Performance Verification Statement for the Satlantic SeaFET pH Sensor

6 / 48

Ref. No. [UMCES] CBL 2015-012
ACT VS15-05
followed by depth profiling (50%), then hand-held portable use (48%), then flow-through
systems (26%). Respondents used a variety of calibration procedures including commercial
buffers (68%), CO2 chemistry (35%), seawater CRMs (23%), pH indicator dyes (18%), and
supplied by manufacturer (13%). The four areas where respondents expressed the greatest
concern over the use of in situ pH sensors were ruggedness (49%), calibration life (46%), level
of measurement uncertainty (43%), and reliability (41%). The complete needs and use
assessment reports can be found at:
INSTRUMENT TECHNOLOGY TESTED
The SeaFET Ocean pH Sensor was developed by Dr. Kenneth Johnson of the Monterey
Bay Aquarium Research Institute (MBARI) and Dr. Todd Martz of the Scripps Institution of
Oceanography, University of California San Diego. Satlantic collaborated with MBARI and
Scripps to make the instrument commercially available to researchers.
The primary sensor element of SeaFET is an ion-sensitive field effect transistor (ISFET).
The advantages of the ISFET include robustness, stability and precision that make it suitable for
ocean pH measurement at low pressure. SeaFET has two potentiometric cells: the internal cell
and the external cell. Both cells are immersed in the sensed medium. The names ‘internal’ and
‘external’ refer to the arrangement of the reference electrodes in each cell.
The internal cell consists of the ISFET as the working electrode and a Ag/AgCl electrode
bathed in a saturated KCl solution/gel as the internal reference electrode. The internal reference
electrode is bathed in a saturated KCl solution/gel so that the chloride concentration that the
electrode ‘sees’ remains relatively constant. The Ag/AgCl electrodes’ primary sensitivity is to
chloride ions. The KCl gel connects to the sensed medium through a porous frit. The reference
electrode electrical potential is proportional to the concentration of chloride of the KCl gel,
which is not expected to vary greatly. The liquid junction at the seawater/KCL interface
generates an electrical potential because ions diffuse through the frit at different rates leading to
a separation of charge.
The external cell also uses the ISFET as the working electrode. The potential of the
external reference electrode varies with the chloride concentration of the sensed medium.
Ordinarily this would not be a good approach for measuring pH because the signal exhibited by
the overall cell potential will be the sum of a chloride signal and a hydrogen/hydroxide ion
signal; however the chloride concentration is easily measured in seawater. The external reference
electrode has been incorporated into the design partly because it does not have a liquid junction
potential, resulting in a more stable reading.
With its on-board data storage capability and internal battery pack, SeaFET can operate
autonomously over long-term deployments. SeaFET has an optional capability of interfacing
with a Sea-Bird SBE37 CTD or an external pump. When integrated with a SBE37 the SeaFET
can perform real-time temperature and salinity corrections. The supplied SeaFETCom software
provides easy set-up and configuration, graphical real-time data display for pre-deployment
checks and interactive sampling, and data re-processing to improve accuracy using ancillary
temperature and salinity measurements.