Hach-Lange SURFACE SCATTER 7 sc Turbidimeter - Manual (Page 61)

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Appendix B Theory of operation
B.1 SS7 sc principle of operation
The Surface Scatter 7 sc Turbidimeter is a sensitive and precise
instrument designed to measure the light scattered by particles
suspended in the sample fluid. The sample flows up through the
turbidimeter body at a rate between 1 and 2 liters per minute (¼ to
½ gallon per minute). As the fluid spills over the top of the
turbidimeter body, a stable, flat surface of fluid forms and becomes
the measuring surface.
Because there is no contact between the fluid being analyzed and
any of the optical surfaces, the instrument can monitor even highly
turbid samples without frequent cleaning. Electrically and
mechanically, the Surface Scatter 7 sc Turbidimeter has been
constructed to ensure reliable operation in adverse environments.
The lamp, powered by a regulated voltage source, provides a high
intensity beam of light that is adjusted to strike the fluid surface at
an angle. Most of the light striking the surface of the fluid is either
reflected into the upper left hand corner of the cabinet and
absorbed, or refracted down into the turbidimeter tube (
Figure 4 on
page 12 ). A small amount of the light is scattered by the particles
suspended in the fluid. Light scattered at 90 degrees from the
incident beam is detected by the detector assembly. The electronic
signal generated by the detector assembly is directly related to the
concentration of particles suspended in the fluid.
Directing the light beam at the fluid surface at an angle and placing
the detector assembly directly over the point where the light enters
the fluid is a very important design feature. Because the light is
scattered at or near the surface, only a minimum amount of
scattered light is absorbed by the fluid before it reaches the
detector assembly. Therefore, the amount of light scattered will
increase with increasing turbidity, regardless of how high the
turbidity becomes. This allows the instrument to measure very high,
as well as very low, levels of turbidity.
B.2 SS7 sc-HST principle of operation
The SS7 sc-HST instrument is designed for high temperature
samples, or in applications where a significant difference between
the sample temperature and the ambient temperature causes
condensation and fogging in the unit. The SS7 sc-HST functions in
the same manner as the SS7 sc, but accommodates samples of
higher temperature and a moist air removal system has
been added.
The moist air removal system contains an air flow multiplier that
creates a vacuum to draw moisture away from the sample tube and
remove the moisture from the enclosure. The moisture removal
system requires the customer to provide a source of air pressure,
such as compressed air, shop air, or an on-site air compressor. The
air line is connected to the top of the flow multiplier
(
Figure 5 , item 1 ).