Evolution of low-cost underwater camera technology

By leveraging cheap technology we aim to dramatically lower the cost of real-time ocean monitoring.  Continuous access to the ocean depends on novel infrastructure and technologies that can expand our monitoring capabilities whilst simultaneously reducing costs.  One such avenue we're continually exploring is low-cost underwater camera technology.

Digital video technology is rapidly improving with emerging possibilities to record high resolution digital imagery over long periods of time and for a much lower cost. Video and image based monitoring in the ocean offers many advantages over traditional diver-based observational techniques. A common fish surveying technique researchers utilise is Baited Remote Underwater Video Stations (BRUVS). These stationary seafloor camera frames use bait to attract fish, capturing the activity on video. Footage is then analysed back in the laboratory where researchers manually perform species ID and individual counts. This technique is extremely labour intensive and a major disincentive against the application of this technology. More commonly, researchers are looking for more efficient ways to capture fish community data. Unlike BRUVS our cameras are designed to be deployed for months at a time live-streaming footage to the cloud.

AusOcean’s previous underwater camera design employed a “hard shell” housing. This means the contents of the body (camera and electronics) are simply enclosed and sealed at sea level: 1 atmosphere of pressure. This permits an extremely basic and cheap housing design that requires minimal materials and tools to fabricate. Unfortunately, we found ourselves with a few problems.

If we neglect deformation of the PVC, internal pressure of the housing should remain equal, i.e. 1 atmosphere. This means however, that a significant pressure difference between the inside and outside of the housing is induced when we deploy it at our target monitoring depth underwater. The surrounding water really wants to move to the lower pressure volume and equalise the difference. If there are any imperfections in the housing seals significant enough to allow water to pass through (a function of fluid viscosity), water will slowly ingress into the housing and fry the electronics.

Hard shell designs are often acceptable in applications where submersion is temporary; think waterproof watches, GoPro cases and waterproof phones. Seals don’t need to be perfect; the ingress is usually slow enough so as to be negligible for small periods of time (these items are usually rated for maximum depths and periods of submersion). Often this means these housings can easily be opened too. But, what if we require continuous submersion over the long term? 

We decided to adapt a fluid-filled pressure-compensating system, a concept conceived by the US Navy unclassified in the early 1970s. This system is now widely accepted as a reliable method of waterproofing for deep or long term ocean applications, particularly where there is motion between interface surfaces. Eliminating the “hard shell” pressure difference reduces the need for vigorous seals as the water doesn't want to enter the housing in the first place.  The housing, or at least part of it, is made flexible; allowing the water to press on the internal “medium” to equalise pressure.

This adaptation uses a mineral oil filled PVC housing with a pressure compensating membrane made from a 3 layer nitrile glove thumb. The pressure compensating membrane is stretched over a pressure plug with a drilled out hole and inserted into a socket on the rear cap. The membrane allows water to push on the internal mineral oil medium equalising the internal pressure. Now that seal quality is less of an issue, threaded connections have been implemented in 4 places, namely, the viewing window, rear cap, cable plug and pressure membrane plug. This allows the housing to be easily opened for repair/maintenance and ensures component reusability. Furthermore, this streamlines the construction process and reduces the amount of time required for assembly.

Mineral oil has very different optical properties which makes it impossible to focus the stock lens on the Raspberry Pi camera board when in contact. By using an Adafruit Pi Camera Board Tripod Case, sealant and a small piece of acrylic, we devised a method for enclosing the camera in an “oil tight” housing to mitigate this issue. It should be noted that it’s much easier to create an “oil tight” enclosure vs a “water tight” enclosure. The tendency for a fluid to leak is a function of viscosity, and mineral oil’s viscosity is about 30 times higher than that of water. A downfall is you cannot use standard sealants; most have a low resistance to mineral oil or other hydrocarbon liquids. For this application we used Sikaflex Tank N: a hydrocarbon resistant sealant used in the chemical and petroleum industries. 

The open end of the camera cable is potted into a pressure plug socket using 2 part epoxy. This ensures oil cannot leak up through the cable sheath resulting in a pressure difference and therefore, potential water ingress. Having the cable potted in a threaded plug allows us to easily swap out the cable for different lengths when required. 

Other than the aforementioned oil medium, interfacing and camera differences, the hardware of the camera and mounting method is much the same. With the addition of the mineral oil and threaded components, the cost of the camera has increased, but it is still significantly cheaper (and easy to make) in comparison to commercial long term underwater cameras.

As of writing this, our first ocean deployed Fluid-filled Pressure-compensating Underwater Camera (FPUC) deployed at Carrickalinga, South Australia has been running well for 2 weeks. This is the first of many more deployments of this new and improved underwater camera design.

Continuous underwater video is valuable from both a scientific and public engagement point of view. Not only does camera technology enhance visualization of underwater environments, it expands our capacity to connect and engage with the wider public remotely.

AusOcean is a not-for-profit ocean research organisation that supports open source practices. Open source approaches to tackling environmental issues means embracing collaborative tools and workflows which enables processes and progress to be fully transparent. A critical aspect of working open is sharing data not only with your immediate team but with others across the world who can learn, adapt and contribute to collective research. By contributing to, and supporting open practices within the scientific community, we can accelerate research and encourage transparency. All tech assembly guides can be found at https://www.ausocean.org/info.

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