Performance Verification Statement for Hach Hydrolab DS5X and HL4 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 Hach Luminescent Dissolved Oxygen sensor incorporated into two different models of the Hydrolab multi-parameter sonde (HL4 and DS5X). The DS5X sonde includes an anti-biofouling wiping system for extended deployments. The HL4 was used in all laboratory testing, the Great Lakes profiling testing, and the Houghton, MI under ice deployment. Both sondes were deployed for the extended field mooring tests in Chesapeake Bay, MD and Kaneohe Bay, HI. Instrument accuracy and precision for the HL4 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 HL4 and reference measurement ranged from -0.315 to 0.304 mg/L over eight of the nine trials. No data was recorded during one of the trials due to an apparent internal power failure. A linear regression of the cross plot between instrument and reference data for all trials combined (n=290; r2 = 0.99; p<0.0001) produced a slope of 1.021 and intercept of -0.181. The absolute precision, estimated as the standard deviation (s.d.) around the mean, ranged from 0.015 – 0.045 mg/L across trials with an overall average of 0.025 mg/L. Relative precision, estimated as the coefficient of variation (CV% = (s.d./mean)x100), ranged from 0.125 – 0.804 percent across trials with an overall average of 0.380%. 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 HL4 stopped logging measurements after 7 days into the test. The overall mean of the differences between HL4 and reference measurements for the 18 comparative observations during the first week was -0.177 (± 0.086) mg/L. 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 for the HL4 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 the field test was conducted under the ice over 104 days with a mean temperature and salinity of 0.7 oC and 0.01. During the pre-tank test the HL4 started reporting errors; the manufacturer was notified but could not get another sensor to Houghton in time for deployment. It was decided to try the malfunctioning instrument, but it returned no data. The DS5X was also not supplied to this site for testing so no instrument results are available for this field test. 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 HL4 was deployed on shore power due to the length of the deployment. Over the 4th of July weekend, the cable was severed underwater, shorting the instrument. Upon return to the manufacturer, only 5 days of data was recovered from the instrument, however, it is not certain whether the instrument stopped functioning on 5/25 or whether data was lost due to the impact of the cord being severed and shorting out the instrument. The DS5X sonde was also deployed at this field sit but a programming error by ACT personnel resulted in the instrument not being fully enabled for the deployment. The average and standard deviation of the measurement difference over the abbreviated deployment range was 0.685 ± 0.322 mg/L with a total range of -0.12 to 1.06 mg/L. The higher than expected offset may indicate the instrument was malfunctioning even at the onset of 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 HACH HL4 stopped functioning on 10/25/15, 33 days into the deployment resulting in a 26% data completion rate. The measured DO range from our 129 discrete reference samples was 3.63 – 9.85 mg/L compared to a range of 2.02 to 10.88 mg/L reported by the HL4. The average and standard deviation of the differences between instrument and reference readings (n=39 of 129 potential observations) were 0.217± .322 mg/L, with a total range in the differences of -0.725 to 0.769 mg/L. The drift in instrument response showed no statistically significant trend over time based on a linear regression of the data (slope = 0.0007 mg/L/d; r2 = 0.0004; p=0.91). A linear regression of the instrument versus reference data (r2 = 0.95; p<0.0001)) had a slope of 1.115 and intercept of -0.514. The HACH DS5X reported data throughout the entire deployment and generated 2827 observations based on its 60 minute sampling interval over the 17 week deployment. However, only 2434of the measurements were considered acceptable based on our approach of excluding values that were more than 2 mg/L from reference sample over a similar timeframe. The accepted data resulted in a data completion rate for this deployment of 86%. Accepted DO measurements by the DS5X ranged from 1.88 to 10.97 mg/L. The average and standard deviation of the differences between instrument and reference readings (limited to ± 2.0 mg/L DO; n=53 of 129 potential observations) were 0.62 ± 0.744 mg/L, with a total range in the differences of -1.923 to 1.970 mg/L. There was a small, but statistically significant, drift in instrument offset over time (slope = 0.019 mg/L/d; r2 = 0.52; p<0.0001). A linear regression of the instrument versus reference data (r2 = 0.872; p<0.0001)) had a slope of 1.143 and intercept of -0.727. Overall, the response of the Hach LDO sensors during field testing were generally consistent across the concentration range within a given test site, as well as over the wide range of DO conditions (4 - 14 mg/L) across sites, regardless of temperature or salinity. A linear regression of the accepted instrument versus reference data (r2 = 0.91 p<0.0001) for the field tests had a slope of 1.151 and intercept of -0.725. The HL4 was evaluated in a profiling field test in the Great Lakes at two separate locations in order to experience both normoxic and hypoxic hypolimnion. In Muskegon Lake, the mperature 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. 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. For Muskegon Lake, the range in measurement differences between instrument and reference was -0.07 to 0.16 mg/L for cast 2 and -0.38 to 0.11 mg/L for cast 3 (cast 1 was aborted and redone as cast 3). For both profiling transitions the HL4 appeared to equilibrate to the new ambient DO conditions by the third sample. For Lake Michigan, the range in measurement differences between instrument and reference was -0.05 to 0.54 mg/L for cast 1 and -0.14 to 0.32 mg/L for cast 2. The equilibration rate differed among the two profiling directions at this site, occurring by 3 minutes for cast 1, but continual drift through the 7th minute during cast 2.