Performance Verification Statement for the RBR XR-420 and XR-620 CTD Salinity Sensors.

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 RBR XR-420 and XR620 CTD salinity sensors 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 XR-620 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 2 o C with R >0.9999, SE = 0.03330 and slope = 1.001. The overall mean and variance of the absolute difference between instrument measured salinity and reference sample salinity for all treatments was -0.0262 ±0.0351 psu. When examined independently, the relative accuracy of the conductivity and temperature sensors were -0.0375 ±0.0458 mS/cm and -0.0045 ±0.0048. 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 and more gradually over the eight weeks at the HI test site. The initial relative accuracy of instrument measured salinity during the first few days of deployment period was -0.036, -0.009, -0.003, and -0.004 psu for FL, GA, HI, and MI test sites, respectively. Variability was too great at the AK test site to precisely define a specific offset. Essentially all of the variability and measurement error was traced to the performance of the conductivity cell. The temperature sensor was accurate and stable throughout all of the deployments. The average offset of the measured temperature relative to our calibrated reference temperature logger was -0.0048, -0.0013, 0.0024, 0.0162, and- 0.0037 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 = 0.997, SE = 0.734 and slope = 0.989. For vertical profiling tests, the instrument response was consistent over all depths and all ambient salinity levels. The average offset in measured salinity was -0.0191 ±0.0096 psu. 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. In general, there was no strong evidence for calibration drift during the period of deployment and results confirmed that any deterioration in instrument performance during field deployments tests 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 any of the laboratory tests, field deployment tests, or 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.