Performance Verification Statement for Real Tech Real Nitrate Analyzer GL Series.

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 Real Tech Real-NO3 and reference measurement across all timepoints for trials C0 – C5 ranged from 0.217 to 0.490 mgN/L, with a mean of 0.185 ±0.168 mgN/L. A linear regression of the measurement difference versus concentration was significant (p=0.0192; r2=0.193), but with a low regression coefficient due to a reversal in direction for the C4 trial. In general, the Real-NO3 increasingly over-predicted concentrations as they increased in the test. 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 0.010 to 0.022 mgN/L across the five trials, and the coefficient of variation ranged from 0.20 to 6.47 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 -0.0880 to 0.4381 mgN/L, with a mean of 0.056 ±0.115 mgN/L. Measurement differences at both C2 and C3 were significantly lower at 5 oC (0.017 and 0.058) versus 20 oC (0.020 and 0.237) (p<0.01). Differences were not statistically significant across temperatures at the C4 level. Similar to test results at 20 oC, the measurement offset increased in a positive direction as concentration increased. 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 0.146 to 0.483 mgN/L, with a mean of 0.272 ±0.095 mgN/L. A linear regression of the measurement differences versus salinity was significant (p=0.004; r2=0.38) with a slope of 0.005 and intercept of 0.184. The average offset at salinity 30 was 0.16 mgN/L higher than the average for the 10 and 20 salinity trials. 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 0.028 to 0.135 mgN/L, with a mean of 0.096 ±0.036 mgN/L. The effect of turbidity on measurement accuracy was mixed, when compared against RO water results, however, the magnitude of over-prediction approximately doubled between the 10 and 100 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 0.099 to 0.482 mgN/L, with a mean of 0.292 ±0.193 mgN/L. The measurement difference increased positively by a factor of four between the 1 and 10 DOC trials. A linear regression of the measurement differences versus DOC concentration was significant (p=0.008; r2=0.43), with a slope of 0.013 and intercept of 0.118. 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 Real-NO3 operated successfully during 31 of the total 32 day deployment, sampling at 5 minute intervals. The instrument shut down on 5/31 and was rebooted on 6/1 per manufacturer’s instructions resulting in the loss of one day of data. The Real-NO3 generated 8827 accepted observations out of a possible 9156 for a data completion result of 96.4%. In total, 11 were omitted as outliers due to extreme range (<-0.01 or >25 mgN/L) and 318 values were missing from the inoperable period. The average and standard deviation of the measurement difference between instrument and reference NO3 measurements for each matched pair (n=47 of a possible 51 observations) over the total deployment was 0.312 ± 1.029 mgN/L with a total range of -3.35 to 1.15 mgN/L. There was no significant trend in measurement difference over time as estimated by linear regression (p= 0.28; r2=0.026). A linear regression of instrument versus reference measurement was highly significant (p<0.0001; r2 = 0.75) with a slope of 0.96 and intercept of 0.38. An 84 day moored field test was conducted in Chesapeake Bay from July 18 to October 10, 2016. The Real-NO3 operated continuously for 69 days until 9/24 when air purge system malfunctioned. The system was bypassed per manufacturer’s instructions and the instrument restarted on 9/30. The instrument returned 22,345 observations out of a possible 24,144 based on approximate 5 minute sampling intervals for a data completion rate of 93%. The average and standard deviation of the measurement difference between instrument and reference NO3 measurements for each matched pair (n=100 of a possible 103 observations) over the total deployment was 0.083 ±0.022 mgN/L, with the total range of differences between 0.018 to 0.166 mgN/L. There no significant trend in measurement difference over time during the deployment (p=0.681; r2=0.002). A linear regression of the data was significant (p=0.0002; r2 = 0.132), with a slope of 0.680 and intercept of 0.085. For the calibration set-up at this field test, the Real-NO3 significantly over-predicted concentrations. A one month long moored field test was conducted in Kaneohe Bay from October 3, 2016 to November 2, 2016. The Real-NO3 was not deployed at HIMB at the manufacturer’s decision.