Turn-key Coastal pH Monitoring with Spotter, Bristlemouth and Sunburst Sensor’s iSAMI-pH

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We are demonstrating integration of Sunburst Sensor’s iSAMI-pH and Sofar Ocean’s Smart Spotter Buoy through the Bristlemouth Dev. kit. The Spotter and the iSAMI are well paired economically/electromechanically. However, the high cost of custom engineering integration still limits these cost effective solutions. With the adoption of Bristlemouth we mitigate costly integration. This strategic development has the opportunity to open up coastal pH monitoring to a wider range of users through the turn-key operability of Sofar’s ecosystem. This ecosystem as a gateway to the demonstrated success of Sunburst Sensor’s spectrophotometric measurement approach enables a higher density of accurate and low drift pH observations in the ocean.

Next Steps:

1. Finalize Data Pipeline
2. Package System for Wet Testing
3. In Water Demonstration

This is a collaboration between UC Berkeley graduate student William Pardis (Elec. Engineering) David Long (Cal Poly San Luis Obispo Dept of Chemistry and Biochemistry) with material support from Sunburst Sensors and Sofar Ocean. The engineering integration and mooring operation is being led by William Pardis. Field testing and evaluation of the instruments is being led by Cal Poly. Potential field sites are Morro Bay, the Cal Poly Pier, and Tetiaroa Atoll in the Society Islands where Dr. Long has field sites led by student researchers.

Big thanks:

Zach J. and Dan B. at Sofar
Jim B. and Mike D. at Sunburst
David L. at Cal. Poly.

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How does the iSAMI-pH work?

We’re trying to put powerful chemistry in the hands of a wider demographic and lower the barrier to entry through standardized integration of well established oceanographic assets. The spotter mooring is a good friend to most of us here on the forum but what is the iSAMI-pH?

Most people are familiar with litmus paper—strips containing pH-sensitive dye that change color when exposed to different solutions. The iSAMI-pH is essentially an automated version of litmus paper, but instead of visual observation, it mixes liquid dye with seawater and analyzes the color change using precise embedded optical electronics. We would call this a spectrophotometric pH method: spectro - (color), photo - (light), metric - (measuring). Here is the dye (m-Cresol Purple) in action:

This method is different from electrochemical methods, like the glass electrode you may have used in a course’s chemistry laboratory (or “ISFETs” used by advanced oceanographers). The advantage of spectrophotometric pH is that, rather than taking a calibrated approach like electrodes, it directly measures hydrogen ions in solution. This calibration free method consequently has very low drift, allowing these sensors to dwell for years while maintaining their accuracy.

Thanks to the exacting work of analytical chemists, this dye’s physical properties have been scrutinized in the seawater matrix, making it highly accurate. It is considered the gold standard for quantitative oceanographic pH measurement [1].

Thanks to Sunburst Sensor’s engineering [2] and attention to detail [3], we have instruments like the iSAMI-pH, which realistically bring this measurement into the real world ocean environment. Here is a nice overview of Sunburst as they were working to compete in the Wendy Schmidt Ocean-X prize. An important note Sunburst Sensor took first place in both categories: Accuracy and affordability.

Stay tuned for a deeper, more quantitative dive into this chemistry and methodology.

References

[1] Dickson, A. G., Sabine, C. L., & Christian, J. R. (2007). Guide to best practices for ocean CO2 measurements. North Pacific Marine Science Organization.

[2] Lai, C. Z., DeGrandpre, M. D., & Darlington, R. C. (2018). Autonomous Optofluidic Chemical Analyzers for Marine Applications: Insights from the Submersible Autonomous Moored Instruments (SAMI) for pH and p CO2. Frontiers in Marine Science, 4, 438.

[3] DeGrandpre, M. D., Spaulding, R. S., Newton, J. O., Jaqueth, E. J., Hamblock, S. E., Umansky, A. A., & Harris, K. E. (2014). Considerations for the measurement of spectrophotometric pH for ocean acidification and other studies. Limnology and Oceanography: Methods, 12(12), 830-839.

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Status Update

A project update on our path toward a standardized, accurate, easy to use and scalable pH monitoring solution. The system is fully integrated, streaming cellular pH data, and buttoned up to go in the drink! This integration in total took roughly 4 evenings (16hr) to get working and it’s a nicely polished product. That is an impressive testament to the work of Sofar and Bristlemouth, their documentation, engineering and our teamwork. Big applause to Sofar/Bristlemouth!

The Technical Details

The Bristlemouth Mote is now an analytical chemist! Performing equilibrium and spectrophotometric calculations in real time. See this in the working Mote firmware available on this public github repository. The readme includes the appropriate wiring of the Subconn whip (MCIL6M) provided with the iSAMI-pH so you can replicate without drooling over datasheets. The individual conductors were cut to length, stripped, twisted, and soldered.

The Mote is also ready to go back to being a submariner! The end cap was adapted with a M20x1.5 hole, loaded with a compression fitting that Sunburst has had good success with on their co-deployed battery packs for shallow deployments.

I am currently running the system for a 24hr test in preparation for our first deployment.

What’s next

Get it in the water! We have three incremental deployments planned with increasing levels of complexity. We are headed to Sofar this Friday (April 18th) for a simple dip test off Pier 28. We will run the instrument over the weekend and collect pH data at 15min intervals. Next a full Mooring deployment domestically, probably down near San Luis Obispo with the support of Cal. Poly - May. Then operations in French Polynesia - June.

Why we are posting

This public documentation is meant to lower the barrier for others interested in iSAMI-pH deployment on Spotter moorings. If you have questions or need support please, I would be excited if you reach out. You can contact me directly by signing up/into the forum and messaging me directly.