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20 st / October 2015 www.sea-technology.com to protect the spool from stones that could hamper the un- winding of the rope upon release of the foat. The foat and the electronic devices are securely attached to the frame with a polyamide cable clamp that fastens the loop of the release system to the wire grate. The whole mooring system weighs 28 kg in the air and 10 kg in water. Electronic Design, Release Device We developed a specifc electronic card according to carefully considered technical specifcations. A low-power microcontroller with an 8 MB fash memory card is con- nected to a temperature and a pressure sensor. It can store up to 420,480 temperature and pressure data samples. The temperature sensor is a precalibrated electronic chip, featur- ing a 0.008° C resolution and requires no future recalibra- tion. One AAA 1.5-V cell is enough to take one temperature and pressure sample per minute for one year, or at longer intervals. There is an LED indicator at the top of the card to facilitate visual inspection of its operating status before deployment. To program data sampling, start data collection or re- trieve data samples, the system is placed in a dock that trig- gers a detection switch. Once detected, the LED indicator starts blinking and the high-frequency data link is powered on. Equipped with a high-frequency USB dongle, a personal computer runs the human-machine interface that connects it to the data-logging system. The plastic tubing, which houses the release mechanism, and the electronic compartment are flled with Marcol 82 insulating oil that allows ambient pressure housing, thereby drastically reducing the cost. A low-cost "burn wire" release mechanism consists of electrolytic erosion of a copper wire loop under low volt- age. The negative circuit is soldered to 16-mm-diameter copper tubing. The negative pole is soldered to a copper wire. The burn wire is mounted in resin to ensure insulation and watertightness. When the electronic device switches on the current, electrolytic erosion is powered by two AAA bat- teries in series and is completed in about 1 hr., releasing the foat and its electronics compartment from the mooring frame. The batteries are located in the mid- dle of the central hose. Laboratory Tests We used the pull test to as- sess the resistance of the copper A classic coastal mooring system with a surface foat was not selected due to the cost of the whole mooring system and the accessories (chain, mooring line, anchor, etc.) re- quired for the system to withstand six months in rough sea conditions. We therefore chose to build a system involving a near- bottom temperature data logger with a ballast and a release device. The most commonly used release devices are based on acoustic signals; a solution we also rejected due to its cost. Therefore, release of the mooring system from the seabed will not be activated by signals sent out from the ship mandated to recover the mooring, but will occur on a specifc date chosen before deployment. Given the uncer- tainty in marine weather forecasts and ship availability, the temperature probe and its foat will nevertheless remain fas- tened to the ballast after the release date to allow recovery a few days later, if necessary. Mechanical Design The positive buoyancy for mooring foatation is provided by a 280-mm-diameter plastic fshing net foat (Nokalon 511), with a central hole of 24 mm. This foat has a net buoyancy of 7.5 kg and a working depth of 950 m. Located on the top of the foat, the electronics are housed in a compartment made of standard PVC parts commonly used for plumbing. The compart- ment is connected to the release device via a fexible plastic tubing that passes through the central hole of the foat. The frame is an ordinary traffc cone (75 cm in height), cut 25 cm above its base. The ballasts consist of four dumb-bell weights of 5 kg each. The weights are ftted with stainless steel bolts that penetrate the sediment and are long enough to maintain the system on the seafoor even in strong current conditions. A stainless steel tube (diameter 30 mm) passes through the cone and is used to hold the spool of polyamide rope (diameter 3 mm, nominal strength 300 kg). The rope is at- tached to the spool at one end and to the foat at the other end via a classic bowline. A stainless steel wire grate is used to reinforce the frame and (Top) (A) Central plastic tubing, electronic compartment on the left side, burn wire on the right side. (B) Electronic card. (C) Batteries. (D) Burn wire. (Bottom) Time series of seabed temperature. Location of the stations is shown on the map in the right panel.