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Tiny Bubble Is Key to Optical Measurement

Photonics Spectra
Jun 1998
Michael D. Wheeler

Engineers at water treatment plants and other facilities that monitor biological and chemical pollutants in water are turning to optical techniques such as spectroscopy. These techniques are not without their drawbacks. Transparent cells holding the sample can become fouled by biological or chemical compounds found in the water, altering how much light is detected after scattering or absorption. This affects the reliability of optical measurements.
To keep these windows clean, engineers have developed cleaning mechanisms including brushes, rigid blades and high-voltage plasma discharges. These techniques reduce the problem but fail to eliminate it.
Mark Johnson, a scientist at North West Water Ltd., a water treatment plant not far from England's Lake District, came up with a solution. He saw how surface tension stops water flowing through a small hole at the bottom of a bucket filled with water. Why not use an air bubble in place of a glass or crystal as an interface in optical measurement?

Versatile technique
For deep immersion measurements, such as is required in brewing and other industries, the surface layer obscures the view into the depths. Johnson developed an "inverted-cup" configuration. Just as a cup or pail captures an air bubble when someone places it down in water, this configuration captured a small pocket of air. This air pocket effectively served as the barrier between the water and a transparent window. When the residue accumulated on the bubble, an air pump created a new bubble.
Johnson developed a second configuration in which he turned the cup upright. For these applications, a pressurized air pocket supports the water above it. Pressure transducers, sensors and air pumps maintain constant pressure and ensure the curvature of the 6-mm-diameter bubble stays the same.
The bubble optical interface system can accommodate most types of optical measurement. Johnson has performed long-path laser scattering measurements and UV spectroscopy to measure concentrations of benzene, chlorine and pesticides. The technique also works for testing pollutants in the near-IR and visible.
"For long-term testing we needed a maintenance-free, noncontact method for testing," Johnson said. The systems in place at North West Waters take measurements at least once an hour.

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