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www.sea-technology.com October 2015 / st 21 loop of the burn wire. Several copper wires of various lengths and shapes mounted in resin have been tested. The best confguration consisted of a 1.2-mm-diameter wire with several small loops inside the resin to enhance electrical resistance. In this confgura- tion, the loop resists 50-kg traction, largely suffcient in regard to the buoy- ancy of the foat. The major drawback of an integrat- ed temperature sensor in an oil-flled compartment is the time constant. It was evaluated as the time required to reach a 63 percent change in tempera- ture. This time was determined in the selected confguration to be 9 min. This duration, albeit long, is satisfactory for the description of bottom temperature, which is known to vary slowly in most environments. More than 25 electronic cards were calibrated in the laboratory. The mean bias was 0.21° C with a standard de- viation of 0.021° C. This bias appeared constant (with a range of 0.03° C) with temperature. This linear behav- ior allows low-cost calibration of new temperature sensors, because one sin- gle-point verifcation provides a fnal precision greater than 0.05° C, which is better than the initial specifcation. In-Situ Tests Several tests were performed on the deployment of the system, and it turned out that a simple drop of the Mastodon mooring system from the deployment ship (stationary) is effcient. The system descends at a constant speed of 0.8 m/s. The aging of the release device was one of our concerns. Given that the mooring is dedicated to coastal mea- surements, biofouling may be trouble- some because it can hamper electroly- sis. A long-term experiment was set up. For more than 18 months, 50 burn wires were placed in a seawater tank at the Ifremer-Brest in-situ testing site. The test tank was outside, exposed to natu- ral light, and seawater was pumped in from the nearby sea. Every three months, a set of eight burn wires suc- cessfully passed the test (electrolytic erosion of the copper wire completed in 1 hr.), proving the reliability of the release device. Our second concern was the ability of the mooring to stay in place under strong current due to the light weight of the system. An experiment was per- formed in Fromveur Passage in the Iroise Sea during a spring tide in 2013, where the current speed can reach 6 kt. The long bolts acted as clamps in the sea bottom, composed of coarse sand/pebble sediment, and the moor- ings stayed in place. However, after release, the foat stayed mostly under- water and the recovery operation was only possible during slack tides. The last experiment to qualify the system was a six-month in-situ experi- ment with nine mooring systems de- ployed at 25-m depth near the Ifremer in-situ testing site. Every two months, three moorings were scheduled to sur- face. After six months, the nine moor- ings were recovered and the data were verifed and proved satisfactory. In-Situ Experiment In the summer of 2014, the frst ex- periment was set up in the Mors Bras region (southern Brittany, France), and 13 Mastodon moorings were deployed near traffc beacons to avoid fshing trawls, from August to the end of Octo- ber. This region has weak to moderate tidal currents, causing summer stratif- cation. All moorings (except one that was recovered on the shore before the end of the experiment) recorded sea- bed temperature for three months. Bot- tom temperature was highly variable on time scales ranging from the tidal cycle (12 hr.) to several days. Some strong spatial gradients lasted a few weeks. At the end of the recording pe- riod, the temperatures became nearly homogeneous after several storms, indicating the end of the stratifcation season. Conclusion, Future Developments Here, we described the devel- opment of an inexpensive mooring system (about €330) to measure the seabed temperature on the European www.n ke-in st ru me n t at ion .com Ac tiv e a ntif o uling E N VI R ON MEN TA L MO NI TO RI NG Mu lti p ar am e t er pro be s SMATCH Aut on om ous M onit or ing s t a t ions Qua l it y w a t e r m onit or i ng L o ng term use Auto m a tic tra nsm issio n Auto m a tic c lea ning Te mp e ra t u re , C o n d uc t ivit y, De p t h , Tu rb id it y, p H, Re d o x, Dis s o lve d o xyg e n , C DOM , C h lo ro p h yll a , P h yc o c ia n in , HAP , H yd ro c a rb o n . SA MBAT W iper c lea ning "We chose to build a system involving a near-bottom temperature data logger with a ballast and a release device."