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Ref. No. [UMCES] CBL 2015-010
ACT VS15-03
EXECUTIVE SUMMARY
The Alliance for Coastal Technology (ACT) conducted a sensor verification study of in
situ pH sensors during 2013 and 2014 to characterize performance measures of accuracy and
reliability in a series of controlled laboratory studies and field mooring tests in diverse coastal
environments. A ten week long laboratory study was conducted at the Hawaii Institute of
Marine Biology and involved week long exposures at a full range of temperature and salinity
conditions. Tests were conducted at three fixed salinity levels (0.03, 22, 35) at each of three
fixed temperatures (10, 20, 30
o
C). Ambient pH in the test tank was allowed to vary naturally
over the first five days. On the sixth day the pH was rapidly modified using acid/base additions
to compare accuracy over an extended range and during rapid changes. On the seventh day the
temperature was rapidly shifted to the next test condition. On the tenth week a repeated seawater
trial was conducted for two days while the temperature was varied slowly over the 10 – 30
o
C
range. Four field-mooring tests were conducted to examine the ability of test instruments to
consistently track natural changes in pH over extended deployments of 4-8 weeks. Deployments
were conducted at: Moss Landing Harbor, CA; Kaneohe Bay, HI; Chesapeake Bay, MD; and
Lake Michigan, MI. Instrument performance was evaluated against reference samples collected
and analyzed on site by ACT staff using the spectrophotometric dye technique following the
methods of Yao and Byrne (2001) and Liu et al. (2011). A total of 263 reference samples were
collected during the laboratory tests and between 84 – 107 reference samples were collected for
each mooring test. This document presents the results of the Idronaut Ocean Seven 305 Plus
CTD which uses a potentiometric pH sensor consisting of two electrochemical cells including
the measuring sensor and the Ag/AgCl reference sensor filled with a saturated KCl gel.
ACT wants to acknowledge that this particular performance verification was not
conducted under ideal conditions. One significant factor was that the company did not have the
resources to send a representative for direct face to face training, and none of the ACT staff had
any prior experience with this instrument. This lack of direct training contributed to one of the
programming errors which caused a loss of test data. In addition, the company acknowledges a
lack of full understanding of the extent of the testing and that refurbishment of the instrument
between test sites could have been done better. It was felt more care should have been taken to
address biofouling and confirmation of the functionality of the reference electrode. It should be
noted the instrument was re-serviced by the company only prior to the Great Lakes field test
since the initial receipt by ACT more than a year earlier. However, the performance verification
was conducted according to the signed testing protocols and at a minimum the instrument was
exposed to some form of CRM prior to deployment to confirm proper working order.
The Idronaut 305 Plus was not operating at the start of the laboratory deployment due to a
battery short, but the error was not discovered and corrected until January 13
th
, which was about
6 days into the brackish water test. Because no seawater results were obtained initially, a brief
additional seawater exposure test was performed at the end of the study. After redeployment on
January 13th, the instrument operated continuously for the next 33 days and generated 3162 pH
measurements at 15 minute intervals. The total range of pH measured by the Idronaut 305 Plus
was 6.899 to 8.276, compared to the range of our reference pH of 6.943 to 8.502. The Idronaut
305 Plus measurements tracked changing pH conditions among all water sources and
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