# Basic Turbidimeter Design and Concepts

## Introduction

Turbidity is described in the *Standard Methods for the Examination of
Water and Wastewater* Method 2130B (EPA Method 180.1) for turbidity
measurement as, "an expression of the optical property that causes light
to be scattered and absorbed rather than transmitted in straight lines
through the sample" (Standard Methods, 1995). This chapter includes a
detailed summary of the various types of instruments used to measure
turbidity and includes descriptions of the physical properties
associated with the measurements of turbidity and design configurations.

As shown in Figure 11-1, modern turbidimeters use the technique of
nephelometry, which measures the amount of light scattered at right
angels to an incident light beam by particles present in a fluid sample.
In general, all modern turbidimeters utilize the nephelometric
measurement principals, but instrument manufacturers have developed
several different meter designs and measurement configurations.

## Turbidimeter Measuring Principles

As light passes through 'absolutely pure' water, the light beams travel
along relatively undisturbed paths. However, some distortion occurs as
light is scattered by molecules present in the pure fluid. As shown in
Figure 11-1, when light passes through a fluid containing suspended
solids, the light beam interacts with the particles, and the particles
absorb the light energy and re-radiate light in all directions.

Particle size, configuration, color, and refractive index determine the
spatial distribution of the scattered light intensity around the
particle. As shown in Figure 11-2, particles much smaller than the
wavelength of the incident light, which is typically expressed in
nanometers (nm), scatter light of approximately equal intensity in all
directions. However, particles larger than the wavelength of the
incident light, form a spectral pattern that results in greater light
scattering in the forward direction (away from the incident light) than
in the other directions. This scattering pattern and intensity of the
light beam transmitted through the sample can also be affected by the
particles absorbing certain wavelengths of the transmitted light (Sadar,
1996).

Since the light scattered in the forward direction is variable depending
on particle size, the measurement of the light transmitted through the
sample yields variable results. In addition, the change in transmitted
light is very slight and difficult to distinguish from electronic noise
when measuring low turbidities. High turbidity samples are also
difficult to measure using transmitted light due to multiple scatter of
the light by many particles in the fluid. To solve these problems,
turbidimeters primarily measure the scatter of light at a 90 degree
angle to the incident beam and relate this reading to turbidity. This
angle is considered very sensitive to light scatter by particles in the
sample. As described later in this chapter, additional light sensors are
also sometimes added to detect light scattered at other angles in order
to improve the instrument range and remove errors introduced by natural
colors and lamp
variability.![](media/image2.png){width="3.2888888888888888in"
height="2.6430555555555557in"}

### Light Source

The basic turbidimeter instrument contains a light source, sample
container or cell, and photodetectors to sense the scattered light. The
most common light source used is the tungsten filament lamp. The
spectral output (band of wavelength light produced) of these lamps is
generally characterized by "color temperature," which is the temperature
that a black body radiator must be operated to produce a certain color.
The tungsten filament lamps are incandescent lamps and are termed
"polychromatic," since they have a fairly wide spectral band that
includes many different wavelengths of light, or colors. The presence of
the various wavelengths can cause interference in the turbidity
measurements as natural color and natural organic matter in the sample
can absorb some specific wavelengths of light and reduce the intensity
of the scattered light (King, 1991).

The tungsten filament lamp is also highly dependent on the voltage of
the lamp power supply. The voltage applied to the lamp determines the
spectral output characteristics produced, making a stable power supplies
a necessity. In addition, as with any incandescent lamp, the output from
the lamp decays with time as the lamp slowly "burns out," making
recalibration of the instrument a frequent and necessary requirement.

To overcome some of the incandescent lamp limitations, some turbidimeter
designs utilize monochromatic light sources, such as light emitting
diodes (LEDs), lasers, mercury lamps, and various lamp filter
combinations. Monochromatic light has a very narrow band of light
wavelengths (only a few colors). By selecting light wavelengths that are
not normally absorbed by organic matter, the monochromatic light source
can be less susceptible to interference by sample color. However, some
of these alternate light sources respond differently to particle size,
and are not as sensitive to small sized particles as the tungsten
filament lamp.

### Sample Volume

Grab samples are typically introduced into bench top turbidimeter
instruments through a transparent sample cell made of glass. These
samples cells, or cuvettes, are usually about 30 milliliters in
capacity. Some on-line turbidimeters utilize the glass sample cell, but
most designs use a flow-through chamber with the light source located
outside the sample. Sample chambers in on-line instruments range from 30
milliliters to over two liters.

