Verification Statement for the YSI 6-series sonde with model 6560 Salinity Sensor.

Abstract

Instrument performance verification is necessary so that effective existing technologies can be recognized, and so that promising new technologies can become available to support coastal science, resource management, and ocean observing systems. The Alliance for Coastal Technologies (ACT) has therefore completed an evaluation of commercially available in situ salinity sensors. While the sensors evaluated have many potential applications, the focus of this Performance Verification was on nearshore moored and profiled deployments and at a performance resolution of between 0.1 – 0.01 salinity units. In this Verification Statement, we present the performance results of the YSI 6600 sonde equipped with the model 6560 salinity sensor evaluated in the laboratory and under diverse environmental conditions in moored and profiling field tests. A total of one laboratory site and five field sites were used for testing, including tropical coral reef, high turbidity estuary, sub-tropical coastal ocean, sub-arctic coastal ocean, and freshwater riverine environments. Quality assurance (QA) oversight of the verification was provided by an ACT QA specialist, who conducted technical systems audits and a data quality audit of the test data. In the lab tests, the YSI 6600 exhibited a strong linear response when exposed to 15 different test conditions covering five salinities ranging from 7 – 34 psu, each at three temperatures ranging from 6 - 32oC (R2 >0.9999, SE = 0.0026 and slope = 0.991). The mean of the absolute difference between instrument measured salinity and reference sample salinity across all laboratory treatments was -0.1678±0.1364 psu. When examined independently, the relative accuracy of the conductivity and temperature sensors were -0.2732 ±0.2392 mS/cm and -0.0737 ±0.0184 oC, respectively. Across all five field deployments, the range of ambient salinity was 0.14 – 36.97. The corresponding conductivity and temperatures ranges for the tests were 0.27 – 61.69 mS cm-1 and 10.75 – 31.14 oC, respectively. Measurement accuracy of the YSI sonde varied across sites and appeared to be affected by both the initial calibration of the sonde as well as the extent of biofouling. For FL, AK, and HI test sites the instrument performance was fairly consistent throughout the deployment period, but a positive offset of between 0.5 -3 psu was observed. At the GA test site the initial accuracy was much better, with an average offset of 0.0084 over the first 10 days, followed by a rapid deterioration as biofouling became extensive. Results for the MI riverine test site were highly accurate and consistent with an average offset to reference samples of -0.0146 psu over the entire deployment. In all field tests, offsets in salinity were directly related to measured conductivity values and temperature measurements remained consistent throughout the deployment with offsets ranging between -0.09 to 0.001 oC. When all field test sites were analyzed compositely for the first 14days of deployment, the instrument response was highly linear (slope = 1.047 and R2 = 0.996) with a standard error of 0.086 psu for the regression. Performance checks prior to deployment and again at the end of the deployment after instruments were thoroughly cleaned of fouling indicated no drift in instrument performance at GA, MI, and HI tests sites. An increased offset was noted in the post-test at AK; however, the time series data did not suggest that accuracy significantly changed over time. For the lab test, one hundred percent of the data was recovered from the instrument and no outlier values were observed. For all five field tests, 100 percent of the data was recovered from each test instrument from the deployment; however, a malfunction in salinity calculations occurred during the posttank test at Florida. A check on the instruments time clocks at the end of the deployment period revealed time differences ranging from 39 seconds slow to 18 seconds fast based against initial settings. We encourage readers to review the entire document for a comprehensive understanding of instrument performance.