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Ref. No. [UMCES] CBL 2016-013
ACT VS16-04
4
At Houghton, MI a field deployment test was conducted under the ice over 104 days with a
mean temperature and salinity of 0.7
o
C and 0.01. The HOBO U26 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 average and standard deviation of the
measurement difference over the total deployment was 0.855 ± 0.116 mg/L with a total range of
0.533 to 1.030 mg/L. The drift rate of instrument offset, estimated by linear regression (r
2
=0.73;
p<0.0001), was 0.003 mg/L/d. This rate would include any biofouling effects as well as any
electronic or calibration drift. A linear regression of the instrument versus reference measurements
over the first month (r
2
= 0.98; p<0.0001) produced a slope of 0.855 and intercept of 2.64
indicating mostly an initial calibration offset.
At Chesapeake Biological Lab, a field deployment test was conducted over 78 days with a
mean temperature and salinity of 25.6
o
C and 10.9. The HOBO U26 generated 7270 observations
over the 11 week deployment, however, only 5314 of the measurements were considered
acceptable based on values that were less than 2 mg/L from any reference sample over a similar
timeframe for a data completion result of 73%. The average and standard deviation of the
difference between instrument and reference measurements for the deployment was -0.211
±0.860 mg/L, with the total range of differences between -1.85 to 0.94 mg/L. The calculated drift
rate in instrument response for the entire deployment period using the accepted data was -0.079
mg/L/d (r
2
= 0.77; p<0.001). A linear regression of all data from the first month (r
2
= 0.262;
p<0.001) produced a slope of 0.594 and intercept of 2.93. The high variability indicate that
biofouling impacts were likely present along with an initial calibration offset.
At Kaneohe Bay, HI a field deployment test was conducted over 121 days with a mean
temperature and salinity of 25.8
and 33.4
o
C. The HOBO U26 reported 11296 observations based
on its 15 minute sampling interval, however, only 11179 of the measurements were considered
acceptable based on excluded values that were more than 2 mg/L from reference samples over a
similar timeframe, for a data completion result of 99%. The average and standard deviation of the
differences between instrument and reference readings (limited to ± 2.0 mg/L DO; n=107 of 129
potential observations) were 0.090 ± .576 mg/L, with a total range in the differences of -1.195 to
1.808 mg/L.
There was a small, but statistically significant, drift in instrument offset (slope =
0.009 mg/L/d; r
2
= 0.18; p=0.009) throughout the deployment period but the scatter resulted in a
very low goodness of fit in the regression. This rate would include any biofouling effects as well as
any electronic or calibration drift. A linear regression of the instrument versus reference
measurements (r
2
= 0.550; p<0.0001)) for the first month of data had a slope of 1.595 and intercept
of -3.102.
Overall, the response of the HOBO U26 RDO sensor during field testing showed good
linearity across all three salinity ranges including freshwater, brackish water, and oceanic water.
The response curves were generally consistent across the concentration ranges observed within
each test site (although more variable at CBL) and relatively consistent over the wide range of DO
conditions (4 - 14 mg/L) across sites. A linear regression of the combined one-month field
deployment measurements (r
2
= 0.94; p<0.0001)) had a slope of 1.106 and intercept of -0.895.
The HOBO U26 was evaluated in a profiling field test in the Great Lakes at two separate
locations in order to experience transitions from surface waters into both normoxic and hypoxic
hypolimnion. In Muskegon Lake, the temperature ranged from 21.0
o
C at the surface to 13.5
o
C in
the hypolimnion, with corresponding DO concentrations of 7.8 and 2.8 mg/L, respectively. In
Lake Michigan, the temperature ranged from 21.0
o
C at the surface to 4.1
o
C in the hypolimnion,