### Photodetector

In turbidimeters, photodetectors detect the light produced from the
interaction of the incident light and the sample volume and produce an
electronic signal that is then converted to a turbidity value. These
detectors can be located in a variety of configurations depending on the
design of the instrument. The four types of detectors commonly used
include photomultiplier tubes, vacuum photodiodes, silicon photodiodes,
and cadmium sulfide photoconductors (Sadar, 1992).

Each of the four types of detectors vary in their response to certain
wavelengths of light. Therefore, if a polychromatic light source is
used, the spectral output of the light source has a direct bearing on
the type and design of photodetector selected for an instrument. The
specification of the photodector is not nearly as critical when a
monochromatic light source is used. In general, with the polychromatic
tungsten filament lamp as a light source, the photomultiplier tube and
the vacuum photodiode are more sensitive to the shorter wavelength light
in the source, making them more sensitive to the detection of smaller
particles. Conversely, the silicon photodiode is more sensitive to
longer wavelengths in the light source, making it more suited for
sensing larger particles. The sensitivity of the cadmium sulfide
photoconductor is between the sensitivity of the photomultiplier tube
and the silicon photodiode.

## Turbidimeter Design Configurations

Several instrument design standards have been developed by various
organizations to attempt to standardize instrument designs and achieve
test results that are accurate and repeatable. These standards govern
the design of the various turbidimeter configurations available today,
which include the single beam design, modulated four beam design,
surface scatter design, and transmittance design. Only the single beam
design, ratio design, and modulated four beam design are approved by
EPA.

### Design Standards

The requirements stated in *Standard Methods* 2130B (see Appendix D) are
similar to the requirements of EPA Method 180.1 (see Appendix C) for
turbidity measurement. The EPA Method 180.1 lists the following design
requirements for turbidimeters:

-   "Light Source: Tungsten-filament lamp operated at a color
    temperature between 2200 and 3000 degree K.

-   Distance traversed by incident light and scattered light within the
    sample tube not to exceed 10 cm.

-   Angle of light acceptance by detector: Centered at 90 degrees to the
    incident light path and not to exceed +/- 30 degrees from 90
    degrees. The detector, and filter system if used, shall have a
    spectral property between 400 and 600 nm (Standard Methods, 1995)."

EPA has recognized one additional standard for turbidimeter design
called GLI Method 2. Like EPA Method 180.1, this standard is applicable
for turbidities in the 0 to 40 NTU range, but may be used for higher
turbidities by diluting the sample. The GLI Method 2 standard requires
that instruments utilize basic nephelometric concepts, but requires the
use of two light sources with a photodetector located at 90-degrees from
each source. This concept, which is often called a modulated four beam
design, pulses the two light sources on and off and utilizes a portion
of the scattered light as a reference signal to arithmetically cancel
errors. A full description of the modulated four beam design is included
later in this Chapter.

The specific apparatus requirements listed in the GLI Method 2 standard
are as follows:

-   The wavelength of the incident radiation shall be 860 nanometers.

-   The spectral bandwidth of the incident radiation shall be less than
    or equal to 60 nanometers.

-   There shall be no divergence from parallelism of the incident
    radiation and any convergence shall not exceed 1.5 degrees.

-   There shall be two light sources and two detectors.

-   The measuring angle between the optical axis of the incident
    radiation and that of the diffused radiation for light pulsed
    through the sample by either light source shall be 90 +\\- 2.5
    degrees.

An additional turbidimeter design standard was developed by the
International Organization for Standardization. ISO 7027 defines the
requirements for a turbidimeter light source with stricter requirements
attempting to produce instruments that have good repeatability and
compare well with other instruments. The specification reads:

"Any apparatus may be used provided that it complies with the following
requirements:

-   The wavelength , , of the incident radiation shall be 860 nm;

-   The spectral bandwidth , , of the incident radiation shall be less
    than or equal to 60 nm;

-   There shall be no divergence from parallelism of the incident
    radiation and any convergence shall not exceed 2.5 degrees;

-   The measuring angle (tolerance on deviation of the optical axis)
    shall be o +/- 2.5 degrees

-   The aperture angle, , should be between 10 and 20 degrees in the
    water sample (ISO, 1990)."

ISO 7027 requires the use of monochromatic light sources such as
tungsten lamps fitted with monochromators and filters, diodes, or
lasers. However, the standard recognizes that many older instruments
have polychromatic light sources, and allows their use for water
treatment monitoring and control, but not for comparison to readings
from other turbidimeters.

