Short answer: build it wet on the outside and dry on the inside. Connect everything on the bench, then take each cable through the wall as a dry-mated IP68 connector or an epoxy-potted penetrator, and rate every entry for twice the working depth. A housing that dives to 20 ft [6 m] carries about 9 psi on each seal. A wet-mate underwater connector costs several times a dry-mate part and buys you nothing unless you must connect or disconnect in the water. The sections below pick the entry for your depth: what IP68 actually promises, the pressure your seals see, when a gland holds, when to pot instead, and the 316L wall that carries it all.
What does an IP68 rating promise underwater?
An IP68 rating promises continuous immersion on conditions agreed between the maker and you: IEC 60529 fixes digit 7 at 1 m [3.3 ft] for 30 minutes and leaves digit 8 open, so the maker prints the depth and the time.
The IP code grades a housing against dust and liquids, and only the second digits 7 and 8 claim any immersion at all. Digit 6 — the second digit on our stainless wall-mount boxes — is powerful jets, a 12.5 mm nozzle at 100 L/min, and it is not an immersion rating. A box that lives on IP66 stays above the waterline.
For US drawings, the NEMA types pair up as follows: Type 6 is temporary submersion, nearest to IP67, and Type 6P is prolonged submersion, nearest to IP68. NEC 110.28 states that IP ratings are not a substitute for enclosure type ratings, so a spec that calls out Type 6P needs the NEMA type on the order as well. The Type 4X versus IP66 comparison shows how the two systems map onto each other.
Every rating in this article is the manufacturer's rating: our enclosures, for example, are rated NEMA 4X / IP66 by the manufacturer; not UL listed — and a connector's depth claim works the same way. Read the printed depth and the printed minutes, not just the two digits; a datasheet that says IP68 with no depth and no time has told you nothing about your job.
The depth-and-time question sits behind the digits: why IP68 ratings carry depth and time limits takes it apart, and the IP rating chart keeps the whole code in one table.
How much pressure does the water add at your depth?
Every 10 m [33 ft] of depth adds 1 bar [14.5 psi], so the example dive loads each seal with about 9 psi and a dive twice as deep with about 18 psi.
The basis is the water column. Fresh water presses 0.433 psi per foot of depth and seawater 0.445 psi, and the load lands on every joint at once: door gasket, gland seal, connector face. A fast cross-check for any part on a datasheet: a component that holds a hard vacuum, 14.7 psi at sea level, holds about a 34 ft [10 m] water column, because 14.7 ÷ 0.433 = 34 ft.
Worked example: a 316L sensor housing for a 20 ft [6 m] vehicle. The head is 20 ft × 0.433 psi/ft = 8.7 psi, call it 9 psi, pressing inward on every joint. Rate each entry at twice the working depth and you order parts published to 40 ft [12 m], about 18 psi, which keeps margin for wave slap, current and a hard landing. In seawater the same dive is 20 ft × 0.445 = 8.9 psi: the same parts, the same decision. The total load is real even when the pressure sounds small: 8.7 psi over a 12 x 10 in [305 x 254 mm] door face is 120 in² × 8.7 = about 1,040 lbf (4.6 kN) pushing the door into its gasket.
Pressure from outside compresses seals rather than peeling them off, which is why shallow immersion kills entries at leak paths, not at the strength of the wall. The failure you should plan against is a wick along a cable, a nicked O-ring or a dry thread — the sections below take each in turn.
Dry-mate or wet-mate: which connector does the job?
Connect dry on the bench: any connector you mate in air only has to hold the water out, while a wet-mate connector earns its several-times-higher price only when you must connect or disconnect under water.
If the housing opens on a bench or in dry dock, a dry-mate pair is the entire requirement: mate it in air, seal it, and the joint rides out the dive. Look for a depth printed for the mated pair — some parts are rated unmated only with caps on — and apply the 2:1 margin from the worked example, so the example vehicle orders connectors published to 40 ft [12 m] or better.
Wet-mate connectors solve a different problem: making and breaking the circuit in the water, on a diver's glove or a manipulator arm. They pay for that with larger shells, positive keying, high mating forces and prices several times the dry-mate equivalent. Oilfield tooling and survey work pay it; a bench-built vehicle or a fixed sensor housing does not.
So the decision is one question: does any joint get made under water? If the answer is no, skip the wet-mate catalog and put the difference into the wall, the pot and the pressure test.
Can a cable gland seal a wall that goes under?
Yes, when the gland is rated for immersion, grips the jacket, and seals to the wall on an O-ring or bonded washer: the weakest part sets the rating, so an IP66 gland on an IP68 connector leaves you an IP66 assembly.
