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Ref. No. [UMCES] CBL 2016-011
ACT VS16-02
3
EXECUTIVE SUMMARY
The Alliance for Coastal Technology (ACT) conducted a sensor verification study of in situ
dissolved oxygen sensors during 2015-2016 to characterize performance measures of accuracy and
reliability in a series of controlled laboratory studies and field mooring tests in diverse coastal
environments. The verification including several months of Laboratory testing along with three
field deployments covering freshwater, estuarine, and oceanic environments.
Laboratory tests of
accuracy, precision, response time, and stability were conducted at Moss Landing Marine Lab.
A
series of nine accuracy and precision tests were conducted at three fixed salinity levels (0, 10, 35)
at each of three fixed temperatures (5, 15, 30
o
C). A laboratory based stability test was conducted
over 56 days using deionized water to examine performance consistency without active biofouling.
A response test was conducted to examine equilibration times across an oxygen gradient of 8mg/L
at a constant temperature of 15
o
C. Three field-mooring tests were conducted to examine the
ability of test instruments to consistently track natural changes in dissolved oxygen over extended
deployments of 12-16 weeks. Deployments were conducted at: (1) Lake Superior, Houghton, MI
from 9Jan – 22Apr, (2) Chesapeake Bay, Solomons, MD from 20May – 5Aug, and (3) Kaneohe
Bay, Kaneohe, HI from 24Sep – 21Jan. Instrument performance was evaluated against reference
samples collected and analyzed on site by ACT staff using Winkler titrations following the
methods of Carignan
et.al. 1998. A total of 725 reference samples were collected during the
laboratory tests and between 118 – 142 reference samples were collected for each mooring test.
This document presents the performance results of PME miniDOT dissolved oxygen sensor using
optical luminescence technology.
Instrument accuracy and precision for the PME miniDOT was tested under nine
combinations of temperature and salinity over a range of DO concentrations from 10% to 120% of
saturation. The means of the difference between the miniDOT and reference measurement
ranged from -0.339 to 0.126 mg/L over all nine trials. There were no consistent trends in
instrument accuracy across salinity ranges. There was a noticeable change in the direction of the
offset across temperature ranges with the average offset equal to -0.23 mg/L for the 4 and 15
o
C
trials compared to a mean offset of 0.11 mg/L for the 30
o
C trials. A linear regression of
instrument and reference measurements for all trials combined data (n=334; r
2
= 0.973; p<0.0001)
produced a slope of 0.98 and intercept of 0.020. Instrument offsets and the linear regression
omitted comparisons that were clearly impacted by contamination of bubbles of the sparging gas
that were trapped on the sensor foil due to its orientation within the tank. The absolute precision,
estimated as the standard deviation (s.d.) around the mean, ranged from 0.005 – 0.013 mg/L across
trials with an overall average of 0.008 mg/L. Relative precision, estimated as the coefficient of
variation (CV% = (s.d./mean)x100), ranged from 0.057 – 0.248 percent across trials with an
overall average of 0.098%.
Instrument accuracy was assessed under a 56 day lab stability test in a deionized water bath
cycling temperature and ambient DO saturation on a daily basis. The overall mean difference
between measurements was 0.034 (s.d. = 0.107) mg/L for 77 comparisons (out of a potential total
of 77). There was a small but statistically significant trend in accuracy over time (slope = -0.002
mg/L/d; r2 = 0.11; p=0.003) indicating very modest performance drift over the 56 days.