01
Vacuum check
Pull a vacuum on the sealed, empty housing and hold it to confirm the O-ring seats are seated correctly before any water is involved.
Subsea enclosure & pressure vessel guide
Selecting a subsea enclosure for marine research payloads.
For a one-off demonstration, an enclosure only has to keep water out for an afternoon. For a research payload that goes back in the water on a repeated field or lab-testing schedule, the same housing becomes a reliability decision: depth rating, material, seal chemistry, and internal fit-out determine whether a season's data collection survives its own hardware. This guide works through subsea enclosure and pressure vessel selection for ROV-mounted and fixed sensor payloads, from tube material through pre-deployment testing.
Housing and end-cap material
Depth rating and material trade-offs.
The base decision is the pressure vessel tube itself, and it is a deployment-pattern question as much as a depth number. A subsea acrylic pressure vessel tube is machined from cast PMMA and gives full optical clarity through the tube wall, at depth ratings that scale down as diameter and length increase (roughly 20–250 m depending on size). A subsea aluminium pressure vessel tube trades that transparency for a flat depth rating to 1,000 m, better resistance to UV and biofouling-cleaning solvents over repeated dockside servicing, and useful thermal conductivity for dissipating heat from embedded compute inside the housing.
End-cap choice follows from whether the payload needs to see out. A camera, light sensor, or any optically coupled instrument needs a viewing window: a subsea acrylic end-cap is a workable, low-cost option for shallow, short-duration work, but repeated tightening against the same acrylic face can indent or crack it, so it suits infrequent disassembly rather than a service-every-cruise workflow. For wide-angle or gimbal-mounted cameras, a domed subsea optical dome end-cap in cleanroom-molded polycarbonate reduces the image distortion and drag a flat window introduces, and where research-grade optical throughput and repeated servicing both matter, a subsea optical glass flange end-cap pairs a hardened borosilicate window (rated around 95% light transmittance) with an aluminium flange body that tolerates far more service cycles than acrylic.
For payloads with no optical requirement — data loggers, CTD strings, hydrophone arrays — go blank rather than paying for a window nobody looks through. A subsea aluminium end-cap is available blank or pre-drilled in M8/M10 patterns for feeding multiple sensor or cable runs through a single cap, and where weight and corrosion resistance matter more than depth margin, an injection-molded subsea polymer flange end-cap combines the flange and cap into one lightweight, glass-filled nylon part for shallower, benign-water research use. Whichever end-cap is chosen, it seats against a subsea aluminium flange, which carries the double O-ring groove that actually makes the tube-to-cap joint watertight.
Mounting the finished tube to the vehicle frame is also a material-linked decision, not an afterthought: an acrylic tube should be secured with a purpose-built subsea acrylic pressure vessel clamp set rather than a rigid rail interface, since repeated clamping directly onto a rail can stress and eventually crack the thinner acrylic wall. An aluminium tube tolerates that rigidity and mounts directly to an aluminium pressure vessel enclosure mounting plate onto the Arca-Swiss rail hardware already common on ROV frames.
Seals and cable entry
Sealing reliability: O-ring compound and gland sizing.
Over a deployment measured in months rather than days, seal chemistry stops being an afterthought. Static face seals on flanges and end-caps are commonly specified in NBR70 (nitrile) — a serviceable, low-cost general-purpose compound for freshwater and moderate-temperature use — but NBR swells and degrades faster than FKM (fluoroelastomer) compounds under sustained saltwater immersion, surface UV exposure between deployments, and contact with biofouling-removal solvents. Bulkhead seals on cable-penetrator threads, which see the most handling of any seal on the housing, are commonly specified in FKM75 for exactly that reason. Before a multi-month program, order a spare O-ring set matched to your specific flange, end-cap and bulkhead sizes, and replace rings at each service interval rather than reusing ones that have already taken a compression set — a ring that has been compressed once does not return to its original cross-section, and its sealing margin on the next cycle is smaller than it looks.
