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Ref. No. [UMCES] CBL 2016-015
ACT VS16-06
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 Xylem EXO2 which measures DO optically based on
quenching of a luminescent dye.
Instrument accuracy and precision for the EXO2 DO sensor 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 EXO2 and reference measurement for the
nine trials ranged from -0.053 to 0.429 mg/L. There was a noticeable increase in the magnitude of
the differences across salinity trials with means of -0.01, 0.10, and 0.35 mg/L for salinities of 0, 10,
and 35, respectively. 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
0.016 – 0.264 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 of the
difference between EXO2 and reference measurements was 0.001 (± 0.326) mg/L for 68 of the
potential 75 sample comparisons. A low power fault in the sonde resulted in no data for the final
three days of the test. There was no significant trend (slope = -0.0007 mg/L/d) 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,
maintained commonly at 15
o
C. The calculated τ
90
was 36 s during high to low transitions and 26 s
for low to high transitions covering a DO range of approximately 8 mg/L at a constant 15
o
C.
However we caution that the sensor was programmed to record at 10 s intervals instead of its
highest possible frequency of 1 s, and that may slightly affect the calculated response time.
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 EXO2 operated successfully throughout the entire