Page 207 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Contamination and Industrial Systems

200 Chapter Nine

materials for these, which are now available with very high quality optical
surfaces at low cost. Their wavelength of operation is limited at short wavelength
to approximately 350nm, but for many laboratory, treatment works, and indus-
trial manual testing applications using spectroscopy, turbidity, and indicator
chemicals, this is sufficient. Although they are often used for repeated measure-
ments, the plastic’s easily damaged optical surfaces make this a risky endeavour
compared with glass or silica cuvettes. In any case, the cost of these measure-
ments is usually dominated by the cost of getting to the site, obtaining a sample,
performing the measurement and thinking about the results, so that disposable
measurement cells contribute only negligible cost in many applications.

9.4.2 Cleaning
For on-line use, where measurements must be performed dozens to thousands
of times per day, cleaning of a higher cost window or cell is a preferable
approach. Chemical and/or mechanical cleaning mechanisms can of course be
added to remove deposited films, either manually or automatically. In all cases
this is expensive to perform, or reduces system reliability, risks more damage
than help, or requires stocks of consumables, but it may still be useful.
Cleaning can be done in many obvious ways, including with water jets, air-
jets, brushes, sponges, and ultrasonic cavitation scouring. Where hard windows
of sapphire, diamond or diamond-like carbon are used, surprisingly tough clean-
ing methods can be used, including steel-blade scrapers, wire brushes, and
abrasive stones. An interesting technique is the use of swirling grit particles
and coated plastic balls, typically kept in motion by high sample flow rates (Fig.
9.9). These are frequently used to scour the electrodes of electrochemical
sensors, but the idea is equally attractive for optical surfaces. In many cases
these can be left in the measurement cell to sand-blast the windows during
sample filling, falling out of the optical path by settlement or magnetic attrac-

Outlet
Low-velocity
stilling region
Grit falls back to
optical chamber
Sapphire
windows
Optical
path
Swirling grit paricles
Fluid inlet
Figure 9.9 Swirling grit particles or neutral-
buoyancy plastic balls can be used to scour the
cuvette windows to remove contamination, and
yet be retained with little loss.

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