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Ref. No. [UMCES] CBL 2016-013
ACT VS16-04
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 Onset’s HOBO U26 Dissolved Oxygen Logger
using the RDO® Basic Technology developed by In-Situ, Inc.
Instrument accuracy and precision for the HOBO U26 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 HOBO U26 and reference measurement ranged from
0.047 to 0.314 mg/L over all nine trials. There was a slight decrease in the magnitude of the
differences with increase of both temperature and salinity levels.
A linear regression of instrument
and reference measurements for all trials combined (n=358; r
2
= 1.00; p<0.0001) produced a slope
of 1.037 and intercept of -0.084.
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 0.036 – 0.268 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 of differences
between instrument and reference measurements was 0.041 (s.d. = 0.339) mg/L for 154
comparisons (out of a potential total of 156). A regression of measurement difference over time
showed a minor drift in response with a slope of -0.004 mg/L/d (r
2
= 0.049; p=0.006). But if two
large negative responses near the end of the test are omitted the regression is not significant
.
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,
maintained commonly at 15
o
C. The calculated τ
90
was 27 s during high to low transitions and 26 s
for low to high transitions covering the 8 mg/L DO range.