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Ref. No. [UMCES] CBL 2015-008
ACT VS15-01
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 265 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 Campbell Scientific Instruments pH
sensor which measures pH using an ion-sensitive field effect transistor.
The CSI pH sensor operated continuously throughout the entire lab test and generated
6294 pH measurements at 15 minute intervals. The total range of pH measured by the CSI was
6.805 to 8.560, compared to the range of our reference pH of 6.943 to 8.502. In general, the CSI
measurements tracked changing pH conditions among all water sources and temperature ranges
including the rapid pH shifts from acid/base additions, but the magnitude and direction of the
offset changed for each water type (Fig.3). The mean of the differences between the CSI pH
measurement and reference pH was -0.019 ±0.319 (N=265), with a total range of -0.668 to
0.624. Instrument measurements conducted after ten weeks with the second seawater trial
showed an increase offset (mean difference = 0.345 ±0.020; N=9) compared to measurements
from the first week (mean difference = 0.084 ± 0.055; 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 CSI operated continuously over the entire
deployment and generated 2579 observations with a measured range in ambient pH from 7.759
to 7.960. The average and standard deviation of the measurement difference between the CSI
and reference pH for the deployment was -0.152 ± 0.023 (N=84), with a total range of -0.253 to -
0.115.
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 CSI operated continuously over 88 days and
generated 4211 observations with a measured range in ambient pH from 7.886 to 8.470. The
average and standard deviation of the differences between CSI and reference pH was 0.221
±0.066 (N=101), with a total range in the differences of 0.076 to 0.316.