Performance Verification Statement for the Eureka Manta2 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 Eureka Manta2 pH sensor which is a glass electrode with a KCl reference electrode. The Manta2 was submitted for testing after the Laboratory study was completed under a new Request for Technologies, therefore only results for the Field testing component are presented. 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. At this site two instrument sondes were deployed with identical pH sensors, however, one was enclosed in a copper mesh screen to serve as an anti-fouling measure and one in a plain sensor guard. The Manta2 with the copper screen operated for 23 consecutive days before experiencing a power failure. Ambient pH measured by this unit ranged from 7.95 to 8.63. The average and standard deviation of the measurement difference between the Manta2 and reference pH was 0.258 ± 0.181 with a total range of -0.014 to 0.551 (N=64). The Manta2 with no anti-fouling operated continuously for the entire 28 days of the deployment, resulting in 2579 observations at 15 minute intervals. Ambient pH measured by this Manta2 sonde ranged from 8.28 to 8.60. The average and standard deviation of the measurement difference between this Manta2 and reference pH was 0.512 ± 0.059 with a total range of 0.333 to 0.584 (N=84). 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 Manta2 sonde operated continuously throughout the first 67 days of deployment measuring at 30 minute intervals, however, a tunicate colonized directly on the pH sensor bulb causing it to fracture and the data beginning on January 30th were not useable for comparisons (pH instantly dropped from 8.1 to 6.6 indicating failure). Ambient pH measured by the Manta2 ranged from 7.85 to 8.39. The average and standard deviation of the measurement difference between instrument and reference was 0.17 ±0.02 with a total range in the differences of 0.12 to 0.20 (N=75). 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 ambient pH range from our 107 discrete reference samples was 8.024 – 8.403. The Manta2 operated successfully over the entire deployment and generated 2756 observations based on its 15 minute sampling interval. Ambient pH measured by the Manta2 ranged from 8.14 to 8.54. The average and standard deviation of the measurement difference between instrument and reference pH was 0.12 ±0.02, with the total range of differences from 0.09 to 0.16 (N=107). 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 pH range from our 98 discrete reference samples was 8.013 - 8.526. The Manta2 operated continuously over the entire deployment and generated 2680 observations based on its 15 minute sampling interval. Ambient pH measured by the Manta2 ranged from 7.86 to 8.51. The average and standard deviation of the difference between instrument and reference pH was -0.07 ± 0.04 with a total range of -0.20 to -0.004 (N=98). A summary plot of all four field tests indicates that the Manta2 responded consistently with good agreement to reference pH measurements at three of the four test sites. No explanation or observed failure was discovered to explain the performance at the Moss Landing test site. Results for the brackish and saline test sites show an expected offset due to calibration and scale differences between NBS buffers and the pHtotal scale reported by the spectrophotometric dye measurement. 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. There were no obvious changes in the differences between instrument and reference measurements during the duration of the mooring test, indicating that biofouling and instrument drift had not significantly affected measurement performance over these deployment durations. Continuous in situ monitoring technologies, such as the Eureka Manta 2, 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.