Performance Verification Statement for Sea-Bird Scientific HydroCAT 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 SBS HydroCAT equipped with an optical dissolved oxygen sensor. Instrument accuracy and precision for the SBS HydroCAT 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 SBS HydroCAT and reference measurement for the nine trials ranged from -0.012 to 0.132 mg/L. There was a noticeable temperature effect in instrument response with offsets going from a mean of -0.082 to 0.103 across the temperature trials of 4-15-30 oC. There was no noticeable pattern to the magnitude or direction of differences across the salinity. 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 0.027 – 0.265 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 difference between SBS HydroCAT and reference measurements was -0.018 (± 0.298) mg/L for 156 comparisons over 56 days of fluctuating temperature and DO conditions in a deionized water bath. There was minimal change in accuracy over time (slope = -0.004 mg/L/d) as indicated by a linear regression of the differences over time ((r2 = 0.049; p=0.006). The manufacturer declined from having the SBS HydroCAT evaluated for response time test as designed in the Verification Protocols, so no results are available. 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 SBS HydroCAT 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 measured DO range from our 142 discrete reference samples was 10.249 to 14.007 mg/L compared to the broader dynamic range of 9.06 to 15.33 mg/L reported by the SBS HydroCAT. The average and standard deviation of the measurement difference between the SBS HydroCAT and reference samples over the total deployment was 0.776 ± 0.223 mg/L with a total range of 0.270 to 1.070 mg/L. There was no measurable trend in instrument offset during the entire deployment (linear regression: r2=0.003; p=0.58) as a result of either biofouling effects or calibration drift. A linear regression of the instrument versus reference measurements over the first month (r2 = 0.99; p<0.0001) produced a slope of 0.947 and intercept of 1.04, indicating an initial calibration offset at the beginning of the deployment. 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 SBS HydroCAT operated successfully throughout the entire deployment and generated 7270 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 4.37 – 10.86 mg/L compared to the broader dynamic range of 2.12 to 12.81 mg/L reported by the SBS HydroCAT. The average and standard deviation of the measurement difference between the SBS HydroCAT and reference measurements for the entire deployment was -0.464 ±0.581 mg/L, with the total range of differences between -1.99 to 0.65 mg/L. The calculated drift rate in instrument response for the entire deployment period was -0.079 mg/L/d (r2 = 0.49; p<0.0001). In contrast, the drift rate for the first 35 days of the deployment was much less, averaging only -0.016 mg/L/d (r2 = 0.54; p<0.001). In both cases the 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 = 0.97; p<0.001) produced a slope of 0.985 and intercept of 0.181. No results are available for the Kaneohe Bay, HI field deployment due to a programming error by ACT staff during the deployment set-up. Overall, the SBS HydroCAT response showed good linearity over the two salinity ranges tested which included fresh brackish water. The response curves were generally consistent across the concentration range within a given test site and over the broader range of DO conditions (4 - 14 mg/L) across sites, with the noted result of a likely initial calibration offset for the freshwater test. A linear regression of the composited data (r2 = 0.997; p<0.0001)) had a slope of 1.043 and intercept of -0.261. The manufacturer declined from having the SBS HydroCAT evaluated under the profiling application as designed in the Verification Protocols, so no results are available.