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Turbidity measurement
ISA Transactions 32 (1993) 397-405 Elsevier
397
Turbidity measurement Ravi Jethra BMMI, Manama, Bahrain
Introduction
A universal indicator of water quality is the presence of dispersed or suspended solids, i.e. particles not in true solution, such as silt, clay, algae and other micro-organisms, organic matter, and other minute particles. The extent to which suspended solids can be tolerated varies, as do the levels at which they exist. Suspended solids obstruct the transmittance of light through water and impart a qualitative characteristic (turbidity) to water. Turbidity is not a direct measure of suspended solids in water but a determination of the scattering effect such solids have on light. Turbidity is a measure of the relative clarity of water. Though there are several methods to determine water contamination, turbidity measurement is important because it is a simple indicator of water quality. Turbidity measurements are crucial in a variety of applications. In the food, pharmaceutical and semiconductor industries, they ensure product quality and the ultra-purity of process water. In the beverages industry they enhance shelf appeal by increasing final product clarity. In the hydrocarbon industry, they provide a final check on the product. Fine droplets of water in a dissimilar liquid will also scatter light, which means that its presence in fluids such as jet fuel can be monitored during discharge and fill operations to prevent water contamination. Definition. Turbidity is an expression of the optical property that causes light to be scattered Correspondence to: Mr. Ravi Jethra, BMMI, P.O. Box 828, Manama, Bahrein.
and absorbed rather than be transmitted in straight lines through the sample. A turbidimeter is an instrument used to measure the "haziness" of a fluid caused by suspended particulate matter. A higher turbidity value indicates more haziness while a lower value indicates greater clarity. Low ranges of turbidity such as those contained in drinking water are measured by a nephelometric turbidimeter. These instruments direct a beam of light through the sample and detect light scattered by the suspended particles. The intensity of scattered light is compared to that of a standard formazin suspension and the result is expressed in nephelometric turbidity units (NTU).
The theory of light scattering
The optical property of turbidity is due to the interaction between light and suspended particles in water. A directed beam of light remains relatively undisturbed when transmitted through absolutely pure water; the molecules in a pure fluid will scatter light to a certain degree. In samples containing suspended solids, the manner in which water interferes with light transmittance depends on:
- the size of the particle; - the shape of the particle; - the composition of particles; and - the wavelength of incident light (Fig. 1). A minute particle interacts with incident light by absorbing the light energy and then radiating the light energy in all directions (point source). This omni-directional re-radiation constitutes the
0019-0578/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
398
R. Jethra
/ Turbidity m e a s u r e m e n t
Stray
~"~1~~ -
I II
I I ~ r
Transmitted Light
Lens
sSct~aer ~ ~
90D°e'tSotier ca
Fig. 1. Sources of stray light.
scattering of the incident light. The spatial distribution of scattered light depends on the ratio of particle size to wavelength of incident light. Particle shape and refractive index also affect the scatter distribution and intensity. The refractive index of a particle is a measure of how it redirects light passing through it from another medium such as the suspending fluid. The refractive index of the suspended particles must be different than that of the sample (suspending) fluid in order for scattering to occur. As this difference increases, scattering becomes more intense. It is inaccurate to relate the measurement of scattered light directly to the concentration of suspended solids, as will be clear from the following. A coloured substance absorbs light energy in certain bands of the visible spectrum, changing the character of both transmitted and scattered light and preventing a certain portion of the scattered light from reaching the detection system. Light scattering intensifies as particle concentration increases. But as scattered light strikes more and more particles, multiple scattering occurs and the absorption of light increases. When the particulate concentration exceeds a certain point, the detectable level of both scattered and transmitted light drops rapidly, marking the upper limit of measurable turbidity. Decreasing the path length of light through the sample reduces the number of particles between the light source and the detector and extends the upper limit of turbidity measurement. Hence, a direct correlation can only be made if suspended solids factors such as size distribution, shape, refractive indices and absorptivities re-
main constant and only the concentration changes. This is impractical in most cases. The greatest misunderstanding of the concepts of turbidity measurement is the tendency to equate the quantity ( m g / L or ppm) of a sample's suspended material with the sample's measured turbidity. Since turbidity is a measurement of the light-scattering properties of a sample's particulates, particulate make-up and instrument sensitivity directly affect light scattering and its detection. For direct measurement of suspended solids, turbidity readings must be supported by auxiliary measurements and a correlation (calibration) curve must be established. Calibrated slit turbidimeter The opeartion of a calibrated slit turbidimeter is based on a visual comparison of the intensity of light scattered by the contained water sample under defined conditions to the intensity of light scattered by a reference sample in the same container. Jackson candle turbidimeter The Jackson candle turbidimeter consists of a special candle and a flat-bottomed glass tube (Fig. 2). Measurements are made by slowly pouring a turbid sample into the tube until the visual image of the candle flame, viewed from the open top of the tube diffuses to a uniform glow. Visual image extinction occurs when the intensity of scattered light equals that of transmitted light. The Jackson candle turbidimeter has serious practical limitations: - it is cumbersome; - it is dependent on human judgement to determine the exact extinction point; - it is not sensitive to very fine particle suspensions; it is incapable of measuring turbidity due to black particles for e.g., charcoal. The higher the intensity of scattered light the higher the turbidity of sample. At high concentrations, multiple scattering interferes with direct scattering. The solution to this problem is to measure the light scattered at -
399
R. Jethra / Turbidity measurement Eye
E
These instruments measuring 9e~°-scatter called nephelometers or nephelometric bidimeters, to distinguish them from generic bidimeters, which measure transmitted or sorbed light.
