Performance Verification Statement for the Aanderaa Data Instruments’ 4319 B Conductivity 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 Aanderaa Data Instruments (AADI) 4319 B 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 ACT QA specialists, who conducted technical systems audits and a data quality audit of the test data. In the lab tests, the AADI 4319 B sensor exhibited a strong linear response when exposed to 15 different test conditions covering five salinities ranging from 7 – 34 psu, each at three temperatures 2 ranging from 6 - 32 oC with R >0.9999, SE = 0.0337 and slope = 0.998. The overall mean and variance of the absolute difference between instrument measured salinity and reference sample salinity for all treatments was -0.0244 ±0.0369 psu. When examined independently, the mean of the offsets for the conductivity and temperature sensors were -0.0468 ±0.0454 mS/cm and -0.0146 ±0.0144 oC. 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. For the HI test site instrument performance was stable for about three weeks before significantly impacted by fouling. The initial relative accuracy of instrument measured salinity during the first few days of deployment period was 0.005, 0.013, -0.054, and -0.034 psu for FL, GA, HI, and MI, respectively. The variability in response was too large in AK to determine any initial offset. 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 -0.0098, 0.0075, -0.0015, 0.0039, and -0.0022 oC for FL, GA, HI, MI, and AK, respectively. When instrument response for the first 14 days of deployment was compared together for 2 all five field sites, a fairly consistent and linear performance response was observed with R = 0.994, SE

= 1.067 and slope = 0.984 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. In general, there was no strong evidence for calibration drift during the period of deployment and the test confirmed that the deterioration of instrument performance during the field deployments was due to biofouling.

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 5 and plus 31 seconds among test sites. We encourage readers to review the entire document for a comprehensive understanding of instrument performance