Comparison
Cable penetrator versus underwater connector
Both take a conductor through a pressure boundary. They fail differently, cost differently and suit different service patterns, and choosing on price alone is how a research platform ends up undivable for a fortnight waiting on a part.
What each one is
A cable penetrator is a fitting that carries a cable through the wall of a pressure housing and seals around it. The cable is continuous: it goes in wet, comes out dry, and the seal is made against the cable's own jacket, either by mechanical compression or by potting it in epoxy. There is no disconnection point.
An underwater connector is a pair that plugs together: a bulkhead half fixed in the housing wall and a cable half that plugs into it. The pressure boundary is sealed by the bulkhead half; the join between the two halves is what you can open. Dry-mate connectors must be mated before immersion; wet-mate connectors can be mated underwater, at considerably more cost and complexity.
For design, the question that matters is whether you will ever need to separate the cable from the housing without opening the housing.
Choose a penetrator when the run is permanent
Penetrators are cheaper per entry, take less bulkhead area, and have fewer things to go wrong: no contact pins to corrode, no mating faces to damage, no O-ring on a connector shell to pinch. For a sensor that lives on the vehicle, a thruster whose cable is not going anywhere, or a power feed between two permanently coupled housings, a penetrator is the correct answer and the connector is over-engineering.
Within penetrators there is a second choice. An epoxy-potted penetrator is permanent. It is reliable and cheap, and to change the cable you replace the whole fitting. A mechanical compression gland can be opened, the cable re-terminated and the gland reused, which suits a research vehicle that changes payload between deployments. The gland costs more and needs the jacket outer diameter measured properly.
The dominant penetrator failure mode is sizing. A compression gland closes on the cable's actual jacket OD; closed on an undersized cable it leaks at the jacket interface, forced over an oversized one it overstresses the elastomer and shortens its life. Measure the jacket with callipers before ordering. The conductor size and the figure from the last harness are both the wrong numbers.
Choose a connector when you need to disconnect in the field
The case for a connector comes from how you operate the vehicle. If a camera, sonar head or sampling module has to come off between dives, if the vehicle ships in pieces, or if a faulty cable must be swapped on deck without opening a housing, a connector pays for itself straight away.
It earns it again on housings that would otherwise be opened repeatedly. Every time a pressure housing is opened, its main O-ring is disturbed, the desiccant is exposed and the assembly has to be re-tested. A connector that lets the cable change without breaking the housing seal removes an entire category of risk from the maintenance cycle.
Connectors have real costs. They cost more per circuit, take more bulkhead space and add contact resistance. They can also fail in ways a penetrator cannot: bent pins, a shell flooded because it was left unmated, and corroded contacts on a mating face dipped in salt water and not rinsed.
Depth rating and seal compound apply to both
Body material sets the depth rating in both cases. Aluminium bodies are the general-purpose choice to a few hundred metres; stainless steel takes the same thread pattern deeper; brass and copper suit shallow, benign freshwater work where cost and weight matter more than depth margin.
Seal compound is the part most often left to chance. Nitrile (NBR) is serviceable and cheap for freshwater and moderate temperature, but it swells and degrades faster than fluoroelastomer (FKM) under sustained saltwater immersion, surface UV between deployments, and contact with biofouling-removal solvents. Bulkhead seals see more handling than any other seal on a housing, which is why they are commonly specified in FKM regardless of the rest of the build.
Whichever you choose, the depth rating of the assembly is the lowest rating in the chain. A 1,000 m penetrator in a 300 m housing gives a 300 m system.
A practical rule
Default to penetrators, and spend the money on connectors only where the operational case is explicit: this cable comes off, in the field, without opening the housing. Count how many entries each end cap can take before you commit, because space for penetrators usually runs out first. If it does, a tube coupling with its own radial ports is often cheaper than a larger housing.
On a mixed platform both belong: penetrators for the permanent internal runs, connectors for the payload interface that changes between deployments.
Sources & basis
What this is based on.
- Depth ratings, body materials and thread sizes for the Blu-Sub penetrator and connector range as listed in the Vebix Automation shop.
- How NBR and FKM compounds behave in salt water and in contact with solvents, from general elastomer selection practice.
Published 16 August 2026. Last revised 16 August 2026. Corrections to sales@
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