Autonomous in situ calibration of ion‐sensitive field effect transistor pH sensors

Abstract

Ion‐sensitive field effect transistor‐based pH sensors have been shown to perform well in high frequency and long‐term ocean sampling regimes. The Honeywell Durafet is widely used due to its stability, fast response, and characterization over a large range of oceanic conditions. However, potentiometric pH monitoring is inherently complicated by the fact that the sensors require careful calibration. Offsets in calibration coefficients have been observed when comparing laboratory to field‐based calibrations and prior work has led to the recommendation that an in situ calibration be performed based on comparison to discrete samples. Here, we describe our work toward a self‐calibration apparatus integrated into a SeapHOx pH, dissolved oxygen, and CTD sensor package. This Self‐Calibrating SeapHOx is capable of autonomously recording calibration values from a high quality, traceable, primary reference standard: equimolar tris buffer. The Self‐Calibrating SeapHOx's functionality was demonstrated in a 6‐d test in a seawater tank at Scripps Institution of Oceanography (La Jolla, California, U.S.A.) and was successfully deployed for 2 weeks on a shallow, coral reef flat (Lizard Island, Australia). During the latter deployment, the tris‐based self‐calibration using 15 on‐board samples exhibited superior reproducibility to the standard spectrophotometric pH‐based calibration using > 100 discrete samples. Standard deviations of calibration pH using tris ranged from 0.002 to 0.005 whereas they ranged from 0.006 to 0.009 for the standard spectrophotometric pH‐based method; the two independent calibration methods resulted in a mean pH difference of 0.008. We anticipate that the Self‐Calibrating SeapHOx will be capable of autonomously providing climate quality pH data, directly linked to a primary seawater pH standard, and with improvements over standard calibration techniques.

Technical note: Stability of tris pH buffer in artificial seawater stored in bags

“…Given the thorough characterization of tris over wide ranges of environmental variables, this contribution can aid in the traceability and intercomparability of pH sensor measurements. While valuable at the current stage of development (as demonstrated by, for example, Lai et al, 2018 andBresnahan et al, 2021), further development would ideally result in a commercially available bag and filling procedure that can yield a rate of pH change less than the climate threshold of 0.003 per year. This will require further tests to identify the source of CO 2 , gas exchange, or microbial respiration, as well as steps to reduce or eliminate these sources.…”

“…This observation highlights the importance of maintaining bag integrity, particularly during use in the field. A successful 2-week field deployment has been conducted using the tris bags described here and a modified SeapHOx in a shallow, coral reef flat (Bresnahan et al, 2021). This 2-week deployment was significantly shorter than the year of storage described here, and further field testing in longer deployments in varied environments is required before widespread use of this technology.…”

“…advancements have led to more routine autonomous pH measurements over the past decade, providing opportunities to fill some gaps in time and space in discrete sampling programs (e.g., Byrne, 2014;Martz et al, 2015;Lai et al, 2018;Wang et al, 2019;Tilbrook et al, 2019). Globally, pH sensors now operate on hundreds of autonomous platforms including moorings and profiling floats, delivering unique data sets in the form of Eulerian and depth-resolved Lagrangian time series (Johnson et al, 2017;Bushinsky et al, 2019;Sutton et al, 2019). While sensors increase data coverage, many sensor-based pH measurements, particularly on moored systems, continue to fall short of both climate and weather levels of uncertainty, as highlighted in the intercomparison tests carried out by the Alliance for Coastal Technologies (ACT, 2012) and by the Wendy Schmidt Ocean Health XPRIZE (Okazaki et al, 2017).…”

“…Independent validation is typically required for autonomous sensors to meet both weather and climate levels of uncertainty. For example, autonomous underway pCO 2 systems (Pierrot et al, 2009), moorings (Bushinsky et al, 2019), and autonomous surface vehicles (Chavez et al, 2017;Sabine et al, 2020) are able to provide climate quality observations with an uncertainty of ±2 µatm because traceable standard gases are frequently measured in situ. For pH measurements on profiling floats (Johnson et al, 2016), sensor performance is validated by comparing to a deep reference pH field that is calculated using empirical algorithms (Williams et al, 2016;Bittig et al, 2018;Carter et al, 2018).…”

Unexpected role of communities colonizing dead coral substrate in the calcification of coral reefs

“…Measurements of pH were performed by the CISME respirometer using a Durafet III combination electrode (Martz et al 2010;Bresnahan et al 2021). All pH were measured and reported on the total scale at in situ temperature.…”