Performance Verification Statement for the In-Situ Troll 9500 pH Sensor.

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 In-Situ Troll 9500 which measures pH using a combination, single-junction electrode that contains a glass pH sensing bulb, replaceable junction and refillable reference saturated KCl electrolyte solution. The Troll 9500 operated continuously throughout the entire lab test and generated 6283 pH measurements at 15 minute intervals. The total range of pH measured by the Troll 9500 was 7.12 to 8.58, compared to the range of our discrete reference samples of 6.943 to 8.502. The Troll 9500 tracked changing pH conditions among all water sources and temperature ranges including the rapid pH shifts from acid/base additions, but the magnitude of the offset changed for each water type (Fig.3). For the complete Lab test, the mean of the differences between the Troll 9500 measurement and reference pH was 0.17 ±0.07 (N=263), with a total range of 0.02 to 0.31. Instrument measurements conducted with the second seawater trial after ten weeks showed a slight increase in the offset (mean difference = 0.31 ±0.004; N=8) compared to measurements from the first week (mean difference = 0.15 ± 0.01; N=27). 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 84 discrete reference samples was 7.933 – 8.077. The Troll 9500 experienced an internal malfunction approximately three hours after logging was initiated on August 26th at 18:00 hours. No useable data was produced to compare to the reference dye results. The Troll 9500 measured pH from 7.94 to 7.98 during the first three hours indicating good operating condition, but then showed an abrupt increase to nearly 14, indicating some internal malfunction. 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 101 discrete reference samples was 7.814 – 8.084. The Troll 9500 only operated over the first three days of the deployment and generated 154 observations with a range in ambient pH from 7.93 to 8.17. (n=10 observations out of a possible 101 for the entire deployment). The average and standard deviation of the differences between the Troll 9500 and reference pH during its operational period was 0.10 ±0.01 (N=10), with a total range of 0.09 to 0.11. 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 107 discrete reference samples was 8.024 – 8.403. The Troll 9500 operated continuously over the entire 30 day deployment and generated 2756 observations; however after the first six hours a significant measurement deviation occurred and continued throughout the test. Only the first 26 instrument measurements and only one matched reference sample pair were considered useable. Ambient pH measured by the Troll 9500 over the first six hours ranged from 8.34 to 8.82. The first sampling pair from March 12th had a difference from the reference pH of -0.002 indicating the probe was in good operating condition at the time of deployment. No summary statistics are calculated for the remaining deployment. 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 98 discrete reference samples was 8.013 to 8.526. The Troll 9500 operated continuously over the 29 days of the deployment and generated 2673 observations with a measured range in ambient pH from 8.24 to 9.57. The average and standard deviation of the difference between the Troll 9500 and reference pH for the entire deployment was 0.60 ± 0.15 with a total range of 0.27 to 0.90. It is unfortunate that internal malfunctions occurred at three of the four field test sites since the sonde appeared to working very well upon initial deployment. As noted by the successful Great Lakes deployment, 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 technologies, such as the Troll 9500, 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.