Performance Verification Statement for the In-Situ Inc. Aqua TROLL 200 sonde.

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 Aqua Troll 200 sonde conductivity sensor evaluated in the laboratory and under diverse environmental conditions in moored and profiling field tests. A total of one laboratory site and five different field sites were used for testing, including tropical coral reef, high turbidity estuary, sub-tropical and 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 Aqua Troll 200 sonde 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 - 32 oC (R2 >0.9999, SE = 0.1013 and slope = 0.991). The overall mean and variance of the absolute difference between instrument measured salinity and reference sample salinity for all treatments was -0.7140 ±0.9885 psu. When examined independently, the relative accuracy of the conductivity and temperature sensors were -0.09093 ±2.4790 mS/cm and -0.0530 ±0.0331 oC, respectively. Across all five field deployments, the range of salinity tested against 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. Extensive and rapid biofouling at the FL and GA test sites severely impacted instrument performance within approximately one week. In addition, there were calibration issues at some test sites as the initial relative accuracy of the instrument during the first few days of deployment period was 0.33, 0.43, -0.033, -0.005 and -0.32 psu for FL, GA, HI, MI, and AK, respectively. Essentially all of the variability and measurement error was traced to the performance of the conductivity cell. The temperature sensor was quite accurate and stable throughout all of the deployments. The average offset of the measured temperature relative to our calibrated reference temperature logger was -0.032, -0.038, 0.021, -0.038, and 0.008 oC for FL, GA, HI, MI, and AK, respectively. When instrument response for the first 14 days of deployment was compared together for all five field sites, a fairly consistent and linear performance response was observed with R2 = 0.995, SE = 0.954 and slope = 0.997. Performance checks were completed prior to field deployment and again at the end of the deployment, after instruments were thoroughly cleaned of fouling, to evaluate potential calibration drift versus biofouling impacts. On several occasions results of these tests were compromised, most likely because of entrainment of air bubbles in the conductivity cell. The manufacturer had alerted us to this concern and we attempted to follow all handling recommendations, however, these results should be viewed as handling errors and not necessarily reflective of instrument performance. In general, there was no strong evidence for calibration drift during the period of deployment.

During this evaluation, no problems were encountered with the provided software, set-up functions, or data extraction at any of the test sites. One hundred percent of the data was recovered from the instrument and no outlier values were observed for all laboratory tests, all field deployment tests, and all tank exposure tests. Lastly, a check on the instruments time clocks at the beginning and end of field deployments showed differences of between minus 3 and plus 11 seconds among test sites. We encourage readers to review the entire document for a comprehensive understanding of instrument performance.