Underwater CTD Sensor —
Construction Surveying

CTDs measure conductivity (a proxy for salinity), temperature, and depth — three readings that underpin almost every piece of marine engineering advice, from material selection to corrosion protection strategy. This instrument was scoped for a specific use case: surveying shallow-water construction sites to a maximum depth of 300m, ahead of underwater build work.

Rather than designing a fully self-contained instrument, the CTD was scoped as one half of a two-part system. A solar-powered surface buoy (referenced against the commercially available VIKING platform) handles power generation, primary data logging, and satellite telemetry — functions that are far easier to provide at atmospheric pressure than 300m down. The submerged pod is reduced to the minimum it needs: sensors, a microcontroller, local backup storage, and power conditioning.

That split was the key early decision. It meant the pressure housing didn't need to carry batteries, solar circuitry, or a transmitter — so it could stay small, simple, and cheap to seal, while the buoy above sustains the system indefinitely.

The housing follows a modular three-piece architecture: a bottom cap carrying the exposed sensors, a centre body forming the sealed dry compartment for the electronics, and a top cap providing the cable/tether interface. The cylindrical profile was chosen deliberately — it's the most efficient geometry for resisting uniform external hydrostatic loading, and it simplifies both manufacture and sensor integration compared to a more complex hull form.

The conductivity probe sits in a recessed pocket at the base, with a 5mm gap in the wall to let water reach the sensing element, while the three pressure sensors are recessed more tightly since their smaller housings don't need the same flow path. Temperature sensors are mounted against the inner wall of the housing rather than exposed directly — heat conducts through the aluminium wall quickly enough to track ambient seawater temperature, without punching another hole through the pressure boundary.

Internally, a sliding frame carries the PCB, battery, and charging electronics on a track that prevents rotation during assembly — so the whole electronics stack can be built, tested, and slid into the shell as a single unit.

Every joint in the housing — the two cap threads and the cable entry — is a potential failure point at 30 bar, so each was treated as its own sealing problem. The caps thread onto the centre body using a single-rotation thread profile, chosen specifically so an operator can't over-rotate the cap and shear the internal wiring during reassembly at sea.

O-ring material is CR (Neoprene) at ~70 Shore A — a general-purpose hardness with good long-term resistance to saltwater and UV exposure — sized against the Parker O-ring handbook for each probe diameter. An L-seal backs up the primary O-ring at the higher-pressure interfaces to resist seal extrusion under sustained load.

Cable entry uses a Blue Robotics WetLink Penetrator: a compression-gland fitting that seals the tether jacket directly, sized to a 4.5mm seal on an M10 bulkhead to match the four-core power/data cable. It's rated well beyond the 300m operating depth, which was a deliberate margin rather than a minimum spec.

The buoy's 4×100W solar array and 200Ah battery bank supply power down a 4-conductor tether, which the CTD converts locally to regulated 5V and 3.3V rails for the electronics. A local 3,500mAh battery and solar-charging board give the pod roughly a month of autonomy on its own reserve — resilience against a tether or telemetry interruption rather than the primary supply.

Data takes the same layered approach: the Raspberry Pi Pico logs to onboard SD card via SPI, and pushes readings to the buoy over an RS-485 link — chosen over a simple UART for its noise immunity across the length of tether involved. The buoy relays compressed summaries to shore by satellite, while the CTD's local card retains full-resolution data as a backup against any comms dropout.