Page 215 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Contamination and Industrial Systems

208 Chapter Nine

An analog feedback loop automatically adjusts the pressure to a constant value,
using a pressure actuator made up of two 40mm diameter Mylar-cone loud-
speakers arranged face to face as a small, fast pump. With a closed loop band-
width of a few hertz, long term stable operation of the meniscus at a fixed
pressure is possible.
The instrument we built (Johnson and Stäcker, 1998) was also fitted with a
high flow air pump, which was actuated periodically to blast the measurement
hole free of fouling. Depending on the degree of fouling, cleaning can be every
few hours, or provide a new window for every measurement, analogous to the
hanging mercury drop electrode so beloved of electro-chemists. The continuous
loudspeaker-pump offers high speed and resolution but has a limited dynamic
range. This can be improved by replacing it with a digital control system driving
two pulsed solenoid valves. One was coupled to a pressurized cylinder at one
bar overpressure, the other to a lightly evacuated cylinder. By rapidly pulsing
the solenoid valves open and closed in less than 3ms, small volumes of air could
be introduced to or extracted from the support chamber. This approach gives
limitless control, allowing automatic filling and emptying of the measurement
chamber. A depth of 50–1000mm was used and long-term noncontact meas-
urements performed.

9.5.3 Falling streams: Transverse and axial
A well-known way to make windowless optical transmission measurements is
the falling stream. Figure 9.16 shows a sample liquid container with a small
hole in the base, through which water leaks out. The water thread that forms

Water Header
inlet tank

Falling Orifice
water-thread
Detector
UV
source

To waste
Figure 9.16 Falling liquid
streams and transverse trans-
mission are used for noncon-
tact transmission measure-
ments of “difficult” liquids such
as waste-water effluents.

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