Sea Technology

FEB 2017

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Page 17 of 72 February 2017 / st 17 major components of the sensor needed to be modified from the original Durafet design to achieve the needed pressure tolerance: the conductive polymer counter electrode was replaced with a titanium tube that surrounded the reference electrode; the ISFET die was placed in a stress-minimizing polyetheretherketone (PEEK) housing; and the liquid/gel Ag/ AgCl reference electrode was replaced with a solid state Ag/ AgCl reference electrode. Calibrating the Deep SeaFET presents challenges in that the stability of the instrument over the ranges of tempera- ture and pressure expected while deployed on a profiling float has to be demonstrated at the same time. To achieve the necessary accuracy and precision, it is not sufficient to characterize the instrument in separate temperature and pressure regimes. Thus, in addition to the substantial me- chanical modifications to the Durafet design, a custom tem- perature-controlled pressure vessel had to be designed and built. Calibrations are performed in a computer-controlled titanium calibration vessel filled with a 0.01 N HCl acid solution of known thermodynamic properties to allow for correction of the sensor response to both temperature and pressure. The calibration procedure runs through a matrix of four temperature and seven pressure points over the range of 5 to 35o C and 0 to 3,000 psi to build the calibration equations for each instrument. One of the key limitations of standard glass electrode pH sensors is their insufficient measurement stability in all me- dia. The Durafet technology by contrast has been proven to be stable in a variety of industrial processes, however, little work had been done to demonstrate its stability in seawa- ter. To determine the measurement stability in seawater, a Deep SeaFET was placed in a continuous-flow tank filled with filtered seawater over a four-month period (May to Sep- tember 2016). The Deep SeaFET was run continuously and delivered pH measurements every 20 seconds throughout the experiment. Validation samples were taken daily and pH was deter- mined in the grab samples using standard pH indicator dye spectrophotometry. The coherence throughout the experi- ment was excellent, with the average difference between the Deep SeaFET and the validation samples within ± 0.005 pH over the entire four-month experiment. The response of the instrument to rapid changes in pH was demonstrated by exchanging the seawater three times over the course of the experiment. Each exchange exposed the instrument to a new seawater sample with a different salinity, tempera- ture and dissolved gas concentration. Inadvertently, we also demonstrated the response of the instrument to the effect of biological activity as the fourth batch of seawater was contaminated. While pH declined much more rapidly over the last week of the experiment, the Deep SeaFET measure- ments tracked the grab samples as well as it had during the earlier, more stable periods. Field Validation Our laboratory work has demonstrated that the Deep SeaFET is stable in seawater for much longer periods than previous methods. One consideration to achieve this stabil- ity in the field is that the solid state Ag/AgCl reference elec- trode requires conditioning in seawater for at least one week prior to deployment to maintain the accuracy of the sensor in natural seawater. S E N S O R S F O R R E S E A R C H & D E V E L O P M E N T | A E R O S PA C E & D E F E N S E | w w w. P C B . c o m / u n d e r w a t e r EXTEND THE LIFE OF YOUR TESTING WITH MINIATURE SENSORS ■ Ideal for use in the lab on space restricted models ■ Robust enough for continuous monitoring at sea ■ Permanently attached submersible cables UNDERWATER TESTING COSTS TIME AND MONEY UNDERWATER TESTING COSTS TIME AND MONEY

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