Page 210 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Contamination and Industrial Systems

Contamination and Industrial Systems 203

selection, amplifier gain-bandwidth product and transimpedance resistor para-
sitic capacitance compensation it is straightforward to achieve adequate detec-
tion bandwidth, over 8kHz.
There are several drawbacks to performing the measurement in situ in a
flowing process stream. First we have only limited control over the sample.
Surface bubbles and floating objects will dominate the received turbidity signal,
and must be removed hydraulically or via their distinctive temporal signals. The
floating ring-tubes used by ornamental fish shops to improve our view of Koi
do a good job here too to still the water surface and deflect bubbles.
Specular reflections of the laser source from the water surface will be much
more intense than the diffusely scattered light, and will give large errors or
even overload, which restricts operation to off-normal axis. If significant surface
ripples are present, the range of reflection angles must be considered and a
large-enough incidence angle provided. If the laser is polarized and the scat-
tering process depolarizes, a crossed polarizer at the receiver can be used to
reduce the specular reflections.
The liquid sample must also be “infinitely deep;” if the incident light propa-
gates significantly through the sample to the channel wall or river bed, erro-
neous results will occur. This typically restricts application to moderately
scattering process, environmental, and wastewater samples. However, these are
just the media that benefit most from reduced contamination rate. An improve-
ment is to offset the source and received beams to put the sensed liquid volume
well under the surface, suppressing surface reflections and avoiding seeing the
back-wall reflection. Finally, the open-beam character of the system brings prob-
lems with ambient light, in particular sunlight reflected from the water surface.
By operating the reflectometer at Brewster’s angle and using a crossed polar-
izer to attenuate the reflection, good suppression of sunlight is possible. This
will generally be backed up by the electronic suppression techniques of Chap. 7.
The instrument is most convenient because of its ease of application. It is not
necessary to provide a sample pumped through pipes, which is expensive and
gives problems of pipe silting and abstraction blockage. It can be set up very
quickly by mounting on available fittings, hand-rails, and furniture, and is
useful even in temporary and covert installations. Last, the small beam means
that measurements can be made where little access is available, for example
through walkway grilles and observation apertures, and even in overflows,
weirs, and pipe outlets. In the right applications, the technique can give tur-
bidity measurements with a far lower cost of ownership than any conventional
pumped-sample or immersed in-line turbidimeter.
Free liquid surfaces are so attractive for scattering-geometry measure-
ments such as turbidity, fluorescence and Raman detection, that it is useful to
make your own. If low fouling is the only benefit sought, much smaller sepa-
rations than 10m are adequate, and commercial turbidimeters are available
from Hach and others with a free-surface provided by a pumped overflow in an
on-line instrument (Fig. 9.11). Surface separations of the order of 50mm are
typical. High repeatability is possible in a well-maintained instrument. Free-

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