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Ref. No. [UMCES] CBL 2015-014
ACT VS15-07
4
each generated 2575 observations at 15 minute intervals. The range in ambient pH measured by
the EXO-pH1 was 7.96 to 8.27 and for the EXO-pH2 was 8.10 to 8.48. The average and
standard deviation of the difference between EXO-pH1 and reference pH over the total
deployment was 0.13 ± 0.04 with a total range of -0.01 to 0.23. The average and standard
deviation of the difference between EXO-pH2 and reference pH over the total deployment was
0.29 ± 0.04 with a total range of 0.15 to 0.41.
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 sonde (which contained two pH sensors)
operated for the first 16 days, but by November 30
th
the battery voltage had dropped to 4.7 volts
and the sonde stopped measuring. Both probes recorded 1445 observations measured at 15
minute intervals during the first 16 days of operation. Ambient pH measured by the EXO-pH1
ranged from 7.90 to 8.33 and for the EXO-pH2 from 7.79 to 8.22. The average and standard
deviation of the difference between EXO-pH1 and reference pH over the total deployment was
0.20 ± 0.02 (N=29), with a total range of 0.17 to 0.23.
The average and standard deviation of
the difference between EXO-pH2 and reference pH over the total deployment was 0.08 ± 0.05
(N=29), with a total range of -0.03 to 0.14.
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. Only one pH sensor was deployed on the EXO sonde for
this deployment. The EXO operated successfully over the entire 30 day deployment and
generated 2759 pH measurements at 15 minute intervals. Ambient pH measured by the EXO
ranged from 8.14 to 8.52. The average and standard deviation of the measurement difference
between the EXO and reference pH was 0.16 ±0.04 (N=107), with the total range of differences
from 0.05 to 0.27.
At Lake Michigan the field deployment test was conducted over 29 days with a mean
temperature and salinity of 21.2
o
C and 0.03. The measured ambient pH range from our 98
discrete reference samples was 8.013 to 8.526. The EXO sonde (which contained two pH
probes) operated continuously over the 29 days of the deployment and each probe generated
2661 pH measurements at 15 minute intervals. The range in ambient pH measured by the EXO-
pH1 was 7.84 to 8.60 compared to 7.88 to 8.61 for the EXO-pH2. The average and standard
deviation of the difference between EXO-pH1 and reference pH over the total deployment was -
0.06 ± 0.05 (N=98), with a total range of -0.22 to 0.04.
The average and standard deviation of
the difference between EXO-pH2 and reference pH over the total deployment was -0.02 ± 0.04
(N=98), with a total range of -0.17 to 0.05.
A comparison of the EXO pH versus reference pH across all sites indicated that the
response for the HI and CBL field tests consistently tracked ambient pH with a noted offset of
approximately 0.13 pH units that represented the expected difference from the NBS buffer
calibrated sonde versus the pH
total
scale measured with the dye reference. In contrast, the
freshwater Great Lakes test showed the expected 1:1 relationship. There was no obvious reason
for the much greater offsets observed during the Moss Landing field test.
Lastly, it is worth emphasizing that the continuous 15 – 30 minute time-series provided
by the test instrument was able to resolve a significantly greater dynamic range and temporal
resolution than could be obtained from discrete reference samples. Continuous in situ