Performance Verification Statement for the Idronaut Ocean Seven 305 Plus CTD.

Abstract

The Alliance for Coastal Technology (ACT) conducted a sensor verification study of in situ pH sensors during 2013 and 2014 to characterize performance measures of accuracy and reliability in a series of controlled laboratory studies and field mooring tests in diverse coastal environments. A ten week long laboratory study was conducted at the Hawaii Institute of Marine Biology and involved week long exposures at a full range of temperature and salinity conditions. Tests were conducted at three fixed salinity levels (0.03, 22, 35) at each of three fixed temperatures (10, 20, 30 oC). Ambient pH in the test tank was allowed to vary naturally over the first five days. On the sixth day the pH was rapidly modified using acid/base additions to compare accuracy over an extended range and during rapid changes. On the seventh day the temperature was rapidly shifted to the next test condition. On the tenth week a repeated seawater trial was conducted for two days while the temperature was varied slowly over the 10 – 30 oC range. Four field-mooring tests were conducted to examine the ability of test instruments to consistently track natural changes in pH over extended deployments of 4-8 weeks. Deployments were conducted at: Moss Landing Harbor, CA; Kaneohe Bay, HI; Chesapeake Bay, MD; and Lake Michigan, MI. Instrument performance was evaluated against reference samples collected and analyzed on site by ACT staff using the spectrophotometric dye technique following the methods of Yao and Byrne (2001) and Liu et al. (2011). A total of 263 reference samples were collected during the laboratory tests and between 84 – 107 reference samples were collected for each mooring test. This document presents the results of the Idronaut Ocean Seven 305 Plus CTD which uses a potentiometric pH sensor consisting of two electrochemical cells including the measuring sensor and the Ag/AgCl reference sensor filled with a saturated KCl gel. ACT wants to acknowledge that this particular performance verification was not conducted under ideal conditions. One significant factor was that the company did not have the resources to send a representative for direct face to face training, and none of the ACT staff had any prior experience with this instrument. This lack of direct training contributed to one of the programming errors which caused a loss of test data. In addition, the company acknowledges a lack of full understanding of the extent of the testing and that refurbishment of the instrument between test sites could have been done better. It was felt more care should have been taken to address biofouling and confirmation of the functionality of the reference electrode. It should be noted the instrument was re-serviced by the company only prior to the Great Lakes field test since the initial receipt by ACT more than a year earlier. However, the performance verification was conducted according to the signed testing protocols and at a minimum the instrument was exposed to some form of CRM prior to deployment to confirm proper working order. The Idronaut 305 Plus was not operating at the start of the laboratory deployment due to a battery short, but the error was not discovered and corrected until January 13th, which was about 6 days into the brackish water test. Because no seawater results were obtained initially, a brief additional seawater exposure test was performed at the end of the study. After redeployment on January 13th, the instrument operated continuously for the next 33 days and generated 3162 pH measurements at 15 minute intervals. The total range of pH measured by the Idronaut 305 Plus was 6.899 to 8.276, compared to the range of our reference pH of 6.943 to 8.502. The Idronaut 305 Plus measurements tracked changing pH conditions among all water sources and temperature ranges including the rapid pH shifts from acid/base additions, but the magnitude and direction of the offset changed for each water type (Fig.3). The mean of the differences between the Idronaut measurement and reference pH was -0.144 ±0.174 (N=181), with a total range of 0.502 to 0.078. At Moss Landing Harbor the field deployment test was conducted over 28 days with a mean temperature and salinity of 16.6oC and 33. The measured ambient pH range from our discrete reference samples was 7.933 – 8.077 (N=84). No results were obtained for the Idronaut 305 Plus for this field deployment test due to a programming error by ACT personnel. It was not possible to repeat the test since only one instrument was available and it needed to be submitted to the next test site. At Kaneohe Bay the field deployment test was conducted over 88 days with a mean temperature and salinity of 24.5oC and 34.4. The measured ambient pH range from our discrete reference samples was 7.814 – 8.084 (N=101). The Idronaut 305 Plus operated continuously for the first twenty seven days (total deployment was 88 days) after which there appeared to be a short in the battery which terminated logging. During its operational period the instrument generated 2756 observations at 30 minute intervals with a measured range in ambient pH from 7.599 to 8.409. The average and standard deviation of the measurement differences between the Idronaut and reference pH was 0.107 ±0.093 (N=62), with a total range of -0.030 to 0.387. At Chesapeake Bay the field deployment test was conducted over 30 days with a mean temperature and salinity of 5.9oC and 12.8. The measured pH range from our discrete reference samples was 8.024 – 8.403 (N=107). The Idronaut 305 Plus only operated during the first four days of the deployment and generated 372 pH measurements at 15 minute intervals with a measured range in ambient pH of 7.798 to 8.135. The average and standard deviation of the measurement difference between the Idronaut and reference pH was -0.366 ±0.042 (N=18), with a total range of -0.425 to -0.247. At Lake Michigan the field deployment test was conducted over 29 days with a mean temperature and salinity of 21.2oC and 0.03. The measured ambient pH range from our discrete reference samples was 8.013 to 8.526 (N=98). The Idronaut 305 Plus operated continuously over the 29 days of the deployment and generated 2671 observations at 15 minute intervals with a measured range in ambient pH of 7.523 to 8.462. The average and standard deviation of the measurement difference between the Idronaut and reference pH was -0.184 ± 0.081 (N=98) with a total range of -0.465 to -0.019. A comparison of the Idronaut 305 Plus pH measurements and dye reference pH across all sites indicated that the instrument tracked changes in ambient pH at all sites but that the response factor (slope) and overall offset from the dye reference pH measurements was different at each site. Lastly although this instrument experienced several power issues during this verification, it is worth emphasizing that the continuous 15 – 30 minute time-series provided by the test instrument was able to resolve a significantly greater dynamic range and temporal resolution than could be obtained from discrete reference samples. Continuous in situ monitoring or profiling technologies, such as the Idronaut 305 Plus, provide critical research and monitoring capabilities for helping to understand and manage important environmental processes such as carbonate chemistry and ocean acidification, as well as numerous other environmental or industrial applications.