Performance Verification Statement for JFE AROUSB AND AROW-USB 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 included 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 (4, 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 results of two different models of the JFE Advantech RINKO optical dissolved oxygen sensors (AroUSB and AroW- USB). Both models were tested in all Laboratory trials and the fast-response AroUSB was used in the field profiling application, while the wiper based AroW-USB was used in the extended field mooring applications. Instrument accuracy and precision for the AroUSB and AroW-USB sensors were tested under nine combinations of temperature and salinity over a range of DO concentrations from 10% to 120% of saturation. The laboratory testing set-up did result in bubbles from the sparging gases used to change DO levels occasionally becoming trapped on the sensor foil and those data where noted were excluded from any comparisons to reference samples. The means of the difference between the AroUSB and reference measurement for the nine trials ranged from -0.277 to 0.265 mg/L. A linear regression of the accepted data (n=377; r2 = 0.965; p<0.0001) produced a slope of 1.015 and intercept of 0.098. For the AroUSB, the absolute precision, estimated as the standard deviation (s.d.) around the mean, ranged from 0.002 – 0.014 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 0.013 – 0.278 percent across trials with an overall average of 0.058%. The means of the difference between the AroW-USB and reference measurements ranged from -0.277 to 0.134 mg/L across all trials. A linear regression of the accepted data (n=257; r2 = 0.976; p<0.0001) produced a slope of 0.969 and intercept of 0.114. The absolute precision for the AroW-USB were ranged from 0.001 – 0.012 across trials, with an overall average of 0.004 and the relative precision ranged from 0.017 – 0.247 percent across trials with an overall average of 0.051%. For the 56 day lab stability test, the overall mean of the differences between AroUSB and reference measurements was 0.001 (± 0.326) mg/L. There was no significant trend in accuracy over time (slope = -0.0007 mg/L/d) that would indicate any type of performance drift over the duration. The overall mean of the differences between AroW-USB and reference measurements was -0.154 (± 0.319) mg/L. There was a minor drift in instrument accuracy over the deployment (slope = -0.006 mg/L/d; r2=0.17) but the goodness of fit was low due to several outliers.For the lab-based functional response time assessment, the calculated τ90 for the AroUSB was 11.8 s during high to low transitions and 7.1 s for low to high transitions covering a DO range of approximately 8 mg/L at a constant 15 oC. However, as noted in the report we incorrectly programmed the sampling rate to 10 seconds which would have a direct impact on the calculated response rate. For the AroW-USB the calculated τ90 was 209 s during high to low transitions and 284 s for low to high transitions for the same conditions. At Houghton, MI the field test was conducted under the ice over 104 days with a mean temperature and salinity of 0.7 oC and 0.01. The measured DO range from our 118 discrete reference samples was 10.25 – 14.01 mg/L compared to a range of 8.669 – 15.076 mg/L reported by the AroW- USB over its 9859 observations conducted continuously at 15 minute intervals. The useable data return for the deployment was 100%. The average and standard deviation of the measurement difference between the AroW-USB and reference samples over the total deployment was 0.170 ±0.057 mg/L with a total range of 0.055 to 0.309 mg/L. A drift rate in instrument response, estimated by linear regression (r2=0.325, p<0.001) of the difference across time, was -0.001 mg/L/d but directionally getting closer to the Winkler reference values. At the Chesapeake Biological Lab, the field test was conducted over 78 days with a mean temperature and salinity of 25.6 oC and 10.9. The measured DO range from our 142 discrete reference samples was 4.370 – 10.858mg/L compared to a range of 2.610 – 14.510 mg/L reported by the AroWUSB over its 7270 continuous observations conducted at 15 minute intervals. The data completion rate for this deployment was 100%. The average and standard deviation of the measurement difference between the AroW-USB and reference samples over the total deployment was -0.056 ±0.131 mg/L with a total range of -0.375 to 0.392 mg/L. There was minor trend in response accuracy over the deployment (slope = -0.002 mg/L/d; r2 = 0.16) but with a low predictive fit. At Kaneohe Bay, HI the field test was conducted over 121 days with a mean temperature and salinity of 25.8 °C and 33.4. The measured DO range from our 129 discrete reference samples was 3.63 – 9.85 mg/L compared to a range of 2.329 – 10.996 mg/L reported by the AroW-USB. Fourteen percent (785 of 5653) of the continuous 30 minute observations fell more than 2 mg/L outside of a natural ambient range as determined by the pattern of Winkler reference samples and were excluded from statistical comparisons. For the accepted data (n=75 of a potential 129 comparisons), the average and standard deviation of the measurement difference between the AroW-USB and reference samples over the total deployment was 0.367 ±0.637 mg/L with a total range of -0.720 to 1.991 mg/L. The drift rate in the instrument offset based on linear regression (r2 = 0.74) was 0.165 mg/L/d throughout the deployment period. Overall, the response of the AroW-USB during field testing showed good linearity across all three salinity ranges including freshwater, brackish water, and oceanic water. The accuracy of the response curve was quite consistent across the concentration ranges observed within each test site and relatively consistent over the wide range of DO conditions (4 - 14 mg/L) across sites. The Aro-USB 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 hird sample, the rosette was quickly profiled into the hypolimnion were samples were collected immediately upon arrival and then each minute for the next 6 minutes. The second trial was performed in the reverse direction. Note for Muskegon Lake cast 1 was aborted due to bottle misfires and repeated as cast 3. In Muskegon Lake, the Aro-USB exhibited a negative bias in the colder, low DO hypolimnion and a positive bias in the warm, high DO surface. Sensor equilibration time was slightly greater going from surface to hypolimnetic conditions. The range in measurement differences between instrument and reference was -0.42 to 0.34 mg/L for cast 2 and -0.75 to 0.27 mg/L for cast 3. In Lake Michigan, the Aro-USB exhibited a positive bias in both portions of the water column but the magnitude was higher in the cold high DO hypolimnion. Sensor equilibration time was similar between both trials, whether equilibrated at surface or depth. The range in measurement differences between instrument and reference was -0.16 to 0.53 mg/L for cast 1 and 0.18 to 0.50 mg/L for cast 2.