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Ref. No. [UMCES] CBL 2016-016
ACT VS16-07
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 In Situ Troll 9000 rugged dissolved oxygen
(RDO) sensor using optical luminescence technology.
Instrument accuracy and precision for the Troll 9000 RDO 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 Troll 9000 and reference measurement
ranged from -0.289 to 0.173 mg/L. There was a small difference in the mean offset for the 4
o
C
trials (mean = 0.05 mg/L) versus the 15 or 30
o
C trials (means = -0.05 and -0.06 mg/L,
respectively). A small response differences was also noted across salinity levels with a mean offset
of 0.05 mg/L for the 0 salinity trials compared to -0.07 and -0.04 mg/L for the 10 and 35 salinity
trials, respectively. A global linear regression of the instrument versus reference measurements for
all trials combined (n=356; r
2
= 0.99; p<0.0001) produced a slope of 1.005 and intercept of -0.075.
The absolute precision, estimated as the standard deviation (s.d.) around the mean, ranged from
0.002 – 0.013 mg/L across trials with an overall average of 0.004 mg/L. Relative precision,
estimated as the coefficient of variation (CV% = (s.d./mean)x100), ranged from 0.021 – 0.268
percent across trials with an overall average of 0.062%.
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 instrument and reference measurements was -0.040 (s.d. = 0.517) mg/L for 75
comparisons. There was no significant trend (linear regression r
2
= 0.009, p=0.41) in accuracy
over time that would indicate performance drift; however the magnitude of offset clearly increased
after approximately 30 days.
A functional response time test was conducted by examining instrument response when
rapidly transitioning between adjacent high (9.6 mg/L) and low (2.0 mg/L) DO water baths,