Performance Verification Statement For the Xylem EXO 2 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 Xylem EXO 2 pH sensor which measures pH using a glass bulb electrode and KCl reference electrode. For most tests two pH sensors were included on the sonde and results are presented separately for each. The EXO-pH1 operated continuously throughout the entire lab test and generated 6286 pH measurements at 15 minute intervals. The total range of pH measured by the EXO-pH1 was 7.04 to 8.50, compared to the range of our reference pH of 6.943 to 8.502. The EXO-pH1 measurements tracked changing pH conditions among all water sources and temperature ranges, and consistently responded to the rapid pH shifts from acid/base additions. The average difference between the EXO-pH1 and reference pH was 0.05 ±0.09 (N=266), with a total range of -0.31 to 0.19. Initial instrument measurements conducted with the second seawater trial on the tenth week exhibited a slightly higher offset (mean difference = 0.17 ±0.004; N=7) compared to measurements from the first week (mean difference = 0.10 ± 0.004; N=28) The EXO-pH2 also operated continuously throughout the entire lab test and generated 6286 pH measurements at 15 minute intervals. The total range of pH measured by the EXO-pH2 was nearly identical at 7.03 to 8.49, and again in close agreement with the range of the reference pH of 6.943 to 8.502. The EXO-pH2 measurements tracked reference pH similarly among all water sources and temperature ranges, and consistently responded to the rapid pH shifts from acid/base additions. The average difference between the EXO-pH2 and reference pH was 0.04 ±0.10 (N=266), with a total range of -0.31 to 0.19. Initial instrument measurements conducted with the second seawater trial on the tenth week exhibited a slightly higher offset (mean difference = 0.17 ±0.003; N=7) compared to measurements from the first week (mean difference = 0.08 ± 0.007; N=28) At Moss Landing Harbor the field deployment test was conducted over 28 days with a mean temperature and salinity of 16.6 oC and 33. The measured ambient pH range from our 84 discrete reference samples was 7.933 – 8.077. The EXO sonde deployed for this field test contained two pH probes and both operated continuously over the 28 days of the deployment and each generated 2575 observations at 15 minute intervals. The range in ambient pH measured by the EXO-pH1 was 7.96 to 8.27 and for the EXO-pH2 was 8.10 to 8.48. The average and standard deviation of the difference between EXO-pH1 and reference pH over the total deployment was 0.13 ± 0.04 with a total range of -0.01 to 0.23. The average and standard deviation of the difference between EXO-pH2 and reference pH over the total deployment was 0.29 ± 0.04 with a total range of 0.15 to 0.41. At Kaneohe Bay the field deployment test was conducted over 88 days with a mean temperature and salinity of 24.5 oC and 34.4. The measured ambient pH range from our 101 discrete reference samples was 7.814 – 8.084. The sonde (which contained two pH sensors) operated for the first 16 days, but by November 30th the battery voltage had dropped to 4.7 volts and the sonde stopped measuring. Both probes recorded 1445 observations measured at 15 minute intervals during the first 16 days of operation. Ambient pH measured by the EXO-pH1 ranged from 7.90 to 8.33 and for the EXO-pH2 from 7.79 to 8.22. The average and standard deviation of the difference between EXO-pH1 and reference pH over the total deployment was 0.20 ± 0.02 (N=29), with a total range of 0.17 to 0.23. The average and standard deviation of the difference between EXO-pH2 and reference pH over the total deployment was 0.08 ± 0.05 (N=29), with a total range of -0.03 to 0.14. At Chesapeake Bay the field deployment test was conducted over 30 days with a mean temperature and salinity of 5.9 oC and 12.8. The measured pH range from our 107 discrete reference samples was 8.024 – 8.403. Only one pH sensor was deployed on the EXO sonde for this deployment. The EXO operated successfully over the entire 30 day deployment and generated 2759 pH measurements at 15 minute intervals. Ambient pH measured by the EXO ranged from 8.14 to 8.52. The average and standard deviation of the measurement difference between the EXO and reference pH was 0.16 ±0.04 (N=107), with the total range of differences from 0.05 to 0.27. At Lake Michigan the field deployment test was conducted over 29 days with a mean temperature and salinity of 21.2 oC and 0.03. The measured ambient pH range from our 98 discrete reference samples was 8.013 to 8.526. The EXO sonde (which contained two pH probes) operated continuously over the 29 days of the deployment and each probe generated 2661 pH measurements at 15 minute intervals. The range in ambient pH measured by the EXOpH1 was 7.84 to 8.60 compared to 7.88 to 8.61 for the EXO-pH2. The average and standard deviation of the difference between EXO-pH1 and reference pH over the total deployment was 0.06 ± 0.05 (N=98), with a total range of -0.22 to 0.04. The average and standard deviation of the difference between EXO-pH2 and reference pH over the total deployment was -0.02 ± 0.04 (N=98), with a total range of -0.17 to 0.05. A comparison of the EXO pH versus reference pH across all sites indicated that the response for the HI and CBL field tests consistently tracked ambient pH with a noted offset of approximately 0.13 pH units that represented the expected difference from the NBS buffer calibrated sonde versus the pHtotal scale measured with the dye reference. In contrast, the freshwater Great Lakes test showed the expected 1:1 relationship. There was no obvious reason for the much greater offsets observed during the Moss Landing field test. Lastly, 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 technologies, such as the EXO, 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.