are turturab-
Photoelectric nephelometer
Scattered Light Is As Intense As Transmitted Light - Image of Flame Disappears At This Depth
Scattered Light
Scattered Light Weak Transmitted Light Strong
tt
__
Length of Arrow Proportional to Intensity of Beam of Light
Flame
The operation of a photoelectric nephelometer is based on instrumental comparison of the intensity of light scattered by the contained static water sample under defined conditions to the intensity of light scattered by a reference standard in the sample container (Fig. 3). Turbidity instruments are required to measure both high and extremely low turbidity levels and an almost infinite range of sample particulate sizes and compositions. The three critical design components of a nephelometer are the light source, the scattered light detector, and the optical geometry. Light source
The most c o m m o n light source is the tungsten-filament lamp. Monochromatic or narrow-band sources (for e.g. LEDs) can be used for applications where specific particle types are present in the sample or a well-characterized light source is necessary. Some of the other light sources are lasers, mercury (discharge) lamps and various l a m p / f i l t e r combinations. Detectors
Candle
When the imposed light signal has interacted with the sample, its response must subsequently
Fig. 2. Jackson candle turbidimeter. Glass
Transm~ed
an angle to the incident light beam and then relate this angle-scattered light to the sample's actual turbidity. Detection at an angle of 90 ° is considered to be the least sensitive to variations in particle size.
n~ II \
~,\ Aperture
I~
/ "-90O-Sc,,tter,d "~"
,~
I'J~L]I
~
Fig. 3.90°-scattered fight nephelometer.
Deteclor
400
R. Jethra / Turbidity measurement
be detected by the instrument. Basically there are four different types of detectors presently used in nephelometers: - the photometer tube; - the vacuum photodiode; - the silicon photodiode; - the cadmium sulphide photoconductor. These detectors differ in their response to a particular wavelength distribution (spectral response characteristics). Both the spectral distribution of the source and the spectral response of the detector are key elements in the performance of a nephelometer. Generally, for a given detector, when the incident light source is shorter in wavelength, the instrument is more sensitive to smaller particles. An instrument's detector affects the response in a similar way. In an instrument, the s o u r c e / d e t e c t o r combination defines the effective spectral characteristics of the instrument and the manner in which it will respond to a sample. The maximum efficiency of the system is obtained when the source and detector are well-matched and their spectral curves have maximum overlap. Lamps and detectors are often the largest source of noise and drift in conventional nephelometers and other optical instruments.
adjustable path length. The use of a short path length can increase the impact of stray light.
Basics
of surface
scatter
measurement
The surface scatter design is based on the principle of the float glass process. In the float glass process, molten glass is poured on the surface of a molten metal. This results in an excellent finish and brilliance on both top and bottom surfaces of glass. The water enters an inclined tube (the turbidimeter body) about halfway between the surface and the bottom and flows upward overflowing a flat weir at the top of the body to form a nearly perfect liquid surface. Light from the instrument is aimed near the centre of this flat surface. A photodetector positioned above the surface at 90 ° to the incident path detects scattered light. The term surface scatter is derived from this positioning of the light source and detector (Fig. 4). A portion of the sample is illuminated. The maximum total light path (incident plus scattered light) in the view of the detector never exceeds a certain limit, generally a few centimeters. This maximum light path would exist only if measuring
Optical geometry Optical geometry incorporates instrument design parameters such as the angle of scattered light detection. Most of the nephelometers used in water and waste water analysis use a 90°-detec tion angle; in addition to being less sensitive to variations in particle size, a 90°-detection angle results in a simple optical system which is free from stray light. The path length traversed by scattered light is a design p a r a m e t e r affecting both instrument sensitivity and linearity. Sensitivity increases as path length increases, but linearity is sacrificed at high particle concentrations due to multiple scattering and absorbance. Conversely the range of linearity is increased by decreasing the path length, but sensitivity is lost at low concentrations. This trade-off can be eliminated with an
turbidity
PHOTOCELL - -
A
LIGHT
~
LENS
/ "",,LIGHT
~%~,' ~. LIGHT
/
INST.
DRAIN
OVERFLOWING ov~ .....
SAIMNPLE
Fig. 4. Surface s c a t t e r t u r b i d i m e t e r .
401
R. Jethra / Turbidity measurement
very clear solutions. As turbidity increases, the light path decreases until the total light path would become a centimeter or less in very turbid water. The surface scatter design has no upper range limit, negligible stray light interference and no loss of sensitivity due to dirty optical surfaces. The range is limited only by the amount of light that can be supplied and the sensitivity of the detector. The advantages of this design are: - the inclined body acts as a trap for heavy solids and also acts as a light trap to minimize stray light; - c o n v e n t i o n a l windows and sample cells are eliminated (and associated problems such as scratches, fogging, biological growth); - there is no need to clean optical components as the sample never comes in contact with optical components.
