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Ref. No. [UMCES] CBL 2015-014
ACT VS15-07
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 Xylem EXO 2 pH sensor which
measures pH using a glass bulb electrode and KCl reference electrode. For most tests two pH
sensors were included on the sonde and results are presented separately for each.
The EXO-pH1 operated continuously throughout the entire lab test and generated 6286
pH measurements at 15 minute intervals. The total range of pH measured by the EXO-pH1 was
7.04 to 8.50, compared to the range of our reference pH of 6.943 to 8.502. The EXO-pH1
measurements tracked changing pH conditions among all water sources and temperature ranges,
and consistently responded to the rapid pH shifts from acid/base additions. The average
difference between the EXO-pH1 and reference pH was 0.05 ±0.09 (N=266), with a total range
of -0.31 to 0.19. Initial instrument measurements conducted with the second seawater trial on
the tenth week exhibited a slightly higher offset (mean difference = 0.17 ±0.004; N=7) compared
to measurements from the first week (mean difference = 0.10 ± 0.004; N=28)
The EXO-pH2 also operated continuously throughout the entire lab test and generated
6286 pH measurements at 15 minute intervals. The total range of pH measured by the EXO-pH2
was nearly identical at 7.03 to 8.49, and again in close agreement with the range of the reference
pH of 6.943 to 8.502. The EXO-pH2 measurements tracked reference pH similarly among all
water sources and temperature ranges, and consistently responded to the rapid pH shifts from
acid/base additions. The average difference between the EXO-pH2 and reference pH was 0.04
±0.10 (N=266), with a total range of -0.31 to 0.19. Initial instrument measurements conducted
with the second seawater trial on the tenth week exhibited a slightly higher offset (mean
difference = 0.17 ±0.003; N=7) compared to measurements from the first week (mean difference
= 0.08 ± 0.007; N=28)
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 EXO sonde deployed for this field test
contained two pH probes and both operated continuously over the 28 days of the deployment and