Performance Verification Statement for Seabird Scientific HydroCycle Phosphate Analyzer.

Abstract

The Alliance for Coastal Technology (ACT) conducted a sensor verification study of in situ nutrient analyzers during 2016 to characterize performance measures of accuracy, precision and reliability. The verification including a week of laboratory testing along with three moored field deployments in freshwater, estuarine, and oceanic coastal environments. Laboratory tests of accuracy, precision, and range were conducted at the University of Maryland’s Chesapeake Biological Laboratory (CBL) in Solomons, MD. A series of five tests were conducted to evaluate performance under controlled challenge conditions including: concentration range, temperature, salinity, turbidity, and dissolved organic carbon. All laboratory tests were conducted in 250 L polypropylene tanks using RO water as the initial matrix, within a temperature controlled room. Instruments sampled from a common, well-mixed, test tank maintained at a documented level of known challenge condition. Instruments were set-up by the manufacturer daily prior to the start of each individual laboratory test, exposed to each test condition for a period of three hours, and programmed to sample at a minimum frequency of 30 minutes. Reference samples were collected every 30 minutes for five timepoints during corresponding instrument sampling times for each test. For the laboratory concentration range challenge the absolute difference between the HydroCycle-PO4 and reference measurement across all timepoints for trials C0 – C5 ranged from 0.0163 to 0.0145 mgP/L, with a mean of -0.0039 ±0.0090 mgP/L. A linear regression of the measurement difference versus concentration was not significant (p=0.36; r2=0.03), however measurement offsets were increasingly negative between C0 and C4, at which point there was a large reversal and the offset became positive. An assessment of precision was performed by computing the standard deviations and coefficients of variation of the five replicate measurements for C1 – C5 concentration trials. The standard deviation of the mean ranged from 0.0005 to 0.0020 mgP/L across the five trials, and the coefficient of variation ranged from 0.14 to 5.78 percent. For the laboratory temperature challenge at 5 oC, the absolute difference between instrument and reference measurement across all timepoints for trials C2 – C4 ranged from -0.0140 to -0.0046 mgP/L, with a mean of -0.0087 ±0.0032 mgP/L. Measurement differences were significantly different for the C2 and C4 trials at 5 versus 20 oC, but the temperature effect was in the opposite direction between those trials. The measurement difference at C3 was nearly identical for the two temperatures. Therefore no clear pattern of a temperature effect on accuracy was observed. For the laboratory salinity challenge performed at the C3 concentration level, the absolute difference between instrument and reference measurement across all timepoints for the three added salinity levels ranged from -0.0124 to 0.0086 mgP/L, with a mean of -0.0018 ±0.0053 mgP/L. There was a statistically significant response to increased salinity with the offsets increasing in a positive direction as salinity increased. A linear regression of the measurement differences versus salinity (p<0.0001; r2=0.70) had a slope of 0.0005 and intercept of- 0.013. The average offset at a salinity of 30 was 0.015 mgP/L higher than at zero salinity. For the laboratory turbidity challenge, performed at the C3 concentration level, the absolute difference between instrument and reference measurement across all timepoints for the two added turbidity levels ranged from -0.0226 to - 0.0113 mgP/L, with a mean of -0.0170 ±0.0047 mgP/L. A linear regression of the measurement differences versus turbidity was significant (p=0.0001; r2=0.68), with a slope of -0.0005 and intercept of -0.010, however the trend line was clearly forced by the large decrease at 100 NTU where the offset was 0.009 mgP/L more negative (under-predicted) than results observed at the 0 and 10 NTU trials. For the laboratory DOC challenge, performed at the C3 concentration level, the absolute difference between instrument and reference measurement across all timepoints for the two added DOC levels ranged from -0.0112 to -0.0074 mgP/L, with a mean of -mgP/L. A linear regression of the measurement differences versus DOC concentration was highly significant (p=0.0005; r2=0.62) with a slope of 0.0002 and intercept of -0.013. The measurement difference generally became less negative as DOC concentration increased. A 32 day field deployment occurred from May 26 through June 27 in the Maumee River, at the facilities of the Bowling Green, Ohio Water Treatment Plant. The HydroCycle-PO4 operated continuously during the entire 32 day deployment sampling at hourly intervals, but of the 763 possible data points 666 were flagged by the instrument as bad. Accepting the flagged data, and omitting 131 outliers (<-0.004 or >0.250), the HydroCycle-PO4 generated 632 observations out of a possible 763 for a data completion result of 82.8%. The average and standard deviation of the measurement difference between instrument and reference PO4 measurements for each matched pair (n=43 of a possible 51 observations) over the total deployment was -0.022 ± 0.029 mgP/L with a total range of -0.110 to 0.033 mgP/L. There was no significant trend in the measurement difference over time as estimated by linear regression (p= 0.20; r2=0.04). A linear regression of instrument versus reference measurement was not significant (p=0.93; r2 = 0.0002) and the HydroCycle-PO4 generally under-predicted concentrations. An 84 day moored field test was conducted in Chesapeake Bay from July 18 to October 10, 2016. The HydroCycle-PO4 had issues communicating with the datalogger, and lost data for the first two days of the deployment. The instrument then stopped reporting data on 9/19 due to the power cable getting frayed by rubbing against the floating dock. The HydroCycle reported 728 of a possible 730 measurements while operational. The average and standard deviation of the measurement difference between instrument and reference PO4 measurements for each matched pair (n=71 of a possible 103 observations) over the total deployment was -0.005 ±0.005 mgP/L, with the total range of differences between -0.018 to 0.003 mgP/L. There no significant trend in measurement difference over time as estimated by linear regression (p=0.89; r2=0.0003) over the deployment period. A linear regression of the data was significant (p<0.0001; r2 = 0.56), but with a slope of only 0.459 and intercept of 0.003. The HydroCycle-PO4 did not accurately measure concentrations above 0.015 mgP/L. A one month long moored field test was conducted in Kaneohe Bay from October 3, 2016 to November 2, 2016. The HydroCycle-PO4 operated successfully for the entire 29 days of the deployment sampling at hourly intervals, and returning 720 of a possible 720 measurements for a data completion result of 100%. However one value was omitted as an outlier due to its value being more than 3x higher than the maximum reference value. The average and standard deviation of the differences between instrument and reference readings over the entire deployment (n=70 out of a possible 71) was -0.0025 ± 0.0012 mgP/L, with a total range in the differences of -0.0048 to 0.0003 mgP/L. There was a small but statistically significant trend in the measurement difference over time (p<0.0001; r2 = 0.54) during the deployment, with a slope of -0.0001 mgP/L/d. A linear regression of instrument versus reference measurements was not significant (p=0.065; r2 = 0.063) and HydroCycle-PO4 did not accurately differentiate concentrations within this fairly narrow 0.004 mgP/L range.