Basics
of the
ratio
method
of turbidity
measure-
ment
The conventional nephelometer cannot account for the effect of colour on turbidity measurement, hence there are limited applications. The application of turbidity measurement to the optimization of water treatment processes has resulted in improved water clarity. As a result, turbidimeters are commonly used at the lowest end of their ranges, making accuracy at very low turbidities essential. The largest source of error at low turbidities is stray light. Stray light introduces a positive error, i.e. the sample is read more turbid than it actually is. If the stray light could be measured, the electronics could be adjusted to compensate. Since experimental determination of stray light is very difficult, the solution lies in designing an optical system with negligible stray light. This forms the basis for the design of a ratio turbidimeter. Colour has a negative interference, attenuating both incident and scattered light, and the turbidity reads lower than it is. The effect is so great that even for moderately coloured samples, conventional nephelometers are useless. The high
Forward
~ mPle Tra nD~s mIFreCl~\ ~° / t , m p
c.. x
~
~Scatter \
Blorror ck Mi 9Oo Detector
Fig. 5. Optical design of ratio turbidimeter.
degree of colour rejection of a ratio turbidimeter opens up new applications, particularly in the food and beverage industry where products are coloured and aesthetic appearance is important. Other areas of application include: metal plating baths, lubricants, vegetable oils, etc. The optical configuration is the key to performance characteristics such as good stability, linearity, sensitivity, low stray light and colour rejection. T h e light beam formed by a lens and aperture, is projected through the cylindrical sample cell and is perpendicular to the cell's axis (Fig. 5). The geometry provides access to both scattered and transmitted light along nearly equivalent paths. As the collimated light beam passes through the sample, some of it will be scattered by turbidity. Before detection, the transmitted beam is reflected from a "black glass" mirror that reflects about 4% of the light at its front surface and absorbs the balance. The mirror serves to attenuate the intense transmitted beam which otherwise might overload the detector. It also eliminates back-scattering towards the sample cell, which could appear as stray light. The scattered light is detected by the "90 °" and "forward scatter" detectors. The remainder of the light passes through the sample as transmitted light. Thus, the instrument is equipped with three light detectors: - to detect light scattered at 90°; - to detect light transmitted through the sample; - to detect light scattered in the forward direction. The turbidity value is derived by determining the ratio of the 90°nephelometric signal against a
R. Jethra / Turbidity measurement
402
weighted sum of the transmitted and forward scatter signals (Fig. 6). This is a key feature in providing excellent long-term stability. In addition to lamp fluctuations, the ratio principle compensates for haze and dust on optics as well as temperature coefficients of detectors and amplifiers. These detectors, operated in a ratio configuration, give the instrument a degree of stability which makes continual calibration unnecessary. Also, the forward-scatter detector helps to provide a linear response over a wide range without sacrificing sensitivity in lower ranges. It is logical to expect that a simple ratio of scattered light to transmitted light would extend the range of linearity because the rays traverse more or less equal distances through the sample and should be affected equally by colour attenuation. However at high turbidity levels, light reach-
ing the detectors is likely to have been scattered more than once. This multiple scattering acts to reduce the distance traversed by the scattered rays while it can only increase the distance traversed by transmitted light. The result is that transmitted light is more attenuated than the scattered light at high turbidities, so that the instrument's response becomes non-linear, and eventually goes "blind" because the increase in light attenuation has a larger effect than the increase in scattering.
Turbidity measurement using fibre optics The sensor utilizes fibre optics to transmit high-intensity pulsed infrared light into the process. This light passes through the quartz window and is scattered by suspended solids in the pro-
RANGE
c
v~A/%
ofo o
~
I INPUT
90OScattered ~ Light ZERO LINEARITY
Forward Scattered Ught
ADC
SPAN
Transmltted~"'w~ Light
1
v
REF
Fig. 6. R a t i o t u r b i d i m e t e r .
TO DISPLAY
403
R. Jethra / Turbidity measurement
cess stream. The back-scattered light is received by two circular rings of fibres arranged in concentric circles on the sensor tip, and is transmitted back to two detectors. Since the m e a s u r e m e n t is based upon a light intensity controlled operation, light received by the inner and outer rings of the fibres is held in a constant ratio by varying the light source intensity according to changes in solids concentration. The light control operation and m e a s u r e m e n t calculations are performed separately. The m e a s u r e m e n t calculation provides a linearized output signal proportional to the concentration.
