Performance Verification Statement for Precision Measurement Engineering miniDOT 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 PME miniDOT dissolved oxygen sensor using optical luminescence technology. Instrument accuracy and precision for the PME miniDOT 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 miniDOT and reference measurement ranged from -0.339 to 0.126 mg/L over all nine trials. There were no consistent trends in instrument accuracy across salinity ranges. There was a noticeable change in the direction of the offset across temperature ranges with the average offset equal to -0.23 mg/L for the 4 and 15 oC trials compared to a mean offset of 0.11 mg/L for the 30 oC trials. A linear regression of instrument and reference measurements for all trials combined data (n=334; r2 = 0.973; p<0.0001) produced a slope of 0.98 and intercept of 0.020. Instrument offsets and the linear regression omitted comparisons that were clearly impacted by contamination of bubbles of the sparging gas that were trapped on the sensor foil due to its orientation within the tank. The absolute precision, estimated as the standard deviation (s.d.) around the mean, ranged from 0.005 – 0.013 mg/L across trials with an overall average of 0.008 mg/L. Relative precision, estimated as the coefficient of variation (CV% = (s.d./mean)x100), ranged from 0.057 – 0.248 percent across trials with an overall average of 0.098%. 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 measurements was 0.034 (s.d. = 0.107) mg/L for 77 comparisons (out of a potential total of 77). There was a small but statistically significant trend in accuracy over time (slope = -0.002 mg/L/d; r2 = 0.11; p=0.003) indicating very modest perform 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 90 s during high to low transitions and 63 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 PME miniDOT 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%. It should be noted that for this deployment a wiping system was not yet available, so some caution should be used in comparisons against the other field test results. The average and standard deviation of the measurement difference over the total deployment was 0.029 ± 0.072 mg/L with a total range of -0.307 to 0.205mg/L. The drift rate of instrument offset, estimated by linear regression (r2=0.373; p<0.0001), was 0.001 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 = 0.97; p<0.0001) produced a slope of 0.92 and intercept of 1.03. 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 PME miniDOT generated 21,810 observations over the 11 week deployment based on its 5 minute sampling interval; however, only 18,173 of the measurements were considered acceptable based on values that were less than 2 mg/L from any minimum reference sample over a similar timeframe and less than 2 mg/L from continuously monitored DO from a nearby independent data sonde. The accepted data resulted in a data completion rate for this deployment of 83%. The average and standard deviation of the difference between instrument and reference measurements for the deployment was -0.40 ±0.702 mg/L, with the total range of differences between -1.90 to 0.86 mg/L. The calculated drift rate in instrument response for the entire deployment period (using the accepted data) was -0.026 mg/L/d (r2 = 0.83; p<0.001). If we consider only the first 35 days of the deployment before any indication of a malfunction, the drift rate was only -0.009 mg/L/d (r2 = 0.34; p<0.001). A linear regression of the instrument versus reference measurements for the first month (r2 = 0.98; p<0.001) produced a slope of 0.968 and intercept of 0.306. 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 PME miniDOT reported 16,957 observations based on its 10 minute sampling interval over the 17 week deployment. Only two instrument value fell outside of an acceptable data range based on ± 2mg/L from any min-max reference sample for essentially a 100% data completion result. The average and standard deviation of the differences between instrument and reference readings (limited to ± 2.0 mg/L DO; n=128 of 129 potential observations) were 0.201 ± .426 mg/L, with a total range in the differences of -1.7021 to 1.441 mg/L. There was a small, but statistically significant, drift in instrument offset (slope = 0.003 mg/L/d; r2 = 0.05; p=0.009) throughout the deployment period. A linear regression of the instrument versus reference measurements for the first month (r2 = 0.97; p<0.001) had a slope of 1.052 and intercept of -0.258. Overall, the response of the PME miniDOT response showed good linearity overall all three salinity ranges including freshwater, brackish water, and oceanic water; but with slightly higher variability for the oceanic test in Kaneohe Bay. Good agreement between instrument and reference measurements was observed over a wide range of DO connditions varying between 4 to 4 mg/L. A linear regression of the composited data (r2 = 0.998; p<0.0001)) had a slope of 0.987 and intercept of -0.150. The PME miniDOT 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 miniDOT exhibited a negative bias in the colder, low DO hypolimnion and a positive bias in the warm, normoxic surface water over both of the trials. The miniDOT appeared to reach equilibration after 7 minutes but still exhibited final offsets of approximately 0.2 mg/L following the profiled transitions. The range in measurement differences between instrument and reference was -0.24 to 0.75 mg/L for cast 2 and -0.57 to 0.14 mg/L for cast 3 (cast 1 was aborted and redone as cast 3). For Lake Michigan, during cast 1 the miniDOT was well matched during surface equilibration and then exhibited a strong negative bias when rapidly transitioned to the cold high DO hypolimnion. The sensor did not fully equilibrate after 7 minutes and ended at -0.8 mg/L against the reference. For cast 2, there was a negative offset of -0.6 mg/L when equilibrated in the hypolimnion and a positive bias when rapidly transitioned into the warm normoxic surface. The sensor appeared to reach equilibration after 7 minutes but with a final offset of around 0.4 mg/L against the reference. The range in measurement differences between instrument and reference was -2.03 to 0.03 mg/L for cast 1 and -0.72 to 1.63 mg/L for cast 2.