The tighter definition of the light source in ISO 7027 eliminates many
of the variables inherent to the polychromatic sources used in the other
standards. However, ISO 7027 does not eliminate the effects of light
source decay or electronic gains and drifts inherent in monochromatic
sources such as LEDs (Lex, 1994). **ISO 7027 is not accepted by EPA for
turbidity analysis for compliance with the IESWTR.**

### Single Beam Design

The single beam design configuration, shown in Figure 11-3, is the most
basic turbidimeter design using only one light source and one
photodetector located at 90 degrees from the incident light. The single
beam design is the oldest of the modern nephelometers and typically is
used with a polychromatic tungsten filament lamp. The design is still in
wide used today and yields accurate results for turbidity under 40 NTU,
provided that samples have little natural color. In fact, many on-line
instruments in use today still utilize the single beam design.

The single beam design does, however, have limited accuracy at higher
turbidities. As turbidity increases and the amount of scattered light
increases, multiple scattering can occur when light strikes more than
one particle as it reacts with the sample fluid. The resulting scattered
light intensity reaching the 90 degree detector can diminish as the
instrument effectively "goes blind." For this reason, a single beam
design conforming strictly to EPA 180.1 does not typically demonstrate
stable measurement capability at high turbidities and is generally only
applicable for turbidity readings from 0 to 40 NTU.
![](media/image3.png){width="3.4854166666666666in"
height="1.9368055555555554in"}

The design of the single beam instrument is also limited by the need for
frequent recalibration of the instrument due to the decay of the
incandescent light source. Because of the polychromatic nature of the
light source, these instruments also can demonstrate poor performance
with samples containing natural color. Since most treated water samples
have low or no color, use of the single beam design is appropriate.

### Ratio Design 

The ratio turbidimeter design expands upon the single beam concept, but
includes additional photodetectors located at other angles than 90
degrees from the incident light. As shown in Figure 11-4, the ratio
design utilizes a forward scatter detector, a transmitted light
detector, and for very high turbidity applications, a back scatter
detector. The signals

Source: Sadar, 1996.![](media/image4.png){width="4.498611111111111in"
height="1.65625in"}

Figure 11-4. Ratio Turbidimeter

from each of these detectors are mathematically combined to calculate
the turbidity of the sample. A typical ratio mathematical algorithm is
as follows (Standard Methods, 1995):

> T= I~90~ / (d~0~ \* I~t~ + d~1~ \* I~fs~ + d~2~ \* I~bs~ + d~3~ \*
> I~90~)

Where:

T = Turbidity in NTU

d~0~, d~1~, d~2~, d~3~ = Calibration Coefficients

I~90~ = 90 Degree Detector Current

I~t~ = Transmitted Detector Current

I~fs~ = Forward Scatter Detector Current

I~bs~ = Back Scatter Detector Current

The use of multiple photodetectors and the ratio algorithm gives the
instrument much better performance with colored samples. The transmitted
light and the 90-degree scattered light are affected almost equally by
the color of the sample because they travel nearly the same distance
through the sample volume. When the ratio of the two readings is taken,
the effects of color absorption on the two readings tend to cancel
mathematically.

### Modulated Four Beam Design

Unlike the single beam and ratio turbidimeters, the modulated four-beam
instrument design utilizes two light sources and two photo detectors.
The two sources and the two detectors are used to the implement theory
of ratio measurements to cancel errors. As shown in Figure 11-5, the
light sources and detectors are located at 90 degrees around the sample
volume (Great Lakes Instruments, undated).
![](media/image5.wmf){width="3.0902777777777777in"
height="1.9166666666666667in"}

This design takes two measurements every 0.5 seconds. In the first
phase, light from source #1 is pulsed directly into photodetector #2.
Simultaneously, photodetector #1 measures the light scattered from this
pulse at a 90 degree angle. In the second phase, light from source #2 is
pulsed directly into photodetector #1. Simultaneously, photodetector #2
measures the light scattered from this pulse at a 90 degree angle. In
both phases, the signal from the photodetector receiving the direct
light signal is the active signal, while the signal from photodetector
measuring scattered light is called the reference signal. The two phase
measurements provide four measurements from two light sources: two
reference signals and two active signals.

The turbidity of the sample is calculated from the four independent
measurements taken from the two light sources using a mathematical
algorithm similar to the algorithm used by the ratio instrument design.
The result is that errors resulting from sample color appear in both the
numerator and denominator of the mathematical algorithm, and the errors
are mathematically canceled.

Like the ratio design, the mathematical algorithm used in the four beam
design allows for more sensitivity in highly turbid samples and extends
the range of the instrument to about 100 NTU. The error cancellation
achieved by the ratio algorithm also makes the instrument very accurate
in the 0 to 1 NTU range.

