Page 208 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Contamination and Industrial Systems

Contamination and Industrial Systems 201

tion during a measurement phase with stopped flow. If ferromagnetic particles
are used, superior flow control may be possible using magnetic field drive from
external coils.
Electrochemical cleaning can also be very effective, by generating reactive
species such as OH-radicals via electrolysis, or by changing pH over a wide range
in a region close to the sensitive windows. Simple heating can even dislodge
some kinds of contaminants. Last, chemical cleaning using liquid- or gas-phase
acid or alkaline chemicals and surfactants can be effective, especially if the
chemical can be warmed. As with disposables, the replenishment of consumable
cleaners brings problems and expense. Ideally sufficient stock is maintained at
the instrument to last between periodic maintenance visits. Windows coated in
toxic substances can supress biogrowth, although sample disposal may become
a problem. Paints containing certain crystalline forms of titania (TiO 2) have
been used to generate chemical radicals under UV illumination. These can be
used for in situ cleaning.

9.4.3 Dissolving windows
Another approach to fouling removal is to use windows that self-clean via a slow
dissolution process. Inorganic salts and special glasses have been used for this,
usually bonded to an insoluble conventional window to avoid complete break-
through if not replaced in time (Carr-Brion, 1996). They must still be consid-
ered to be consumables, which will contribute to running costs, and the surface
quality of the window will be worse than a conventional (new) window. For
systems that can tolerate some imperfection in the optical surfaces such as the
integrating sphere described below self-cleaning is an elegant approach.

9.4.4 Automation
Automated systems also exist which combine cleaning with replacement. By
using a pair of windows, one of which is in operation while the second is being
cleaned, high data availability can be guaranteed. Changeover can be arranged
to be automatic. The use of two windows means that more time can be spent
cleaning, for example with long soaking in aggressive cleaners or with ultra-
sonic agitation. Automatically disposable windows are also possible. For
example, tapes of plastic films can be moved continuously or in steps into the
measurement cell, allowing periodic change of fouled windows. Last, we can try
to avoid windows completely by operating the measurements windowless.

9.5 Windowless Measurements

9.5.1 Through-surface measurements
The highest stability, reliability, and independence of fouling contamination
is likely when we can completely avoid touching the sample. This is easily
possible in certain scattering-geometry measurements. Figure 9.10 shows a
laser turbidimeter designed to measure through the free surface of a process

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