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Instrument Development

posted Jul 6, 2011, 1:32 PM by mikutas   [ updated Dec 20, 2012, 11:22 AM by Matthew Alford ]

Mooring Technology

Deployment of the HOT Profiling Mooring, Hawaii Current moored profiling technology is severely limited by battery power. This means that if we want to observe with a profiling mooring for a year, one must suffice with daily profiles - or worse. Since internal waves, which contain about half the ocean's energy, have higher frequencies than this, we wish to profile at least once an hour or so - resulting in a short (44-day) time series. To improve on this situation, we are developing a next-generation moored profiling system. With Bruce Howe and Time McGinnis (also at APL), we are modifying a McLane profiler, to be inductively charged from an external power supply. This can be a cabled observatory node, or a large moored battery pack for deployment at non-cabled sites. We are also developing the needed inductive, acoustic and radio communication technologies to allow real-time telemetry back to a nearby ship or shore.

We are also working with McLane to add new instruments and capability to the McLane whenever we can, beginning with different gearbox ratios for faster profiling speeds, rechargeable battery packs, a Nitrate sensor, and a Nortek aquadopp velocity sensor.  Working with our colleagues at OSU, we have successfully integrated chi-pods for direct turbulence measurements into two of our profiler.

Finally, check out our state-of-the-art mooring system we maintain off the Washington Coast.

Fiber-optic Salinity/Density Sensor: the ocean refractometer

refractometer image  Understanding the smallest motions in the ocean, which have scales smaller than a millimeter, is important to advance our understanding of the climate, since circulation models cannot resolve turbulence and have to parameterize it. These scales are poorly understood because they are so small, and because salinity is very hard to measure. Using technology modified from AIDS research, Alford developed a sensor half the diameter of a human hair to sense these smallest-scale fluctuations in ocean refractive index. These are closely related to salinity fluctuations, which to-date have been measured by conductivity and temperature measurements. Measuring salinity from one small sensor reduces "spiking" resulting from mismatched sensor responses. Testing began summer 2001 in Puget Sound, Washington, and initial results showed that in fact the so-called Batchelor cutoff for salinity could be resolved. In addition, however, the sensor responds to the velocity signal of the turbulence, making it an optical "shear probe," but precluding resolution of low-frequency signals. I was able to reduce, but not eliminate, this effect.
Schematic diagram of a coupler used as a refractometer. Light shone into one fiber enters the 1-cm long, 30-µm-radius fused section in the center, and couples into the two output fibers in a ratio which depends sensitively on the refractive index in a one-wavelength-wide (~ 1.5 µm) surrounding radius.