### Surface Scatter Design

As turbidity increases, light scattering intensifies and multiple
scattering can occur as light strikes more than one particle as it
interacts with the fluid. Light absorption by particles can also
significantly increase. When particle concentration exceeds a certain
point, the amount of transmitted and scattered light decreases
significantly due to multiple scattering and absorption. This point is
known as the optical limit of an instrument.

The surface scatter design utilizes a light beam focused on the sample
surface at an acute angle. As shown in Figure 11-6, light strikes
particles in the sample and is scattered toward a photodetector that is
also located above the sample surface. As turbidity increases, the light
beam penetrates less of the sample, thus shortening the light path and
compensating for interference from multiple scattering. The reported
range of surface scatter instruments is about 0 to 9999 NTU, although
these instruments are best suited for measuring high turbidities such as
are present in raw water and recycle streams (Hach Corporation, 1995).
These designs are not approved by
EPA.![](media/image6.png){width="2.729861111111111in"
height="2.9270833333333335in"}

### Transmittance Design

Instruments utilizing a transmittance design are often referred to as
turbidimeters, but these instruments do not measure true turbidity of
water in NTUs. These instruments are better termed "absorptometers" as
they measure the amount of light transmitted through a sample rather
than the amount of light scattered by a sample. Light transmittance is
measured by introducing a light source to a sample volume and measuring
the relative amount of light transmitted through the sample volume to a
photodetector located opposite the light source. Transmittance values
are reported as 0 to 100 percent of the incident light source
transmitted through the sample. The use of absorptometers in water
treatment has generally been restricted to monitoring spent filter
backwash water to determine relative cleanliness of the filter media
(Hach Corporation, 1995). These designs are not approved by EPA.

## Types of Turbidimeters

There are three common types of turbidimeters employed today. These are
referred to as bench top, portable, and on-line instruments. Bench top
and portable turbidimeters are used to analyze grab samples. Bench top
units are typically used as stationary laboratory instruments and are
not intended to be portable. On-line instruments are typically installed
in the field and continuously analyze a sample stream spilt off from a
unit process. Measurement with these units requires strict adherence to
the manufacturer's sampling procedure to reduce errors from dirty
glassware, air bubbles in the sample, and particle settling.

### Bench Top Turbidimeters

Most bench top turbidimeters are designed for broad applications and
have the capability to measure highly colored samples as well as samples
with high turbidities. The most popular bench top turbidimeters used
today utilize the ratio design, but may have options for back scatter
detectors or monochromatic light sources. Many ratio bench top
instruments also have the capability to turn off the ratio calculation
so that measurements can be made using the single beam design. Older
bench top instruments may be of the single beam design, and some have
analog rather than digital displays. Bench top units are used
exclusively for grab samples and require the use of glass cuvettes for
holding the sample volume.

### Portable Turbidimeters![](media/image8.png){width="2.386111111111111in" height="1.2743055555555556in"}

Portable turbidimeters are similar to the bench top units, except that
they are designed for portable use and are battery operated. Portable
turbidimeters are available in a variety of designs, including the
single beam and ratio designs. The accuracy of portable instruments is
comparable to the bench top units, but the resolution of low turbidity
reading may only be 0.01 NTU as compared to the 0.001 NTU resolution of
bench top units (Hach Corporation, 1995).

Portable turbidimeters are designed for use in the field with grab
samples. These instruments are designed to be rugged and capable of
withstanding the affects of moving the instrument as well as variable
field conditions. However, since these instruments are inherently
susceptible to damage or disturbance from dropping, abuse, or
environmental conditions such as dust, these units are not appropriate
for the process monitoring and reporting tasks normally accomplished by
bench top units or on-line turbidimeters.

However, portable instruments are useful for measuring turbidity at
remote locations such as at sampling points in the watershed upstream of
a water treatment plant, or at a remote raw water intake location.
Portable instruments are also useful for conducting special process
studies, such as backwash recycle characterization or distribution
system analysis that may be accomplished more readily and accurately in
the field rather than conducting analysis after transporting a sample to
a laboratory.

### On-Line Turbidimeters

The on-line instruments used in the water treatment industry typically
utilize the single beam or modulated four beam design. On-line ratio
turbidimeters are also available, but their use has not been as
extensive as the single beam and modulated four beam designs. On-line
surface scatter turbidimeters are often used for raw water monitoring
and transmittance-type absorptometers have been used for filter backwash
monitoring.

