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Ref. No. [UMCES] CBL 2016-012
ACT VS16-03
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 results of the SBS HydroCAT equipped with an optical dissolved
oxygen sensor.
Instrument accuracy and precision for the SBS HydroCAT 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 SBS HydroCAT and reference measurement
for the nine trials ranged from -0.012 to 0.132 mg/L.
There was a noticeable temperature effect in
instrument response with offsets going from a mean of -0.082 to 0.103 across the temperature trials
of 4-15-30
o
C. There was no noticeable pattern to the magnitude or direction of differences across
the salinity.
The absolute precision, estimated as the standard deviation (s.d.) around the mean,
ranged from 0.003 – 0.013 mg/L across trials with an overall average of 0.006 mg/L. Relative
precision, estimated as the coefficient of variation (CV% = (s.d./mean)x100), ranged from 0.027 –
0.265 percent across trials with an overall average of 0.074%.
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 SBS HydroCAT and reference measurements was -0.018 (± 0.298) mg/L for 156
comparisons over 56 days of fluctuating temperature and DO conditions in a deionized water bath.
There was minimal change in accuracy over time (slope = -0.004 mg/L/d) as indicated by a linear
regression of the differences over time ((r
2
= 0.049; p=0.006).
The manufacturer declined from having the SBS HydroCAT evaluated for response time
test as designed in the Verification Protocols, so no results are available.
At Houghton, MI the field test was conducted under the ice over 104 days with a mean
temperature and salinity of 0.7
o
C and 0.01. The SBS HydroCAT operated successfully throughout
the entire 15week deployment and generated 9859 observations based on its 15 minute sampling
interval for a data completion result of 100%.
The measured DO range from our 142 discrete
reference samples was 10.249 to 14.007 mg/L compared to the broader dynamic range of 9.06 to
15.33 mg/L reported by the SBS HydroCAT. The average and standard deviation of the