A low-cost, open-source oceanographic instrument to measure our changing oceans

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The ocean belongs to everyone.

The tools necessary to study, explore, and understand the ocean are often inaccessible to the vast majority of ocean users. By nurturing a community of open-source hardware developers, scientists, and ocean stakeholders, we want to change that.

We are developing a fleet of open-source oceanographic tools, including the OpenCTD, a vital tool for studying the ocean, to empower researchers, educators, and citizen scientists. A CTD is a device that measures salinity, temperature, and depth in a water column. It allows scientists to explore the subtle, often invisible dynamics of a water body. But CTDs are expensive, which creates a barrier to entry for many ocean stakeholders. An open-source alternative will allow citizen scientists to participate in the collection of vital ocean data.

The ocean belongs to all of us. Let’s ensure that everyone has access to the tools needed to understand it.

Conductivity, temperature, and depth (CTD). These three measurements allow scientists to unlock ocean patterns hidden beneath the sea's surface. The ocean is not uniform, it its filled with swirling eddies, temperature boundaries, layers of high and low salinity, changing densities, and other physical characteristics invisible to an observer floating upon its surface. To reveal these patterns oceanographers employ a CTD--an oceanographic instrument that, at its core, measures temeprature, salinity, and depth down a water column. The CTD is the workhorse of oceanographic research.

CTDs are expensive, and that expense can act as a barrier-to-entry for researchers with limited budgets, scientists from developing nations, citizen oceanographers, and students of all levels interested in learning more about their local coasts and waterways.

The OpenCTD is a low-cost, open-source CTD suitable for both educators and scientists. The platform is built using reasonably accessible parts and is powered by an Arduino-based microcontroller. Source codes and building instructions are freely available. The device, sufficeintly accurate for scientific research, can be constructed for approximately $300, and is effective to 200 meters depth.

Why 200 meters? For many coastal regions, 200 meters of water depth covers the majority of the ocean that is accessible by small boat and generally the maximum depth of the continental shelf. The OpenCTD is targeted to people working in this niche, where entire research projects can be conducted for less than the cost of a commercial CTD. The OpenCTD is scalable, and someone with the inclination and expertise can adapt an OpenCTD to operate in deeper waters.

The OpenCTD is the flagship project in the Oceanography for Everyone citizen science community.

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  • Storms a brewin'

    andrew.david.thaler09/06/2016 at 18:46 0 comments

    We deployed two OpenCTDs in a tidal estuary ahead of Hurricane Hermine.


    Hurricane Hermine is a tropical cyclone that passed over our field site in coastal Virginia on September 4, 2016. To test the longevity of the batteries and the durability of the OpenCTD during long term deployments, I placed two units in our Test Ditch (a tidally influence drainage ditch connected to the Mobjack Bay estuary that is conveniently located on our property), where they remained for 74 hours.

    Photo Narrative

    Photo of deployment siteDeployment site.

    Photo of deployment site at peak floodDeployment site at time of peak flooding.

    Photo of wider flood zoneWider flood zone to show extent of flooding.

    Photo of deployment site at recoveryDeployment site at time of recovery.

    First look at the data

    Hermine deployment for CTD 2This is the cleaned data from the second unit, which used a cheaper conductivity probe. It was deployed at dead low tide 12 hours before Hermine was supposed to hit.

    Temperature: Temperature is relatively stable and declines predicatably as air temperature declined during the storm, and then rose once the storm passed near the end of the deployment. Two rapid drops can been seen in the first 24 hours of deployment, these correspond to heavy rainfall.

    Salinity: Salinity climbs rapidly as Hermine drives salty seawater up into the Chesapeake and then Mobjack Bay. It declines slightly in conjunction with the heaviest rainfall, and then rises. It fluctuates slightly with changes in sea level, but remains high throughout the deployment.

    Depth: This is a lot of flooding for the region. Low tide during the storm is almost as high as pre-flood high tide. Because this is a drainage ditch, tides don't fall like they normally would, since runoff continuously adds more water during the storm, leading to low-tide spikes rather than gradual curves as seen druing the first and last low tide.

    As important as sensor validation against other equipment, these readings are consistent with observed environmental conditions during the time of deployment.


    One salinity probe appears to have lost calibration (mid-level graphite probe), furthur investigation will determine if the data is recoverable. This particular unit has been used for several dozen test deployments and has been pretty beaten up. It is likely that conductivity calibrations will need to be conducted more often than originally anticipated.

    Lessons Learned

    • 2,000 mAh batteries last for approximately 44 hours of continuous use on a full charge.
    • Conductivity needs to be calibrated more frequently.

  • Update from the International Marine Conservation Congress

    andrew.david.thaler08/22/2016 at 15:00 0 comments

    Andrew presented the OpenCTD and the larger Oceanography for Everyone movement in a keynote at the Fourth International Marine Conservation Congress in St. John's, Newfoundland this August. A transcript for the entire talk can be found here: Ocean Outreach in an Evolving Online Ecosystem: Science is Storytelling but the relevant section begins with part 3: Exploration wants to be shared.

