The first time I went to test an OpenCTD head-to-head against a hand-held commercial CTD, something was wrong. It was early in the OpenCTD development, but we were deep enough into the weeds that we had what we thought was a pretty good calibration process. The OpenCTD looked good against our standards. But the devices were far, far apart. Something was off. Way off.
In Ecology and conservation researchers should adopt open source technologies, published this month, the authors advocate for a greater inclusion of open-source hardware tools in conservation and ecology research. For years I’ve been banging the drum of the low-cost, open-source CTD, a tool that unlocks the fundamental measurements needed for all ocean science research, for a fraction the cost of commercial alternatives.
There are a ton of great reasons to go the open-source hardware route, especially following the model we’ve built with the OpenCTD to prioritize cost reductions and accessibility of materials, but one that I didn’t expect until the data started rolling in was in quality and transparency of data.
Back on the pier, we’re puzzling over outputs from two CTDs. I pulled out my trusty hand refractometer (the classics never go out of style). We summoned water quality data from the nearest NOAA buoy. The OpenCTD was dead on. The commercial CTD was off. Not by a lot. Not even by enough to make someone look at the data and think “huh, that’s not right”. It was only because we we doing head-to-heads that the disparity was apparent. So we grabbed another one off the shelf. And another.
I’ll get to the punchline: handheld CTDs, whether commercial or DIY, are used and abused in the field far more than their shipboard siblings. We toss them out of kayaks, cast them on fishing lines, forget them in the marsh. They go out of calibration. The oceanographers among you will say “yes, Andrew, of course. That’s how this works. CTDs need to be calibrated, often.” But we still trust them. We grab them off the shelf, we toss them into our field bags, we drop them in the ocean, and we diligently record the data.
Over the last ten years, I’d say about half of the commercial handheld CTDs we’ve compared OpenCTDs against were far enough out of calibration to be detectable. I’ll freely acknowledge that’s not a fair comparison. The OpenCTDs were fresh from production, and usually been calibrated that day, in the same place, under the same conditions. The commercial handhelds were whatever was available, often sitting around because they weren’t currently being used for field work.
Closed source hardware is opaque, preventing researchers from fully understanding how the equipment operates. This makes identifying systematic errors difficult, especially if the manufacturer has a monopoly on the technology so that users have no alternatives for comparison.
Hsing et al. 2024.
The problem lies in the fact that, for the vast majority of these handheld devices, calibration services are provided by the manufacturer, not the user. To ensure that your handheld CTD is accurate, in most cases it needs to be sent back to the manufacturer for annual maintenance and calibration. For researchers working on a budget, working in economically depressed regions, working in places where reliable shipping services don’t exist, that prospect becomes untenable. This is one reason we’re trying to develop calibration centers of excellence through the Coastal Observing Lab in a Box project. It’s the reason that a huge amount of OpenCTD development over the last few years has been geared towards developing detailed calibration protocols.
I am absolutely not arguing that we have a crisis in ocean data production or that there’s a critical problem with commercial handheld CTDs. The YSI Castaway is a great little device. I’ve used hundreds of Hobo loggers in my career. Seabirds are rock solid instruments.
Handheld CTDs are more often than not used to produce data to supplement ecology, conservation, and management, rather than the primary data that goes into things like global ocean models. We accept that they are noisier and, even with the OpenCTD, explicitly state that the data quality is insufficient for chemical or physical oceanography. I am arguing that we need to be more thoughtful in how data is produced and what it means when the people producing the data are dependent on third-parties to maintain and calibrate their tools.
These tools are made by humans. These tools are used by humans. We are imperfect plankters drifting through a sea of data produced almost entirely by other imperfect plankters. We are limited by our time and our attention and our finances and our foresight. More often than most field scientists would care to admit, if the data looks right, it’s good enough.
Ecology and conservation researchers should adopt open source technologies by Pen-Yuan Hsing, Brianna Johns, and Amanda Matthes is available, open-access, from Frontiers in Conservation Science.
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