Performance Verification Statement for Campbell Scientific CS547A and OBS-3A Salinity Probes.

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 CS547A salinity probe evaluated in the laboratory and under diverse environmental conditions in moored field tests. In addition the OBS-3A salinity probe was tested in a vertical profiling application. A total of one laboratory site and five different field sites were used for testing, including tropical coral reef, high turbidity estuary, subtropical 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 CS547A exhibited a linear but variable response when exposed to 15 different test conditions covering five salinities ranging from 7 – 34 psu, each at three temperatures ranging from 6 2 - 32 oC (R >0.963, SE = 1.954 and slope = 0.927). The overall mean and variance of the absolute difference between instrument measured salinity and reference sample salinity for all treatments was 2.33 ±2.03 psu. When examined independently, the relative accuracy of the conductivity and temperature sensors were 2.52 ±2.35 mS/cm and -0.0394 ±0.0721 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. With the exception of the MI freshwater test site, instrument performance was impacted by calibrations issues as well as impacts from biofouling. For the MI deployment the average offset in salinity was -0.033 ±0.010 and performance was stable throughout the deployment. For the marine test sites the initial offset in measured salinity during the first week of deployment ranged from 0.4 – 3.0 psu. The measurement error was due almost exclusively from the conductivity sensor and the temperature sensor response was quite stable and accurate at all sites. The average offset of the measured temperature relative to our calibrated reference temperature logger was 0.026, -0.009, 0.092, 0.008, and 0.079 oC for FL, GA, HI, MI, and AK, respectively. When instrument response was compared together for the first 14 days of deployment at all five field sites, a fairly consistent and linear performance response was observed with R2 = 0.982, SE = 1.789 and slope = 0.982. This response was also very similar to that observed for the lab studies. For the vertical profiling application, the OBS-3A tracked vertical changes in salinity closely and the average offset over the two profiling tests was 0.74 ± 0.19 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. There were often significant changes in the measured accuracy for salinity of between 1-3 psu. It is not possible to directly identify whether these changes resulted from calibration drift or if we could simply not completely remove the impacts of biofouling during our post-deployment cleaning. 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 1 and plus 3 seconds among test sites. We encourage readers to review the entire document for a comprehensive understanding of instrument performance.