Performance Verification Statement for Onset’s HOBO U26 Dissolved Oxygen Sensors.

Abstract

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 oC). 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 oC. 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; r2 = 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 (r2 = 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 oC. The calculated τ90 was 27 s during high to low transitions and 26 s for low to high transitions covering the 8 mg/L DO range. At Houghton, MI a field deployment test was conducted under the ice over 104 days with a mean temperature and salinity of 0.7 oC 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 (r2=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 (r2 = 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 oC 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 (r2 = 0.77; p<0.001). A linear regression of all data from the first month (r2 = 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 oC. 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; r2 = 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 (r2 = 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 (r2 = 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 oC at the surface to 13.5 oC 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 oC at the surface to 4.1 oC in the hypolimnion, with corresponding DO concentrations of 8.6 and 12.6 mg/L, respectively. Two profiling trials were conducted at each location. The first trial involved equilibrating test instruments at the surface (3m) for ten minutes and then collecting three Niskin bottle samples at one minute intervals. Following the third sample, the rosette was quickly profiled into the hypolimnion where samples were collected immediately upon arrival and then each minute for the next 6 minutes. The second trial was performed in the reverse direction. For Muskegon Lake, the range in measurement differences between instrument and reference was -0.15 to 1.78 mg/L for cast 2 and - 0.92 to -0.17 mg/L for cast 3 (cast 1 was aborted and redone as cast 3. For Lake Michigan, the range in measurement differences was -3.03 to 0.40 mg/L for cast 1 and -0.81 to 3.05 mg/L for cast 2. The equilibration rate of the sensor differed across each of the profiles with no obvious trend across temperature or DO levels.