On-line instruments typically sample a side stream split off from the
treatment process. The sample flows through the on-line instrument for
measurement and then is wasted to a drain or recycled through the
treatment process. Supervisory Control and Data Acquisition (SCADA)
instrumentation and remote telemetry can also be connected to on-line
instruments to collect data for trending analysis or to control
automated treatment actions based on the turbidities measured. The use
of SCADA with turbidity measurement is discussed in Chapter 4.

Typical sample flow rates through on-line instruments range from about
0.1 to 1.0 liter per minute. Some single beam on-line turbidimeters do
not contain a glass sample container. The light source is located above
the sample volume, which has an optically flat surface as it flows over
a weir. The photodetector is submerged within the sample volume and
requires frequent cleaning to prevent fouling. Most on-line four beam
instruments used in the water industry contain a sealed flow-through
sample volume with windows at each of the light sources and
photodetectors. These surfaces must also be cleaned frequently to
prevent fouling.

Most on-line instruments contain bubble traps to eliminate air bubbles
from the sample that might interfere with the turbidity readings. Bubble
traps are typically baffled chambers that allow air bubbles to rise to
the sample surface prior to the sample entering the measurement chamber.
The volume of the sample chamber varies significantly between the single
beam and four beam design due mostly to the design of the bubble trap.
Single beam devices typically include a bubble trap within the sample
chamber, making the sample volume in excess of two liters. Several other
on-line instruments use sample volumes as small as 30 milliliters.

## References

1.  California Department of Health Services. 1996. *Particle Counting
    Guidelines.* California Department of Health Services.

2.  GLI (Great Lakes Instruments). undated. *Technical Bulletin Number
    T1 -- Turbidity Measurement.* Rev. 2-193, Great Lakes Instruments.

3.  Hach Corporation. 1995. *Excellence in Turbidity Measurement.* Hach
    Corporation.

4.  International Organization for Standards (ISO). 1990. *International
    Standard ISO 7027 -- Water Quality -- Determination or Turbidity.*
    ISO.

5.  King, K. 1991. *Four-Beam Turbidimeters for Low NTU Waters.* Great
    Lakes Instruments.

6.  Lex, D. 1994. Turbidity Technology Turns on the High Beams. *Intech
    Engineer's Notebook.* 41(6):36

7.  Sadar, M.J. 1996. *Understanding Turbidity Science*. Technical
    Information Series -- Booklet No. 11. Hach Company.

8.  Standard Methods. 1995. *Standard Methods for the Examination of
    Water and Wastewater,* nineteenth edition. America Public Health
    Association, AWWA, Water Environment Federation. Franson, M.H., A.D.
    Eaton, L.S. Clesceri, and A.E. Greenberg (editors). American Public
    Health Association, AWWA, and Water Environment Federation. Port
    City Press, Baltimore, MD.

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**TABLE OF CONTENTS**

11\. Basic Turbidimeter Design and Concepts
[1](#__RefHeading___Toc448538864)

11.1 Introduction [11-1](#introduction)

11.2 Turbidimeter Measuring Principles
[11-1](#turbidimeter-measuring-principles)

11.2.1 Light Source [11-2](#light-source)

11.2.2 Sample Volume [11-3](#sample-volume)

11.2.3 Photodetector [11-3](#photodetector)

11.3 Turbidimeter Design Configurations
[11-3](#turbidimeter-design-configurations)

11.3.1 Design Standards [11-4](#design-standards)

11.3.2 Single Beam Design [11-5](#single-beam-design)

11.3.3 Ratio Design [11-6](#ratio-design)

11.3.4 Modulated Four Beam Design [11-7](#modulated-four-beam-design)

11.3.5 Surface Scatter Design [11-7](#surface-scatter-design)

11.3.6 Transmittance Design [11-8](#transmittance-design)

11.4 Types of Turbidimeters [11-8](#types-of-turbidimeters)

11.4.1 Bench Top Turbidimeters [11-9](#bench-top-turbidimeters)

11.4.2 Portable Turbidimeters [11-9](#portable-turbidimeters)

11.4.3 On-Line Turbidimeters [11-10](#on-line-turbidimeters)

11.5 References [11-11](#references)

LIST OF FIGURES (manually completed)

Figure 11-1. Scattered Light at 90 11-1

Figure 11-2. Angular Patterns of Scattered Light from Particles of
Different Sizes 11-2

Figure 11-3. Basic Nephelometer 11-5

Figure 11-4. Ration Turbidimeter 11-6

Figure 11-5. Modulated Four-Beam Turbidimeter 11-7

Figure 11-6. Surface Scatter Turbidimeter 11-8

Figure 11-7. Bench Top Turbidimeter 11-9

Figure 11-8. Portable Turbidimeter 11-9