Gland cable penetrators are the second common failure point, and the failure is almost always a sizing mismatch against the cable rather than a defect in the component itself: a compression gland has to close on the cable's actual jacket outer diameter, not the OD used on a previous harness. A deep-sea M10 gland cable penetrator covers a wide cable range across six size variants at up to 300 m in aluminium or 1,000 m in stainless steel, while a mid-depth M8 gland cable penetrator gives a slimmer profile for tight bulkhead spacing on compact payload boxes. For benign, shallow freshwater trial work where cost and weight matter more than depth margin, a low-depth M10 gland cable penetrator uses a brass/copper body rated to 100 m. In every case, measure the cable's actual jacket OD with calipers before ordering — a gland closed on an undersized cable leaks at the jacket interface, and one forced over an oversized cable overstresses the elastomer and shortens its working life well before the next scheduled service.
Validation before deployment
Testing before fieldwork.
A pressure vessel that has never actually seen pressure before its first dive is an unverified assumption, not a validated instrument. Labs running repeat deployments get more reliable results treating pressure testing as a standing pre-cruise step rather than a one-time qualification done at first purchase. A bulkhead pressure testing tube validates an individual threaded seal, penetrator, or plug in isolation: the sample threads into each end of the clear acrylic test fixture, the assembly is pressurized inside a compatible chamber, and the transparent body allows a direct visual check for water ingress afterward — before that seal is committed to a full enclosure assembly where a leak is harder to isolate. For validating a complete, assembled enclosure with its cable runs and connectors together, a water pressure testing chamber — rated to 10 MPa (roughly 1,000 m equivalent) with an 18-port hatch cover for feeding through live cabling — exercises the whole system exactly as it will be loaded in the field, rather than each component on its own.
02
Static pressure hold
Hold the housing at or beyond its rated depth for a fixed soak time in the test chamber, not just a momentary spike to the target pressure.
03
Cycle test
Repeat the pressurize/depressurize cycle for seals and penetrators that will see multiple deployments, since a single successful hold does not confirm long-term fatigue behaviour.
04
Visual inspection
Open the housing and inspect O-ring seats, desiccant, and any acrylic components for moisture, indentation, or stress marks before signing off for the field.
Payload layout
Internal fitting and payload organisation.
Once the housing and seals are settled, the inside of the tube still has to hold electronics still against vibration and handling shock without shorting against the bore wall or crowding the O-ring grooves at either end. An electronic tray set for enclosure pressure vessels gives multi-board payloads a shelf structure sized to the tube's internal diameter, with pre-tapped mounting holes rather than field-drilled ones that risk swarf inside a sealed housing. Where a single insulated deck is enough — a controller board and a small breakout, for instance — an insulated PCB board shelf provides double-sided, electrically insulated mounting space and doubles as a layout reference while the payload is still being designed. For components too large or irregularly shaped for the tray-and-shelf system, single-point enclosure tray shelf supports mount around them with a smaller footprint than the double-point tray.
Longer sensor strings — multiple loggers, a hydrophone array, or a battery bank sharing a housing with a compute module — outgrow a single tube length quickly. A subsea enclosure tube coupling joins two 115 mm tube sections while adding sixteen radial M10 ports of its own, moving additional cable-penetrator density away from the already-crowded end-caps. Where the payload is built around embedded compute — a Jetson- or Raspberry Pi-class board rather than a sensor string — a box-form subsea watertight enclosure box trades the tube's cylindrical simplicity for a rectangular footprint that is easier to populate with multiple boards and connectors at fixed, known positions.
Sources & basis
What this is based on.
- Depth ratings, materials, diameters and weights for the Blu-Sub enclosure, end cap, flange and penetrator ranges as published in the Vebix Automation catalogue.
- Elastomer behaviour under saltwater immersion and solvent exposure — general selection practice for NBR and FKM compounds.
Published 7 August 2026. Last revised 16 August 2026. Corrections to sales@
Need help specifying a configuration?
Discuss your enclosure spec with Vebix Automation.
Enclosure specification is easiest to get right once, before hardware is machined and O-rings are ordered in the wrong compound. Vebix Automation supplies the Blu-Sub subsea enclosure range covered in this guide and can help confirm depth rating, seal compound, gland sizing, and internal fit against your payload before you commit to a configuration.