The four-beam (ratiometric) method The four-beam method uses two light sources and two photodetectors. The two light sources and the two photodetectors are spaced at 90 ° intervals around a circular sample chamber. After a prefixed short time interval, the microprocessor-based electronics accomplishes two measurement phases, and the reading is calculated. In the first phase, light source A momentarily pulses a light b e a m directly into photodetector B. Simultaneously photodetector A measures the light scattered at 90 °. In the second phase, light source B momentarily pulses a light b e a m directly onto photodetector A. Simultaneously, photodetector B measures the scattered light at 90 °. Every time a light source is illuminated it
provides both an active signal and a reference signal. Similarly, the two photodetectors alternate in reading either the active or the reference signal. The two-phase m e a s u r e m e n t provides four i n d e p e n d e n t m e a s u r e m e n t s from two light sources, using direct strength readings from two detectors and 90 ° scattered light readings from the same two detectors (Fig. 7). The software uses a ratiometric algorithm to calculate the turbidity value from these four readings. The error effects appear in both the numerator and the denominator, and hence are cancelled. The four-beam ratio method (by using two light sources and a four-beam system) cancels all error terms derived from aging or fouling of components, and reduces errors due to colour factors. This method also offers the practical advantage that light sources and detectors need not be matched to provide accurate measurements.
Problems in turbidity measurement In practice, significant problems can introduce interferences and errors that reduce the accuracy of any instrument. Stray light
Stray light is defined as extraneous light not due to sample scattering but still detected by the nephelometer. It reaches the photocell because
Light Source A
Light Source A
Photo Detector A
Source B
Light Source B
Photo Detector B
Photo Detector A
Photo Detector B
Fig. 7. Four-beam system:principle of operation.
404
R. Jethra / Turbidity measurement
of reflections from all kinds of causes. It results in difficulties in measuring low levels of turbidity. Stray light must be measured to be reduced. Stray scattered light may be produced at the entrance and exit window areas of a glass sample cell used to isolate the sample fluid from the light source and detector. Dust, scratches and smudges, condensation or a film on the sample cell all scatter light as do cell imperfections. This scattered light may strike the exit window and be transmitted light or may reflect off other areas of the cell. In both cases, this extraneous light reaches the detector as stray light. To keep stray light to a minimum, sample cells must be kept clean and free of scratches. The use of a sample cell in an instrument that monitors continuously can be particularly cumbersome. Solids in the sample fluid, either organic or inorganic, tend to grow or plate out on the sample cell surfaces, forming a film that blocks or absorbs light entering the cell. Circumventing this problem requires regular cleaning of optical surfaces in contact with the sample, electronic compensation for attenuated light or an instrument design that minimizes contact between optical system and sample or a combination of these alternatives. Air bubbles Air bubbles introduce measurement errors. Air bubbles in the sample reflect and scatter light to a higher degree than most solid particles. Larger bubbles are the most common source of erratic or noisy signals from a turbidimeter when they rise through or swirl in the light path. Extremely small bubbles tend to be suspended in a sample and rise very slowly, so the instrument detects them as turbidity. Most liquid samples contain various dissolved gases whose solubility depends on sample temperature and pressure. As the temperature of a solution increases, solubility decreases and excess gas comes o u t of the solution in the form of bubbles. These bubbles tend to grow on particles or irregularities at the sample cell surface until they are large enough to rise to the surface or are sheared away by fluid motions.
Bubble removal from a sample requires mechanical means. Bubble interference can also be controlled by the use of electronic circuitry designed to reject a momentary increase in the signal that might be caused by bubbles. Bubbleinduced error signals are prevented by a combination of an integral mechanical trap and bubble rejection software. The most important practical consideration in turbidity measurement is the difference in measured values among different instruments that have been calibrated with the same standard material. This is due to the difference in spectral characteristics of the light s o u r c e / d e t e c t o r combination giving different values for the same sample.
Applications
Water Water treatment regulations require potable water to have a turbidity of 0 - 1 NTU, as high turbidity promotes the growth of microorganisms, reducing the effectiveness of chlorination and increasing health hazards. Waste water Waste water turbidity must be kept to a minim u m (0-250 NTU). The organic and inorganic material in sewage and industrial wastes have a detrimental effect on the quality of water. Power utilities Modern boilers require the water to be virtually free of all suspended solids, usually 0 - 1 NTU. The quality of the steam is determined by the purity of water. Other applications Other areas of application are: - food p r o c e s s i n g / b e v e r a g e manufacturing (e.g. detecting trace contaminants in fructose syrup); textile industry (e.g. sour waste); - pulp and paper (e.g. monitoring the clarity of mild acids); chemical and pharmaceutical industries (e.g.
R. Jethra / Turbidity measurement
monitoring microbiological cultures in antibiotics production); mineral and metal refining and processing; - computer and electronic component manufacturing. -
In order to design a suitable turbidity analysis system, the following points need to be considered: - type of measurement required; - composition of the solution whose turbidity is to be measured; - tendency of the solids to deposit; - normal and maximum temperature and pressure at the point of measurement; material compatibility of the fluid; - distance between sensor and electronics unit; -
405
- accuracy required (measurement and control); alarm contacts (number and rating); - power requirements; instrument mounting configuration. -
-
R e f e r e n c e s :
[1] Richard D. Vanous, Introduction to the Ratio Turbidimeter, Technical Information Series, Hach Co. [2] Clifford C. Hach, Principles of Surface Scatter TurbMity Measurement, Technical Information Series, Hach Co. [3] Clifford C. Hach, Richard D. Vanous and John M. Heer, Understanding Turbidity Measurement, Technical Information Series, Hach Co. [4] Terry L. Englehardt, "Measuring turbidity--Translating theory to practical application", Fluid~Part. Sep. J. 5 (1992).
