Performance Verification Statement for XYLEM EXO2 Sonde 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 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 oC. The calculated τ90 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 oC. 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 oC and 0.01. The EXO2 operated successfully throughout the entire 5week deployment and generated 9859 observations based on its 15 minute sampling interval for a data completion result of 100%. The measured DO range from our 142 discrete reference samples was 10.249 to 14.007 compared to the full dynamic range of 9.01 to 14.03 mg/L observed by the EXO2. The average and standard deviation of the measurement difference between the EXO2 and reference samples over the total deployment was 0.37 ±0.10 mg/L with a total range of -0.06 to 0.56 mg/L. A drift rate in accuracy, estimated by linear regression (r2=0.85) of the difference across time, was 0.009 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 0.747 and intercept of 3.64 resulting from a rapid change in response accuracy during the first few weeks. At 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 4.37 – 10.86 mg/L compared to the broader dynamic range of 3.11 – 14.85 mg/L reported by the EXO2 over its 6027 accepted observations. The sonde had two brief periods of interruption on July 8th and July 12th, and then quit operating permanently on July 31st for a data completion result of 83% of expected. The average and standard deviation of the measurement difference between the EXO2 and reference samples over the total deployment was 0.15 ±0.22 mg/L with a total range of -0.69 to 0.79 mg/L. There was no significant trend in accuracy over time (slope = 0.0007 mg/L/d; r2 = 0.002) over the deployment. At Kaneohe Bay, HI the field test was conducted over 121 days with a mean temperature and salinity of 25.8 and 33.4 oC. The EXO2 operated successfully throughout the deployment and generated 5653 observations conducted continuously at 30 minute intervals for a data completion result of 100%. The measured DO range from our 142 discrete reference samples was 3.63 – 9.85 mg/L compared to the broader dynamic range of 1.97 – 10.50 mg/L observed by the EXO2. The average and standard deviation of the measurement difference between the EXO2 and reference samples over the total deployment was -0.10 ±0.23 mg/L with a total range of -1.39 to 1.08 mg/L. There was no significant drift in instrument offset (slope = 0.00017 mg/L/d; r2 = 0.0008) throughout the deployment period. Overall, the EXO2 response showed good linearity across all three salinity ranges including fresh, brackish, and oceanic water, covering an ambient DO range of 4 – 14 mg/L. There was however a slightly higher offset for the cold freshwater test in Houghton, MI, and the overall variability was slightly higher for the oceanic test in Kaneohe Bay. The EXO2 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. Note for Muskegon Lake cast 1 was aborted due to bottle misfires and repeated as cast 3. In Muskegon Lake, the EXO2 exhibited a negative bias in the colder, low DO hypolimnion and a positive bias in the warm, high DO surface. Sensor uilibration time was slightly greater going from surface to hypolimnetic conditions. The range in measurement differences between instrument and reference was -0.13 to 0.18 mg/L for cast 2 and -0.28 to 0.23 mg/L for cast 3. In Lake Michigan, the EXO2 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 -0.32 to -0.03 mg/L for cast 1 and -0.20 to 0.11 mg/L for cast 2.