources are alternately pulsed. Likewise, the two photodetectors alternate in reading either the active signal or reference signal. This two phase measurement provides four independent measurements from two light sources, using direct strength readings from two detectors and 90° scattered light readings from the same two detectors. The microprocessor uses a ratiometric algorithm to calculate the turbidity value from these four readings. Mathematically, this means that error effects appear both in the numerator and the denominator-and thus are cancelled.

figure 6The modulated four-beam method cancels all error terms derived from aging or fouling of components, and reduces errors due to color factors. This method also offers the practical advantage that light sources and detectors need not be matched to provide accurate measurements.Figure 6 - Dual-Beam Method Turbidimeter Design

Reproduced with the permission of Great Lakes Instruments, Inc.Figure 8 - Comparison Between USEPA and ISO Turbidity Standards

1 Standard Methods for the Examination of Water and Wastewater, APHA, AWWA and WPCF, 16th edition, 1985. Figure 2 - Angular Patterns of Scattered Intensity for Three Basic Particle Sizes 22Brumberger, et al, Light Scattering, Science and Technology, November, 1968, page 38. Document #1234 On-line Publishing ServiceU.S.A. and Canada For Technical Assistance: U.S.A. and Canada

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