397
Turbidity measurement Ravi Jethra BMMI, Manama, Bahrain
Introduction
A universal indicator of water quality is the presence of dispersed or suspended solids, i.e. particles not in true solution, such as silt, clay, algae and other micro-organisms, organic matter, and other minute particles. The extent to which suspended solids can be tolerated varies, as do the levels at which they exist. Suspended solids obstruct the transmittance of light through water and impart a qualitative characteristic (turbidity) to water. Turbidity is not a direct measure of suspended solids in water but a determination of the scattering effect such solids have on light. Turbidity is a measure of the relative clarity of water. Though there are several methods to determine water contamination, turbidity measurement is important because it is a simple indicator of water quality. Turbidity measurements are crucial in a variety of applications. In the food, pharmaceutical and semiconductor industries, they ensure product quality and the ultra-purity of process water. In the beverages industry they enhance shelf appeal by increasing final product clarity. In the hydrocarbon industry, they provide a final check on the product. Fine droplets of water in a dissimilar liquid will also scatter light, which means that its presence in fluids such as jet fuel can be monitored during discharge and fill operations to prevent water contamination. Definition. Turbidity is an expression of the optical property that causes light to be scattered Correspondence to: Mr. Ravi Jethra, BMMI, P.O. Box 828, Manama, Bahrein.
and absorbed rather than be transmitted in straight lines through the sample. A turbidimeter is an instrument used to measure the "haziness" of a fluid caused by suspended particulate matter. A higher turbidity value indicates more haziness while a lower value indicates greater clarity. Low ranges of turbidity such as those contained in drinking water are measured by a nephelometric turbidimeter. These instruments direct a beam of light through the sample and detect light scattered by the suspended particles. The intensity of scattered light is compared to that of a standard formazin suspension and the result is expressed in nephelometric turbidity units (NTU).
The theory of light scattering
The optical property of turbidity is due to the interaction between light and suspended particles in water. A directed beam of light remains relatively undisturbed when transmitted through absolutely pure water; the molecules in a pure fluid will scatter light to a certain degree. In samples containing suspended solids, the manner in which water interferes with light transmittance depends on:
- the size of the particle; - the shape of the particle; - the composition of particles; and - the wavelength of incident light (Fig. 1). A minute particle interacts with incident light by absorbing the light energy and then radiating the light energy in all directions (point source). This omni-directional re-radiation constitutes the
0019-0578/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
398
R. Jethra
/ Turbidity m e a s u r e m e n t
Stray
~"~1~~ -
I II
I I ~ r
Transmitted Light
Lens
sSct~aer ~ ~
90D°e'tSotier ca
Fig. 1. Sources of stray light.
scattering of the incident light. The spatial distribution of scattered light depends on the ratio of particle size to wavelength of incident light. Particle shape and refractive index also affect the scatter distribution and intensity. The refractive index of a particle is a measure of how it redirects light passing through it from another medium such as the suspending fluid. The refractive index of the suspended particles must be different than that of the sample (suspending) fluid in order for scattering to occur. As this difference increases, scattering becomes more intense. It is inaccurate to relate the measurement of scattered light directly to the concentration of suspended solids, as will be clear from the following. A coloured substance absorbs light energy in certain bands of the visible spectrum, changing the character of both transmitted and scattered light and preventing a certain portion of the scattered light from reaching the detection system. Light scattering intensifies as particle concentration increases. But as scattered light strikes more and more particles, multiple scattering occurs and the absorption of light increases. When the particulate concentration exceeds a certain point, the detectable level of both scattered and transmitted light drops rapidly, marking the upper limit of measurable turbidity. Decreasing the path length of light through the sample reduces the number of particles between the light source and the detector and extends the upper limit of turbidity measurement. Hence, a direct correlation can only be made if suspended solids factors such as size distribution, shape, refractive indices and absorptivities re-
main constant and only the concentration changes. This is impractical in most cases. The greatest misunderstanding of the concepts of turbidity measurement is the tendency to equate the quantity ( m g / L or ppm) of a sample's suspended material with the sample's measured turbidity. Since turbidity is a measurement of the light-scattering properties of a sample's particulates, particulate make-up and instrument sensitivity directly affect light scattering and its detection. For direct measurement of suspended solids, turbidity readings must be supported by auxiliary measurements and a correlation (calibration) curve must be established. Calibrated slit turbidimeter The opeartion of a calibrated slit turbidimeter is based on a visual comparison of the intensity of light scattered by the contained water sample under defined conditions to the intensity of light scattered by a reference sample in the same container. Jackson candle turbidimeter The Jackson candle turbidimeter consists of a special candle and a flat-bottomed glass tube (Fig. 2). Measurements are made by slowly pouring a turbid sample into the tube until the visual image of the candle flame, viewed from the open top of the tube diffuses to a uniform glow. Visual image extinction occurs when the intensity of scattered light equals that of transmitted light. The Jackson candle turbidimeter has serious practical limitations: - it is cumbersome; - it is dependent on human judgement to determine the exact extinction point; - it is not sensitive to very fine particle suspensions; it is incapable of measuring turbidity due to black particles for e.g., charcoal. The higher the intensity of scattered light the higher the turbidity of sample. At high concentrations, multiple scattering interferes with direct scattering. The solution to this problem is to measure the light scattered at -
399
R. Jethra / Turbidity measurement Eye
E
These instruments measuring 9e~°-scatter called nephelometers or nephelometric bidimeters, to distinguish them from generic bidimeters, which measure transmitted or sorbed light.
