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Ref. No. [UMCES] CBL 2015-011
ACT VS15-04
3
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 In-Situ Troll 9500 which measures
pH using a combination, single-junction electrode that contains a glass pH sensing bulb,
replaceable junction and refillable reference saturated KCl electrolyte solution.
The Troll 9500 operated continuously throughout the entire lab test and generated 6283
pH measurements at 15 minute intervals. The total range of pH measured by the Troll 9500 was
7.12 to 8.58, compared to the range of our discrete reference samples of 6.943 to 8.502. The
Troll 9500 tracked changing pH conditions among all water sources and temperature ranges
including the rapid pH shifts from acid/base additions, but the magnitude of the offset changed
for each water type (Fig.3). For the complete Lab test, the mean of the differences between the
Troll 9500 measurement and reference pH was 0.17 ±0.07 (N=263), with a total range of 0.02 to
0.31. Instrument measurements conducted with the second seawater trial after ten weeks
showed a slight increase in the offset (mean difference = 0.31 ±0.004; N=8) compared to
measurements from the first week (mean difference = 0.15 ± 0.01; N=27).
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 Troll 9500 experienced an internal
malfunction approximately three hours after logging was initiated on August 26
th
at 18:00 hours.
No useable data was produced to compare to the reference dye results. The Troll 9500 measured
pH from 7.94 to 7.98 during the first three hours indicating good operating condition, but then
showed an abrupt increase to nearly 14, indicating some internal malfunction.
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 Troll 9500 only operated over the first three
days of the deployment and generated 154 observations with a range in ambient pH from 7.93 to
8.17. (n=10 observations out of a possible 101 for the entire deployment). The average and