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Ref. No. [UMCES] CBL 2016-014
ACT VS16-05
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 Hach
Luminescent Dissolved Oxygen
sensor
incorporated into two different models of the Hydrolab multi-parameter sonde (HL4 and DS5X).
The DS5X sonde includes an anti-biofouling wiping system for extended deployments. The HL4
was used in all laboratory testing, the Great Lakes profiling testing, and the Houghton, MI under
ice deployment. Both sondes were deployed for the extended field mooring tests in Chesapeake
Bay, MD and Kaneohe Bay, HI.
Instrument accuracy and precision for the HL4 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 HL4 and reference measurement ranged from -0.315 to
0.304 mg/L over eight of the nine trials. N
o data was recorded during one of the trials due to an
apparent internal power failure.
A linear regression of the cross plot between instrument and
reference data for all trials combined (n=290; r
2
= 0.99; p<0.0001) produced a slope of 1.021 and
intercept of -0.181. The absolute precision, estimated as the standard deviation (s.d.) around the
mean, ranged from 0.015 – 0.045 mg/L across trials with an overall average of 0.025 mg/L.
Relative precision, estimated as the coefficient of variation (CV% = (s.d./mean)x100), ranged from
0.125 – 0.804 percent across trials with an overall average of 0.380%.
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 HL4 stopped logging
measurements after 7 days into the test. The overall mean of the differences between HL4 and
reference measurements for the 18 comparative observations during the first week was -0.177 (±
0.086) mg/L.
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
for the HL4 was 27 s during high to low
transitions and 26 s for low to high transitions covering the 8 mg/L DO range.