are turturab-
Photoelectric nephelometer
Scattered Light Is As Intense As Transmitted Light - Image of Flame Disappears At This Depth
Scattered Light
Scattered Light Weak Transmitted Light Strong
tt
__
Length of Arrow Proportional to Intensity of Beam of Light
Flame
The operation of a photoelectric nephelometer is based on instrumental comparison of the intensity of light scattered by the contained static water sample under defined conditions to the intensity of light scattered by a reference standard in the sample container (Fig. 3). Turbidity instruments are required to measure both high and extremely low turbidity levels and an almost infinite range of sample particulate sizes and compositions. The three critical design components of a nephelometer are the light source, the scattered light detector, and the optical geometry. Light source
The most c o m m o n light source is the tungsten-filament lamp. Monochromatic or narrow-band sources (for e.g. LEDs) can be used for applications where specific particle types are present in the sample or a well-characterized light source is necessary. Some of the other light sources are lasers, mercury (discharge) lamps and various l a m p / f i l t e r combinations. Detectors
Candle
When the imposed light signal has interacted with the sample, its response must subsequently
Fig. 2. Jackson candle turbidimeter. Glass
Transm~ed
an angle to the incident light beam and then relate this angle-scattered light to the sample's actual turbidity. Detection at an angle of 90 ° is considered to be the least sensitive to variations in particle size.
n~ II \
~,\ Aperture
I~
/ "-90O-Sc,,tter,d "~"
,~
I'J~L]I
~
Fig. 3.90°-scattered fight nephelometer.
Deteclor
400
R. Jethra / Turbidity measurement
be detected by the instrument. Basically there are four different types of detectors presently used in nephelometers: - the photometer tube; - the vacuum photodiode; - the silicon photodiode; - the cadmium sulphide photoconductor. These detectors differ in their response to a particular wavelength distribution (spectral response characteristics). Both the spectral distribution of the source and the spectral response of the detector are key elements in the performance of a nephelometer. Generally, for a given detector, when the incident light source is shorter in wavelength, the instrument is more sensitive to smaller particles. An instrument's detector affects the response in a similar way. In an instrument, the s o u r c e / d e t e c t o r combination defines the effective spectral characteristics of the instrument and the manner in which it will respond to a sample. The maximum efficiency of the system is obtained when the source and detector are well-matched and their spectral curves have maximum overlap. Lamps and detectors are often the largest source of noise and drift in conventional nephelometers and other optical instruments.
adjustable path length. The use of a short path length can increase the impact of stray light.
Basics
of surface
scatter
measurement
The surface scatter design is based on the principle of the float glass process. In the float glass process, molten glass is poured on the surface of a molten metal. This results in an excellent finish and brilliance on both top and bottom surfaces of glass. The water enters an inclined tube (the turbidimeter body) about halfway between the surface and the bottom and flows upward overflowing a flat weir at the top of the body to form a nearly perfect liquid surface. Light from the instrument is aimed near the centre of this flat surface. A photodetector positioned above the surface at 90 ° to the incident path detects scattered light. The term surface scatter is derived from this positioning of the light source and detector (Fig. 4). A portion of the sample is illuminated. The maximum total light path (incident plus scattered light) in the view of the detector never exceeds a certain limit, generally a few centimeters. This maximum light path would exist only if measuring
Optical geometry Optical geometry incorporates instrument design parameters such as the angle of scattered light detection. Most of the nephelometers used in water and waste water analysis use a 90°-detec tion angle; in addition to being less sensitive to variations in particle size, a 90°-detection angle results in a simple optical system which is free from stray light. The path length traversed by scattered light is a design p a r a m e t e r affecting both instrument sensitivity and linearity. Sensitivity increases as path length increases, but linearity is sacrificed at high particle concentrations due to multiple scattering and absorbance. Conversely the range of linearity is increased by decreasing the path length, but sensitivity is lost at low concentrations. This trade-off can be eliminated with an
turbidity
PHOTOCELL - -
A
LIGHT
~
LENS
/ "",,LIGHT
~%~,' ~. LIGHT
/
INST.
DRAIN
OVERFLOWING ov~ .....
SAIMNPLE
Fig. 4. Surface s c a t t e r t u r b i d i m e t e r .
401
R. Jethra / Turbidity measurement
very clear solutions. As turbidity increases, the light path decreases until the total light path would become a centimeter or less in very turbid water. The surface scatter design has no upper range limit, negligible stray light interference and no loss of sensitivity due to dirty optical surfaces. The range is limited only by the amount of light that can be supplied and the sensitivity of the detector. The advantages of this design are: - the inclined body acts as a trap for heavy solids and also acts as a light trap to minimize stray light; - c o n v e n t i o n a l windows and sample cells are eliminated (and associated problems such as scratches, fogging, biological growth); - there is no need to clean optical components as the sample never comes in contact with optical components.
