Sea Technology

JUN 2014

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42 st / June 2014 www.sea-technology.com known glass electrode. Two pH buffers of known and verifed values are used to obtain the liner func- tion between the pH and cell potential, thus the pH of the sample can be derived from the calibration data. With the in-situ calibration method, inaccu- racy caused by signal drift from the pH electrode (iridium oxide) is explicitly addressed. It is appar- ent that the practical determination of pH requires the electrode to interact with the target sample and the buffers in series under the same pressure-tem- perature condition, while simultaneously record- ing the cell potential. To achieve this, a computer-controlled and automated fow- through strategy for fuid delivery and transfer is required. The dimension of the pH Calibrator is 70 by 28 by 28 cen- timeters without the sensor probe. The system weighs about 20 kilo- grams in seawater, with a depth rating of 4,500 meters. The instrument is constructed with two main modules. The sensor probe module is composed of four solid-state electrodes, a pH electrode (iridium oxide), dissolved hydrogen electrode (Pt), dissolved hydrogen-sulfde elec- trode (silver sulfde) and reference electrode (silver chloride), and a temperature sensor (thermistor). The sensor unit allows the simul- taneous measurement of multiple parameters. The sensing elements of the elec- trodes are sealed in a small sen- sor cell, with a volume of about 1 milliliter, by a precompressed Vi- ton sealant immersed in silicone oil of a pressure-balanced damp. This design obtains a very high rate of buffer displacement and the reliable seal of the electrodes under pressure. The sensing tip of the electrodes and thermistor are set on the same axial plane in the sensor cell to limit the ef- fects of temperature and chemi- cal gradients. The sensor probe is tethered with communication cable and fexible fow tubing to the hydraulic module of the system to permit fne-scale mea- surement of the environment by a submersible manipulator arm. The hydraulic module con- sists of pumps and valves for fow control, and therefore can be op- erated remotely with power from a seafoor cabled observatory or as a stand-alone device using battery power for shorter-term deployments. In both cases, standardization with onboard fuids of known redox, pH and sodium-chloride activity can be activated at any Indeed, to our knowledge, this has not been satisfactorily accomplished for any electro- chemical measurements performed to date. Using a novel experimental approach, however, we have completed a series of critical improvements in this area and have completed the development of an in-situ calibration system. The concept of coupled in-situ pH measurement and calibration has been our main objective related to seafoor observatory efforts. Thus, a new device, which we refer to as "pH Calibrator," was developed as the next generation of in-situ sensor instrumentation. This self-calibrating sensor system can be operated on the seafoor or in the wa- ter column to carry out the long-term monitoring of pH of seawater or diffuse hydrothermal fuid. During the development of the in- strument, several technical criteria were considered: reliability, power consumption, range and consumption rate of pH buffers, remote operation, and environmental factors that could affect length of deployment. The re- sulting self-calibrating pH system can be deployed by various methods, such as Lander system or ROVs. The unit is especially important with the parallel development of seafoor cabled obser- vatories, which can provide the power needs for calibration inherent to the long-term acquisition of high-quality chemical sensor data in challenging chemical and physical environments. Instrumentation In-situ calibration of the iridium-oxide electrode is gen- erally similar to that used for lab calibration of the well- Deployment of the pH Calibrator in the MARS cabled ocean observatory, Lost City hydrothermal diffuse fuids site, and elevator and winch deploy- ment for pH profling of ocean water masses near TAG vent feld. JUN2014.indd 42 6/4/14 12:48 PM

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