Feature ArticleMoored Profiler for High-Frequency Monitoring of Oceanic Microstructures
By Víctor Villagrán O.
Department of Geophysics
University of Concepción
César Hormazabal F.
Oceanographic Equipment Technician
Professor of Oceanography
Department of Oceanography
University of Concepción
Modern underwater vehicles, such as AUVs, Argo floats and ocean gliders, are marking a new era of observing the ocean, serving marine researchers and the oil, military and fishing industries. In contrast to these vehicles, moored profilers (also called yo-yo profilers) have a very high vertical resolution and can potentially study microstructures in the water column. However, their development has been limited to universities and a few research institutions, and most devices remain in the prototype stage.
This article discusses the development of a variable buoyancy yo-yo profiler intended to study the oxygen minimum zone (OMZ) in the eastern South Pacific. The prototype, the Sistema Autonomo de Mediciones Yo-Yo (SAMY), was developed by the University of Concepción. It is the result of reengineering a 1980s profiler developed at the Niels Bohr Institute at the University of Copenhagen for a group headed by Gary Shaffer and Henning Hundahl. Almost 30 years later, the instrument was donated to the University of Concepción. The new version of the profiler was intended to update the instruments and the main controller so it could offer a more open and versatile platform. The work was initially limited to the electronic architecture, but some mechanical and hydraulic issues had to be reviewed when weaknesses of the original design were observed in field campaigns.
Design and General Features
The main components of the profiler are the control unit, the hydraulic unit and the external bladder. The control unit, completely redesigned in the SAMY project, communicates with all sensors in the sampling process, sends commands to the hydraulic unit to control vertical position, stores data and communicates with users for configuration and data transfer. Sensors and buoyant material are attached to the profiler structure, and a hydrodynamic plastic shield encloses the system.
The SAMY control unit uses a VIPER PC/104 processor from the Arcom Group, which is now part of Eurotech (Amaro, Italy). Although the VIPER consumes very little power, its sleeping mode is not as good as less complex microprocessors. Accordingly, the SAMY architecture includes a small microprocessor for the sleep mode, during which the device consumes less than 30 microamps. The microprocessor is also used to communicate with the older electronics in the hydraulic unit. The processor runs an embedded Linux operating system. Its activity depends on three programming levels. On the first, deepest level, basic functions are written in C/C++; the second level, which can call first-level functions, is script language; and the third, highest level is determined by configuration parameters established by the user in text files. The timing of many activities is reported in diagnostic output files.
The buoyancy of the system changes when the oil moves between the external bladder and the oil tank placed inside the hydraulic unit. To move the profiler up the water column, the hydraulic unit pumps oil out to the external bladder until the internal tank reaches a certain level, which ultimately defines a profiling speed. To move the profiler down the water column, an internal valve on the hydraulic unit is opened, and the pressure difference allows oil to come back into the internal tank.
After field testing, SAMY’s rolling system and the internal tank were redesigned to provide more robust operation. The following describes each component’s upgrades.
Rolling System. The initial design of the profiler used two rolling wheels to guide the mooring rope. This design had the risk of critical failure if there was an excessive angle on the line, insufficient buoyancy or other problems with the rolling system. To fix this issue, the two rollers were replaced with two pairs of tackles that allow the rope to pass between them. Additionally, the new mooring design minimized excessive angles on the line.
Internal Tank. The internal tank is a semirigid rubber container that can extend or contract vertically, depending on whether the oil is coming in or out of the hydraulic unit. To control the oil volume flow, there is a rotative potentiometer mounted on the top of the container. In this way, electric resistance (or turns) can be related to the height of the container. During shallow testing at 15 meters’ depth, it was found that because irregularities in the rubber container produce bad alignment, this part of the system was not robust and had the potential risk of disconnecting from the tethered pole. To solve this issue, the internal tank was contained in a secondary container (similar to a piston-system), and the potentiometer was replaced by a linear displacement potentiometer.
The SAMY’s sensors are designed to accurately characterize the water column. They include a 16-hertz SBE 49 CTD from Sea-Bird Electronics (Bellevue, Washington), a HyperOCR hyperspectral ocean color radiometer from Satlantic Inc. (Halifax, Canada), a Vector acoustic Doppler current profiler from Nortek AS (Rud, Norway) and a FLNTU fluorescence sensor from WET Labs Inc. (Philomath, Oregon). To continue this article please click here.
Víctor Villagrán O. is an electronics engineer with a master’s degree in science. Since 2000, he has been in charge of developing instruments at the Medición Innovación y Desarrollo Geofísico laboratory for geophysics research in multidisciplinary teams.
César Hormazabal F. is an oceanographic equipment and sea moorings technician. Since 1997, he has been working in several research programs and is now a part of SeaHorse, a company dedicated to technical support in geophysics.
Osvaldo Ulloa is an oceanography professor at the Department of Oceanography and the Center for Oceanographic Research at the University of Concepción in Chile.