Basics
of the
ratio
method
of turbidity
measure-
ment
The conventional nephelometer cannot account for the effect of colour on turbidity measurement, hence there are limited applications. The application of turbidity measurement to the optimization of water treatment processes has resulted in improved water clarity. As a result, turbidimeters are commonly used at the lowest end of their ranges, making accuracy at very low turbidities essential. The largest source of error at low turbidities is stray light. Stray light introduces a positive error, i.e. the sample is read more turbid than it actually is. If the stray light could be measured, the electronics could be adjusted to compensate. Since experimental determination of stray light is very difficult, the solution lies in designing an optical system with negligible stray light. This forms the basis for the design of a ratio turbidimeter. Colour has a negative interference, attenuating both incident and scattered light, and the turbidity reads lower than it is. The effect is so great that even for moderately coloured samples, conventional nephelometers are useless. The high
Forward
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Fig. 5. Optical design of ratio turbidimeter.
degree of colour rejection of a ratio turbidimeter opens up new applications, particularly in the food and beverage industry where products are coloured and aesthetic appearance is important. Other areas of application include: metal plating baths, lubricants, vegetable oils, etc. The optical configuration is the key to performance characteristics such as good stability, linearity, sensitivity, low stray light and colour rejection. T h e light beam formed by a lens and aperture, is projected through the cylindrical sample cell and is perpendicular to the cell's axis (Fig. 5). The geometry provides access to both scattered and transmitted light along nearly equivalent paths. As the collimated light beam passes through the sample, some of it will be scattered by turbidity. Before detection, the transmitted beam is reflected from a "black glass" mirror that reflects about 4% of the light at its front surface and absorbs the balance. The mirror serves to attenuate the intense transmitted beam which otherwise might overload the detector. It also eliminates back-scattering towards the sample cell, which could appear as stray light. The scattered light is detected by the "90 °" and "forward scatter" detectors. The remainder of the light passes through the sample as transmitted light. Thus, the instrument is equipped with three light detectors: - to detect light scattered at 90°; - to detect light transmitted through the sample; - to detect light scattered in the forward direction. The turbidity value is derived by determining the ratio of the 90°nephelometric signal against a
R. Jethra / Turbidity measurement
402
weighted sum of the transmitted and forward scatter signals (Fig. 6). This is a key feature in providing excellent long-term stability. In addition to lamp fluctuations, the ratio principle compensates for haze and dust on optics as well as temperature coefficients of detectors and amplifiers. These detectors, operated in a ratio configuration, give the instrument a degree of stability which makes continual calibration unnecessary. Also, the forward-scatter detector helps to provide a linear response over a wide range without sacrificing sensitivity in lower ranges. It is logical to expect that a simple ratio of scattered light to transmitted light would extend the range of linearity because the rays traverse more or less equal distances through the sample and should be affected equally by colour attenuation. However at high turbidity levels, light reach-
ing the detectors is likely to have been scattered more than once. This multiple scattering acts to reduce the distance traversed by the scattered rays while it can only increase the distance traversed by transmitted light. The result is that transmitted light is more attenuated than the scattered light at high turbidities, so that the instrument's response becomes non-linear, and eventually goes "blind" because the increase in light attenuation has a larger effect than the increase in scattering.
Turbidity measurement using fibre optics The sensor utilizes fibre optics to transmit high-intensity pulsed infrared light into the process. This light passes through the quartz window and is scattered by suspended solids in the pro-
RANGE
c
v~A/%
ofo o
~
I INPUT
90OScattered ~ Light ZERO LINEARITY
Forward Scattered Ught
ADC
SPAN
Transmltted~"'w~ Light
1
v
REF
Fig. 6. R a t i o t u r b i d i m e t e r .
TO DISPLAY
403
R. Jethra / Turbidity measurement
cess stream. The back-scattered light is received by two circular rings of fibres arranged in concentric circles on the sensor tip, and is transmitted back to two detectors. Since the m e a s u r e m e n t is based upon a light intensity controlled operation, light received by the inner and outer rings of the fibres is held in a constant ratio by varying the light source intensity according to changes in solids concentration. The light control operation and m e a s u r e m e n t calculations are performed separately. The m e a s u r e m e n t calculation provides a linearized output signal proportional to the concentration.
The four-beam (ratiometric) method The four-beam method uses two light sources and two photodetectors. The two light sources and the two photodetectors are spaced at 90 ° intervals around a circular sample chamber. After a prefixed short time interval, the microprocessor-based electronics accomplishes two measurement phases, and the reading is calculated. In the first phase, light source A momentarily pulses a light b e a m directly into photodetector B. Simultaneously photodetector A measures the light scattered at 90 °. In the second phase, light source B momentarily pulses a light b e a m directly onto photodetector A. Simultaneously, photodetector B measures the scattered light at 90 °. Every time a light source is illuminated it
provides both an active signal and a reference signal. Similarly, the two photodetectors alternate in reading either the active or the reference signal. The two-phase m e a s u r e m e n t provides four i n d e p e n d e n t m e a s u r e m e n t s from two light sources, using direct strength readings from two detectors and 90 ° scattered light readings from the same two detectors (Fig. 7). The software uses a ratiometric algorithm to calculate the turbidity value from these four readings. The error effects appear in both the numerator and the denominator, and hence are cancelled. The four-beam ratio method (by using two light sources and a four-beam system) cancels all error terms derived from aging or fouling of components, and reduces errors due to colour factors. This method also offers the practical advantage that light sources and detectors need not be matched to provide accurate measurements.
