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MODEL CLARITY II
5.5 CHOOSING TURBIDITY OR TOTAL SUSPENDED SOLIDS
5.5.1 Purpose
This section describes how to do the following:
1. Configure the analyzer to display results as turbidity or total suspended solids (TSS).
2. Choose units in which results are to be displayed.
3. Select a time period for signal averaging.
4. Enable or disable bubble rejection software.
5.5.2 Definitions
1. TURBIDITY. Turbidity is a measure of the
amount of light scattered by particles in a sam-
ple. Figure 5-4 illustrates how turbidity is meas-
ured. A beam of light passes through a sample
containing suspended particles. The particles
interact with the light and scatter it in all direc-
tions. Although the drawing implies scattering
is equal in all directions, this is generally not the
case. For particles bigger than about 1/10 of
the wavelength of light, scattering is highly
directional. A detector measures the intensity of
scattered light.
Measured turbidity is dependent on instrumen-
tal conditions. In an attempt to allow turbidities
measured by different instruments to be com-
pared, two standards for turbidity instruments
have evolved. USEPA established Method
180.1,
and
the
Organization established ISO 7027. EPA
Method 180.1 must be used for reporting pur-
poses in the United States. Figure 5-5 shows
an EPA 180.1 turbidimeter. Figure 5-6 shows
an ISO 7027 turbidimeter.
EPA Method 180.1 requires that:
A. The light source be a tungsten lamp operat-
ed with a filament temperature between 2200 and 2700 K.
B. The detector have optimum response between 400 and 600 nm (approximates the human eye).
C. The scattered light be measured at 90º±30º with respect to the incident beam.
D. The total path length of the light through the sample be less than 10 cm.
Requirements A and B essentially restrict the measurement to visible light. Although the most of the energy
radiated by an incandescent lamp is in the near infrared, keeping the filament temperature between 2200 and
2700 K, ensures that at least some energy is available in the visible range. Further specifying that the detec-
tor and filter combination have maximum sensitivity between 400 nm (violet light) and 600 nm (orange light),
cements the measurement in the visible range. Wavelength is important because particles scatter light most
efficiently if their size is approximately equal to the wavelength of light used for the measurement. The longer
the wavelength, the more sensitive the measurement is to larger diameter particles and the less sensitive it is
to smaller diameter particles.
26
International
Standards
FIGURE 5-4. Turbidity Sensor — General
FIGURE 5-5. Turbidity Sensor — EPA 180.1
SECTION 5.0
PROGRAMMING THE ANALYZER
continued on following page
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