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Ref. No. [UMCES] CBL 2015-012
ACT VS15-05
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 Satlantic SeaFET which measures
pH using an ion-sensitive field effect transistor.
Due to an electronics problem with one of the circuit boards there was no reportable data
for the SeaFET. The instrument was confirmed to be operating over a three day period prior to
deployment within the test tank but then failed immediately upon the start of the test. The failure
was discovered only upon final retrieval so unfortunately no replacement was attempted.
At Moss Landing Harbor the field deployment test was conducted over 28 days with a
mean temperature and salinity of 16.6
o
C and 33. The measured ambient pH range from our 84
discrete reference samples was 7.933 – 8.077. The SeaFET operated successfully over the entire
deployment and generated 2576 measurements with a measured range in ambient pH from 7.795
to 8.106. The average and standard deviation of the measurement difference between the
SeaFET and reference pH for the total deployment was -0.008 ± 0.029 (N=84) with a total range
of -0.107 to 0.063.
At Kaneohe Bay the field deployment test was conducted over 88 days with a mean
temperature and salinity of 24.5
o
C and 34.4. The measured ambient pH range from our 101
discrete reference samples was 7.814 – 8.084. The SeaFET operated successfully over the entire
deployment and generated 4211 observations with a measured range in ambient pH from 7.617
to 8.077 The average and standard deviation of the differences between the SeaFET and
reference pH for the deployment was -0.014 ±0.009 (N=101), with a total range of -0.042 to
0.019.
At Chesapeake Bay the field deployment test was conducted over 30 days with a mean
temperature and salinity of 5.9
o
C and 12.8. The measured pH range from our 107 discrete
reference samples was 8.024 – 8.403. The SeaFET operated successfully over the entire
deployment and generated 2775 observations with a measured range in ambient pH from 7.611