  • Rhode Island Deployment #1

    andrew.david.thaler07/11/2016 at 20:17 0 comments

    [from Kersey Sturdivant]

    The OpenCTD was deployed in the nearshore off Peabody Beach, Newport, Rhode Island, alongside a commercial YSI Model 6600 datasonde for validation. Peabody Beach is a small embayment that directly feeds into the Atlantic Ocean. This marine environment was one of the first deployments of OpenCTD in a euhaline environment.

    A few lessons learned. The OpenCTD is slight buoyant so a small fishing weight was attached to keep it at depth. Over a 24 hour period, no leaking occurred, reaffirming the integrity of the OpenCTD housing. Measurements made by OpenCTD (including the conductivity sensor) were with-in 2% of commercial datasonde measurements. Future test will attempt to deploy the OpenCTD deeper, and alongside multiple commercial water quality datasondes.

  • Second Deployment at Blue Marsh Farm

    andrew.david.thaler07/11/2016 at 13:27 0 comments

    We're back out at the drainage ditch for a second 12-hour deployment. Here are the results:

    Some observations: We had a freak torrential rain, so yes, the conductivity really was effectively zero during this deployment, you can even see when most of the rain came down by the sudden acceleration of depth to the left. Also, the rapid dip at the end is the device coming out of the water and sitting on the porch to dry.

  • Long-term Deployment Preliminary Tests

    andrew.david.thaler07/07/2016 at 20:33 0 comments

    I deployed one of the prototypes in our local tidal estuary over a 24-hour period to assess the behavior of the instrument. Readings were validated against commercial sensors and found to be within 2% accuracy.

    A few interesting things to note: The weird "shakes" at the beginning, and then twice near the end of the series, are me, knocking the device. A few hours after deployment I checked it for leaks, and then later, near the end of deployment, I knocked it a few times to check if it had filled with water. The housing and electronics remained completely dry.

    The steep drops or rises appear to be caused by the device sliding up and down on the rebar stake. Right now the instrument is slightly buoyant, and when the tide comes in, it lifts the device with it. Future deployments will use a more robust locking system to keep the CTD in place.

  • #HackTheOcean: A Weekend of CTD Building

    andrew.david.thaler07/07/2016 at 20:27 0 comments

    Over a long weekend in early June, Kersey and I gathered on my farm in coastal Virginia to construct and test 3 OpenCTDs and compile the ultimate build guide for OpenCTD.

    3 OpenCTDs ready to dive.

    The build guide, which is now complete, can be found at the Oceanography for Everyone GitHub repository: OpenCTD Introduction and Build Guide.

    After a series of field tests, these CTDs will be distributed to potential users for further shakedown trials.

  • Field Trial on Lake Superior

    andrew.david.thaler07/07/2016 at 20:17 0 comments

    Welcome Aboard the R/V Blue Heron

    Field testing and rapid prototyping of open-source hardware on Lake Superior.


    This summer, as part of the UNOLS Chief Scientist Training Cruise program, I was invited to join 4 other early career researchers on Lake Superior, aboard the University of Minnesota Duluth's flagship research vessel, the Blue Heron. During this 3 day cruise, I had the opportunity to test the OpenCTD, push the OpenROV to its breaking point, and send the Niskin3D into the field for the first time. In addition, I brought my trusty Printrbot with me to explore the feasibility of rapid prototyping research materials while at sea.

    The Sites

    I deployed my equipment at four sites: Sterner B, West Mooring, Castle Danger, and, via small inflatable, the Apostles archipelago. The Niskin3D performed flawlessly, though the OpenROV it was mounted to suffered catastrophic flooding at 100 meters. The OpenCTD was deployed from 30 to 140 meters without issue. It logged perfectly on all casts and the 140m deployment at West Mooring is the cleanest data set, by far. I actually didn't expect the data to look as clean as it does.

    The only downside is that, because Lake Superior is the second freshest large lake in the world, the salinity was too low to measure, so we still need to put the conductivity circuit through its salty paces.

    The Story

    I blogged the whole adventure on OpenExplorer. Go check it out!

    The Validation Instrument

    Blue Heron carries a SeaBird Model 911 plus CTD.

    Lessons Learned

    • The OpenCTD logs too much. By which I mean you set it up in the dry lab, attach it the the winch, and then wait for the deployment. All the while, it's logging ambient, resulting in thousands of trash data points to clean up. A simple magnetic switch would solve that problem nicely.
    • with mineral oil, the tube is functionally incompressable and we had no leaks, despite several casts to various depths.
    • Casts need to be slow. On the order of 0.15 m/s slow. The temperature probes are not fast.
    • You need to let the OpenCTD sit on the surface for a while, like 10 minutes if it's been in the hot sun, in order to give the probes time to equilibrate.
    • The data was significantly cleaner than I expected for a first sea trial.

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  • 1
    Step 1

    Comprehensive build instructions can be found in the OpenCTD GitHub repository: OpenCTD

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