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Ref. No. [UMCES] CBL 2015-009
ACT VS15-02
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 Eureka Manta2 pH sensor which is
a glass electrode with a KCl reference electrode. The Manta2 was submitted for testing after the
Laboratory study was completed under a new Request for Technologies, therefore only results
for the Field testing component are presented.
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. At this site two instrument sondes were deployed
with identical pH sensors, however, one was enclosed in a copper mesh screen to serve as an
anti-fouling measure and one in a plain sensor guard. The Manta2 with the copper screen
operated for 23 consecutive days before experiencing a power failure. Ambient pH measured by
this unit ranged from 7.95 to 8.63. The average and standard deviation of the measurement
difference between the Manta2 and reference pH was 0.258 ± 0.181 with a total range of -0.014
to 0.551 (N=64). The Manta2 with no anti-fouling operated continuously for the entire 28 days
of the deployment, resulting in 2579 observations at 15 minute intervals. Ambient pH measured
by this Manta2 sonde ranged from 8.28 to 8.60. The average and standard deviation of the
measurement difference between this Manta2 and reference pH was 0.512 ± 0.059 with a total
range of 0.333 to 0.584 (N=84).
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 Manta2 sonde operated continuously
throughout the first 67 days of deployment measuring at 30 minute intervals, however, a tunicate
colonized directly on the pH sensor bulb causing it to fracture and the data beginning on January
30
th
were not useable for comparisons (pH instantly dropped from 8.1 to 6.6 indicating failure).
Ambient pH measured by the Manta2 ranged from 7.85 to 8.39. The average and standard
deviation of the measurement difference between instrument and reference was 0.17 ±0.02 with
a total range in the differences of 0.12 to 0.20 (N=75).