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Ref. No. [UMCES] CBL 2015-013
ACT VS15-06
reference samples was 7.814 – 8.084 compared to a range of 7.678 – 8.220 reported by the
SAMI-pH for 4211 observations conducted continuously at 30 minute intervals. The average
and standard deviation of the measurement difference between the SAMI-pH and reference
samples over the total deployment was -0.014 ±0.016 with a total range in the differences of -
0.068 to 0.025. The useable data return for the deployment was 100%.
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 compared to a range of 8.121 – 8.479 reported by the
SAMI-pH for 1842 observations conducted continuously at 15 minute intervals during the first
20 days of operations. Following this date, the instrument reported 175 non-determined values
and exhibited a noticeable increased offset so the remaining data was considered compromised.
The average and standard deviation of the measurement difference between the SAMI-pH and
reference samples for the period up to April 1 was 0.041 ±0.033 (n=69), with the total range of
differences between -0.086 to 0.114. The reported data return for the deployment was 94% but
again only 69% of the data was considered fully reliable.
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. Two SAMI-pH units were deployed at this test
site, one equipped with MetaCresol Purple (mCP) as the indicator dye and one with Phenol Red
(PR) as the indicator dye. The Phenol Red method was considered to be experimental for the
company at the time and was being tested for freshwater applications. The measured range in
ambient pH for the 98 reference samples was 8.013 – 8.526. The original range in pH reported
by the SAMI-pH (PR) was 6.606 – 9.305 for 2564 observations conducted continuously at 15
minute intervals. After removing fifty-one observations that were identified as outliers based on
a difference between the paired SAMI-pH units of more than two times the mean, the measured
range was reduced to 7.658 – 8.687. The average and standard deviation of the measurement
difference between the SAMI-pH (PR) and reference samples for the edited dataset was 0.021
±0.047 with a range of -0.095 to 0.177. The omitted data represents 1.7 % of the returned data
but represents a more realistic summary of the expected accuracy and uncertainty for the
technology.
The measured range of the SAMI-pH (mCP) samples was 7.813 – 8.405 based on 2665
observations conducted continuously at 15 minute intervals. The average and standard deviation
of the measurement difference between the SAMI-pH (mCP) and reference samples over the
total deployment was range was -0.162 ±0.045 and the total range of the differences was
between -0.333 to -0.068. The useable data return for this unit was 100%.
A summary plot of the SAMI-pH versus dye reference pH for all sites indicated that the
SAMI-pH responded consistently among all test sites which covered a salinity range of
freshwater to full seawater and temperatures from 3-26
o
C. As expected there was a consistent
offset in results reported within the Great Lakes test for the SAMI-pH with MetaCresol Purple
indicator dye compared with the unit operating with the Phenol Red dye indicator. The SAMI-
pH (PR) results closely match the reference samples since ACT also used Phenol Red as the
indicator dye for their analysis.
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
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