Performance Verification Statement for the Campbell Scientific Instruments 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 265 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 Campbell Scientific Instruments pH sensor which measures pH using an ion-sensitive field effect transistor. The CSI pH sensor operated continuously throughout the entire lab test and generated 6294 pH measurements at 15 minute intervals. The total range of pH measured by the CSI was 6.805 to 8.560, compared to the range of our reference pH of 6.943 to 8.502. In general, the CSI 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 CSI pH measurement and reference pH was -0.019 ±0.319 (N=265), with a total range of -0.668 to 0.624. Instrument measurements conducted after ten weeks with the second seawater trial showed an increase offset (mean difference = 0.345 ±0.020; N=9) compared to measurements from the first week (mean difference = 0.084 ± 0.055; 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 CSI operated continuously over the entire deployment and generated 2579 observations with a measured range in ambient pH from 7.759 to 7.960. The average and standard deviation of the measurement difference between the CSI and reference pH for the deployment was -0.152 ± 0.023 (N=84), with a total range of -0.253 to 0.115. 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 CSI operated continuously over 88 days and generated 4211 observations with a measured range in ambient pH from 7.886 to 8.470. The average and standard deviation of the differences between CSI and reference pH was 0.221 ±0.066 (N=101), with a total range in the differences of 0.076 to 0.316. 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 CSI operated successfully over the entire 30 day deployment with a measured range in ambient pH from 7.693 to 7.979. A programming error initiated by ACT personnel during set-up resulted in the shutter being closed during every other observation, so the useable number of observations was reduced from 2756 to 1378. No comparative data with reference samples were lost due to this programming error. The average and standard deviation of the measurement difference between the CSI and reference pH was 0.398 ±0.040, with the total range of differences from -0.472 to -0.316. 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 CSI-pH operated continuously over 29 days and generated 2680 observations with a range in ambient pH from 7.240 to 7.946. The average and standard deviation of the difference between CSI and reference pH was -0.676 ± 0.109 (N=98), with a total range of -0.894 to -0.517. A comparison of the CSI pH versus reference pH across all sites indicated that the response was variable among test conditions. The CSI under reported pH by an average of 0.15, 0.40, and 0.68 pH units at Moss Landing, Chesapeake Bay, and the Great Lakes respectively. In contrast the CSI over reported pH relative to the dye reference measurement by 0.22 for the Hawaii 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 CSI, 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.