Development Of In Situ Calibrators For Long Term Chemical Measurement In Hydrothermal Fluid Environments

Ever since the discovery of hydrothermal springs on the Galapagos Rift in1977, the researches on hydrothermal activities have been of great interest to marine scientists. Hydrothermal envirouments are characterized by great spatial and temporal gradients, thus, in situ measurement techniques are better to contrain the chemical evolution of the system. A variety of electrochemical sensors have been developed for deep sea applications, many successful due to their simplicity and low cost, such as pH, H2, H2S and S2-electrodes. For long-term applications, signal drift and poor stability at high pressure are caused by irreversible compositional change to the surface phase of the electrodes over time, after which the electrodes require in situ calibration. To address this issue, we have developed two novel in situ calibrators using fluid control techniques for sensor calibration and chemical measurement. In situ chemical data can be calculated and verified from constraints imposed by the Nernst relationship between measured cell potentials. Sea trials have successfully demonstrated the effectiveness of this in situ calibration.This paper first presents the design of two key components of the calibrator:the fluid control system and the sensor head. Based on the field application requirements, two in situ fluid control systems for the calibrators are developed. The first is used for in situ long term measurement and the second is used for in situ on line measurement. A key feature of these two systems is that they are capable of conducting a two-point calibration of the electrodes along with measurement of a target fluid under the deep sea environment. The research indicates that at a certain flow rate, the positive piston pump should increase the dispense volume to minimize the pressure variation. In the sensor head, the electrodes is sealed inside the small sensor cell with an internal volume of only～1.03mL, which was specifically designed to facilitate access to the fluid sample, while minimizing calibration time. The electrodes are sealed by a pre-compressed Viton sealant which also provides reliable seal under pressure. The in situ long term sensor head is integrated with a custom-designed valve to isolate the hydrothermal fluids during calibration. The cone angle stem and Viton seat contact seal of the integrated valve is FEA (Finite element analysis) analyzed and tested. The high temperature pH measurement test and sea trials indicate that the sensor head is suitable to investigate the hydrothermal fluids under100℃. The in situ on line sensor head has a Φ2.8cm×34cm sensor body. The sensor cell is located at the most forward tip of the sensor body. This design facilitates the deployment of the sensor head in the environments of hydrothermal vent organisms at temperatures up to150℃.Numerical and FEA models of Endeavour hydrothermal cheminy demonstrate the spatical temperature distribution among the vent field. The vent temperature gradients of hydrothermal fluids depend on the initial temperature and flow rate of the cheminy orifice. Using the temperature and flow rate gradients on the surface of the sensor heads above the hydrothermal chimney, the transient thermal convection process can be obtained by numerical calculations. As the temperature variations will increase the uncertainties of the in situ data, the calibrators should begin in situ measurement when the sensor heads achieve the temperature equilibrium with the ambient envirouments. The resistance of the valve solenoid varies with temperature which affects force output, thus the duty cycle of the sensor head is calculated to avoid over heating that is very helpful when building the in situ working strategy.Finally, two in situ calibrators:in situ long term calibrator and in situ on line calibrator were built up according to the researches mentioned above, each containing a fluid control system and a Data-logging system. By low-power design techniques, the long term in situ calibrator can carry out6month continuous measurement on the seafloor. Also in situ long term calibrator has been reconfigured and tested on the Monterey Accelerated Research System (MARS) at890m depth. The new instrument was successfully tested in the deep ocean for seawater pH measurement up to the depth of3200m. The two days field test at Lost City hydrothermal filed demonstrates successful operation and auto-calibration of the pH sensor at the depth of778m. The development of the novel auto-calibration instrument, however. transcends simple application to pH, while also servers as a testbed for other chemical sensors for verification of data under the same conditions. The pH calibrator has passed the Wet Node Simulator tests in seawater tank at the Monterey Bay Aquarium Research Institute (MBARI), and will be further tested on the MARS cabled observatory, which will provide continuous power and data connection.