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geochemistry

Happy Samples

Wu, Shi-jun, Can-jun Yang, Hao-cai Huang, and Ying Chen, 2014, Development of an electric control gas-tight sampler for seafloor hydrothermal fluids, Appl Phys & Eng, v. 15, no. 2, p. 120-129. Doi: 10.1631jzus.A1200233.

Introduction:

Nearly forty years ago hydrothermal vents were first discovered. Their role in the earth’s heat budget and the diverse habitats they support have been studied ever since. As exploration of the seafloor broadens sightings of vent systems is expanding beyond the mid ocean ridges to seamounts and volcanic islands. These off-ridge vents differ from the originally discovered deep sea (>2000 meters) black smokers and sub-aerial vents. By investigating the more recently discovered vent systems scientists hope to deepen their understanding of the ocean’s role in the global heat budget.

Accurate sampling is important for meaningful investigations of these fluids. Hydrothermal vent fluid can be particularly difficult to sample because it easily diffuses and mixes with sea water, a major source of contamination. To avoid contamination to the best of their ability, scientists can monitor sampling with oxygen and temperature probes to assure the least-like-ocean waters are collected. A second difficultly of sampling vent fluids, specifically those that are in shallow water, is that most fluid samplers are designed for deep water vent systems. A major restriction to the usable depth is that they are designed for use by a submersible and manipulator. As of 2005, a common method for sampling fluids from shallow vents includes syringes and bottles, tactics that make it difficult to avoid gas exchange and sample contamination. More recently a group of scientists and engineers from Zhejiang University in China set forth to design a fluid sampler that can work in shallow and deep water environments.

The design:

Jan_15 figure 1Key features of the sampler design are maneuverability, an electric control sampling valve that operates in automatic and manual modes, and a built in temperature monitor with display. The final design (Figure 1) is 15.4” long, 5.1” wide, and 9.1” high. It weighs in at 19.8 lbs in air and 15.4 lbs in seawater. The sampler casing can withstand 3 times the pressures expected at 4000m water depth. The controller, designed by Texas Instruments, does not consume a lot of power and its size and adaptability conserve space. The sampling cylinder has a piston design which doubles to regulate flow rate when used with underwater vehicles.  The electric control sampling valve is similar to isobaric samplers such that it is designed to preserve the fluid sample at in situ pressure while transported to the surface. The sampling valve is required to prevent leakage of seawater into the sampling chamber, and with stand high temperatures and corrosive fluids. The corrosion resistant and leak proof materials used are titanium allow (Ti-6Al-4V) and polyetheretherketone (PEEK). Usage of PEEK requires precaution when sampling high temperature fluids (>400°C); the sample rate must be slow enough to allow the fluid time to cool to <260°C. The sampler can manage one sample at a time. After each sample the actuator must be reloaded and the sampling valve and chamber need to be cleaned and the piston repositioned.

Results:

Two samplers (Figures 2a and 2b) were deployed with scuba divers to sample vents 10-30 meters deep near Kueishantao Inlet. The first deployment successfully recovered samples from both samplers at a vent 18 meters deep (Table 1). The second deployment to a vent only 8 meters deep was unsuccessful at collecting a sample, a fault attributed to fluid pressure too weak to move the piston. Greasing the O-rings and evacuating the chamber are simple proposed solutions to this problem when sampling at shallow depths.Jan 15_table 1Jan 15_Figure3Jan_15 figure 2

 

 

 

Conclusion:

The design accomplishes the goal to create a gas tight sampler that may be used at shallow water vents.   In field observations highlighted that the sampler is successful at collecting samples with little to no gas-exchange contamination from sea water.   Laboratory experiments confirm that even at low pressures (<10 meters of the water column) the sampler can still be used to collect fluids, permitted the sampler is correctly set up.  As with many mechanics improvements are sure to come, but in the meantime this gas-tight sampler is a credible option for vent fluids sampling at any depth.

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