Development of an In Situ pH Calibrator in Deep Sea Environments

Abstract

The pH Calibrator is an instrument for in situ calibration and measurement of chemical species (pH) in aqueous fluids at elevated pressure. The calibration device consists of two interactive systems: a data-logging sensor system and an automated fluid de- livery system. The solid-state pH sensor array is sealed in a flow cell to decrease measurement response time. The automated fluid de- livery system consists of a pump and process control valves, which intermittently deliver seawater samples for measurement. Following measurement cycles, the pH buffer solutions are drawn into the sensor cell to perform two-point calibration. Here, we describe the initial steps in its conversion from a battery-powered instrument for short-term application into a unit that can be used to take full advantage of cabled-powered observatories that will soon be coming online at a number of locations in marine environments. Accordingly, we made use of the continuous power supply and TCP/IP network capability intrinsic to the Monterey Accelerated Research System (MARS)-cabled ocean observatory, Monterey Bay, CA, to reconfigure the unit in anticipation of future seafloor deployments. The flow through system has been further optimized to reduce dead volume effects in the calibration process, while the mechanical system can now better tolerate increases in fluid discharge pressure and flow rate for long-term operations. Furthermore, the sensor head is now equipped with a novel valve to enhance operations in hydrothermal diffuse flow environments. To gain full access to the MARS-cabled observatory, the dc-dc power converter module and Ethernet to serial module are integrated to the electronics. The pH Calibrator has passed the wet node simulator tests in a seawater tank at the Monterey Bay Aquarium Research Institute, moving the project closer to fulfilling long-term objectives for marine studies.