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12 st / October 2015 www.sea-technology.com different probes for three different depths, for example, a fow-through system facilitates the same sampling specifcations with just one probe and three intake tubes all connected to a single water pump. Although the in-situ method pro- vides for accurate results at the onset of delivering the probe into the water, over time these results begin to skew due to the continuous exposure to these rich marine environments, result- ing in biofouling. Data show that an open-water in- strument begins to fall apart after only a month in the water. Without regular maintenance, the open-water sensors foul quickly and produce unusable data and eventually become compro- mised to the point of not being able to collect any data and simply fat lining all data points. On the other hand, the fow-through system drastically curbs this sort of data degradation. Since the sensors are kept dry and out of harsh marine elements, they require much less maintenance and have longer feld lives in general. It should also be noted that the fow-through method of sampling and testing proves more effective for instru- mentation that requires a high degree of precision. For instance, using three instruments for three different depths is inherently less precise than using one instrument to measure three dif- ferent samples. And, some data simply depend more on precision than ac- curacy. For example, "difference" in- struments, such as algae sensors, use fuorescence to detect algal densities in the water column and, using tra- ditional methods, this would call for three separate instruments for each of the three depths in question. However, the nature of this method incorporates some element of measurement error since small differences get lost in mea- surement noise and instrument toler- ances, thereby diluting the validity of comparing data captured by each of the sensors. On the other hand, the fow- through sampling method greatly im- proves precision, allowing a single instrument to make measurements on samples pulled from multiple depths. For example, incorporating three inde- pendent instruments could vary data as much as +/- 2.0 mg/L, whereas using a single instrument may offer repeatabil- ity as low as +/- 0.2 mg/L. Maintenance Regardless of the system type, how- ever, if the platforms are not main- tained, collecting sound data over an extended period of time would not be possible. As with all feld-deployed equipment, a set of standard mainte- nance procedures should be sched- uled at regular intervals depending on the type of buoy in service. As cost of maintenance is a signifcant concern, it makes sense to consider the differ- ence in upfront cost from one type to another. For example, buoys that are confgured with in-situ sensors can be as high as 28 percent of the upfront cost of the unit due to the nature of the measurement method. The longer the sensors are exposed to warm-water environments the more fouling can be expected, while cold water presents its own challenges as freezing becomes a concern. However, the bulk of the cost, both monetarily and from a risk perspective, for maintaining these kinds of buoys is the requirement for commercial divers to enter the water and conduct sub- surface cleaning of the in-situ sensors. Flow-through platforms, on the other hand, do not require regular diver in- tervention to maintain good standing operation. Instead, all maintenance can be carried out by technicians above sea level. There may at some point be a need for a diver to get involved to inspect ca- bles, suction lines, or simply have a look at the underside of the buoy, but it is typically a much less frequent endeavor. Based on this comparative reduction in maintenance procedures, that which is performed on fow-through systems ac- counts for just 12 percent of the upfront cost of the platform. Communications One of the unique aspects to de- ploying remote-monitoring equip- ment at this point in our technologi- cal history is the ease at which data can be transferred wirelessly from even the most remote locations. As cellular communication is beginning to replace radio in critical systems by combining redundant service pro- viders, it is increasingly useful as a reliable alternative. Satellite service is also becoming a more affordable method of data transfer and can be used in a much wider range of appli- cations, including coastal monitoring platforms. By incorporating relatively simple all-inclusive telemetry units