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.