Problems in turbidity measurement In practice, significant problems can introduce interferences and errors that reduce the accuracy of any instrument. Stray light
Stray light is defined as extraneous light not due to sample scattering but still detected by the nephelometer. It reaches the photocell because
Light Source A
Light Source A
Photo Detector A
Source B
Light Source B
Photo Detector B
Photo Detector A
Photo Detector B
Fig. 7. Four-beam system:principle of operation.
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R. Jethra / Turbidity measurement
of reflections from all kinds of causes. It results in difficulties in measuring low levels of turbidity. Stray light must be measured to be reduced. Stray scattered light may be produced at the entrance and exit window areas of a glass sample cell used to isolate the sample fluid from the light source and detector. Dust, scratches and smudges, condensation or a film on the sample cell all scatter light as do cell imperfections. This scattered light may strike the exit window and be transmitted light or may reflect off other areas of the cell. In both cases, this extraneous light reaches the detector as stray light. To keep stray light to a minimum, sample cells must be kept clean and free of scratches. The use of a sample cell in an instrument that monitors continuously can be particularly cumbersome. Solids in the sample fluid, either organic or inorganic, tend to grow or plate out on the sample cell surfaces, forming a film that blocks or absorbs light entering the cell. Circumventing this problem requires regular cleaning of optical surfaces in contact with the sample, electronic compensation for attenuated light or an instrument design that minimizes contact between optical system and sample or a combination of these alternatives. Air bubbles Air bubbles introduce measurement errors. Air bubbles in the sample reflect and scatter light to a higher degree than most solid particles. Larger bubbles are the most common source of erratic or noisy signals from a turbidimeter when they rise through or swirl in the light path. Extremely small bubbles tend to be suspended in a sample and rise very slowly, so the instrument detects them as turbidity. Most liquid samples contain various dissolved gases whose solubility depends on sample temperature and pressure. As the temperature of a solution increases, solubility decreases and excess gas comes o u t of the solution in the form of bubbles. These bubbles tend to grow on particles or irregularities at the sample cell surface until they are large enough to rise to the surface or are sheared away by fluid motions.
Bubble removal from a sample requires mechanical means. Bubble interference can also be controlled by the use of electronic circuitry designed to reject a momentary increase in the signal that might be caused by bubbles. Bubbleinduced error signals are prevented by a combination of an integral mechanical trap and bubble rejection software. The most important practical consideration in turbidity measurement is the difference in measured values among different instruments that have been calibrated with the same standard material. This is due to the difference in spectral characteristics of the light s o u r c e / d e t e c t o r combination giving different values for the same sample.
Applications
Water Water treatment regulations require potable water to have a turbidity of 0 - 1 NTU, as high turbidity promotes the growth of microorganisms, reducing the effectiveness of chlorination and increasing health hazards. Waste water Waste water turbidity must be kept to a minim u m (0-250 NTU). The organic and inorganic material in sewage and industrial wastes have a detrimental effect on the quality of water. Power utilities Modern boilers require the water to be virtually free of all suspended solids, usually 0 - 1 NTU. The quality of the steam is determined by the purity of water. Other applications Other areas of application are: - food p r o c e s s i n g / b e v e r a g e manufacturing (e.g. detecting trace contaminants in fructose syrup); textile industry (e.g. sour waste); - pulp and paper (e.g. monitoring the clarity of mild acids); chemical and pharmaceutical industries (e.g.
R. Jethra / Turbidity measurement
monitoring microbiological cultures in antibiotics production); mineral and metal refining and processing; - computer and electronic component manufacturing. -
In order to design a suitable turbidity analysis system, the following points need to be considered: - type of measurement required; - composition of the solution whose turbidity is to be measured; - tendency of the solids to deposit; - normal and maximum temperature and pressure at the point of measurement; material compatibility of the fluid; - distance between sensor and electronics unit; -
405
- accuracy required (measurement and control); alarm contacts (number and rating); - power requirements; instrument mounting configuration. -
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R e f e r e n c e s :
[1] Richard D. Vanous, Introduction to the Ratio Turbidimeter, Technical Information Series, Hach Co. [2] Clifford C. Hach, Principles of Surface Scatter TurbMity Measurement, Technical Information Series, Hach Co. [3] Clifford C. Hach, Richard D. Vanous and John M. Heer, Understanding Turbidity Measurement, Technical Information Series, Hach Co. [4] Terry L. Englehardt, "Measuring turbidity--Translating theory to practical application", Fluid~Part. Sep. J. 5 (1992).