Page 199 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Contamination and Industrial Systems

192 Chapter Nine

In some processes designed to precipitate chemical species, such as the
manganese-removal stages of drinking water treatment, highly opaque, physi-
cally hard and adherent films can build up on instrument windows in a few
hours (in this case black, highly absorbing films of manganese dioxide). Even
in clean drinking water supplies, nutrients and oxygen are always present,
allowing bacteria to grow and form “biofilms.” Again, this growth can be a fast
process. Ultrasmooth window surfaces of polished sapphire or highly inert
materials such as transparent PTFE and other fluoropolymers may delay the
onset of film growth, but once a few bacteria have taken hold, further growth
is often rapid. Films can range from predominantly transparent, sticky poly-
mers to hard, opaque layers with combined inorganic constituents from the
fluids themselves and from corrosion products of the pipe-work.
Reference-beam configurations, and in particular the four-beam approach
repeated in Fig. 9.1, are widely used to counter window fouling in these indus-
trial sensors. We calculate the ratio of “short” to “long” path intensities or scat-
tered to transmitted intensities:
È PP ˘ L L )
11 22
a
Q = = e - ( 11 22
LL - 12 21
Í Î PP ˚ ˙
21
12
which appears to be independent of source and detector window fouling. The
basis of the intensity compensation scheme is that the intensities measured
along paths L 11 and L 12 are perfectly proportional, no matter what is the level
of fouling, and the compensation scheme should be perfect. In practice, the

Sources Detectors
D 1 (b) Turbidity
L 11
S 1 measurement
a Source S
L 12 1

L 21
D 2 s
L 22
S 2
D 1

(a) Absorption Source S 2
measurement

Detectors
D 2
Figure 9.1 Four-beam compensation systems are used for (a) absorption and (b) scat-
tering measurements.

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