A compression gland seals twice: the elastomer ring grips the cable jacket, and an O-ring or bonded washer under the hex seals the thread into the wall. Both joints carry the head, so both go on the spec. Match the clamping range to the jacket, not to the conductor count — a 0.35 in [9 mm] jacket needs a gland whose published range covers 9 mm — and the M20 thread is the common choice for multi-conductor cable through a stainless wall. Dope the male thread with an immersion-rated sealant and torque the hex to the gland maker's value.
For the part itself, picking an M20 SS316 IP68 gland for 40 m depth walks the pressure-to-rating check on a real gland, and what watertight vs gas-tight gland ratings prove explains what the marking on the bag was actually tested for.
A molded plug or an overmolded connector will not pass a gland's jacket seal; for those cables use a bulkhead connector on the wall or pot the entry, and getting a connector through a cable gland without leaks covers the awkward cases. A rubber grommet in the hole protects a jacket from a sharp edge but is not a pressure seal — rate the entry, not the grommet.
Cut the holes before anything else: the cutout designer lays out the gland positions, checks spacing against the gasket line, and exports the layout with the quote.
When is a potted penetrator the better entry?
For the deepest service, or an entry you will never open, pot it: a penetrator bonded into the wall with epoxy has no O-ring to relax, and a sound pot shrugs off the 9 psi the example dive asks of it.
The pot replaces every dynamic seal with one static bond, which is why potted penetrators and sealed pin plates are the standard entry on housings that live under water. The bond has to be made well, and four habits get it there:
- Key the surfaces. Abrade or score every bonded face: a wire-brushed bore gives the epoxy something to grip, and a polished bore gives it nothing.
- Break the wick. Cut the jacket back so conductor insulation does not run continuously through the pot; water travels along strands and under insulation, and one wet path floods the housing. Stop water wicking through cable into an enclosure shows the cuts and the dams.
- Make the bond long. About 5 jacket diameters of pot depth is a sound shop rule, so a 0.35 in [9 mm] jacket wants a 1.8 in [45 mm] bond; that fits a short through-bulkhead boss on any wall 2 in [50 mm] or deeper.
- Cure before launch. A typical room-temperature epoxy reaches its published properties in about 24 h at 20 °C (68 °F); follow the potting compound's datasheet rather than the clock on the wall.
Penetrators with sealed pins — glass-to-metal or epoxy-potted — land the wires inside and mate to a dry-side connector, so you keep a disconnect without giving the water an entry. That style covers the middle ground: nothing to torque, no connector face under water, and a bond you can bench-test before the housing ever gets wet.
Which enclosure wall and finish survive immersion?
Order 316L: its molybdenum lifts pitting resistance over 304, 304 already pits at room temperature in the crevice under a gasket or a tired gland, and above about 60 °C (140 °F) both grades risk chloride stress-corrosion cracking.
Under water the attack is crevice corrosion, and the crevices you control are the ones you cut and the ones you bolt. Keep cut edges passivated, keep the gasket line continuous, and lay entries away from corners where water sits. For a housing with factory-cut entries in 316L, custom stainless enclosures with factory cutouts is the starting point; the approval drawing is where you check seals, mounting and rear clearance before anything is cut.
Continuous immersion, above all in warm seawater, is a duty beyond any catalog splash rating. Write the depth, the medium and the time into the order and agree them with the manufacturer — that written agreement is literally what the second digit 8 means. Continuous submersion enclosures: depth and rating limits walks that conversation in detail.
Which entry wins for each depth?
Match the entry to the duty, and the shallow jobs stay cheap: splash and hose-down take a standard box, an occasional 1 m [3.3 ft] dunking takes IP67 parts, and continuous immersion takes a dry-mate IP68 pair or a pot.
| Duty | Head at the seals | Entry that holds | Wall |
|---|---|---|---|
| Rain, washdown, hose | none | Type 4X / IP66 box with IP66 glands | 304 or 316L |
| Occasional dunking to 1 m [3.3 ft] for 30 min | about 1.4 psi | IP67-rated entries, dry-mated | 304 or 316L |
| Continuous immersion to 20 ft [6 m] | about 9 psi | Dry-mated IP68 connector or epoxy-potted penetrator, published to 40 ft [12 m] | 316L |
| Continuous immersion past 100 ft [30 m], or mating in the water | 43 psi and up | Wet-mate connector system | 316L, duty agreed with the manufacturer |
A wet-mate connector buys nothing below about 100 ft [30 m] unless a diver or a tool has to make the joint in the water. Below that line, mate dry on the bench and carry the water at the wall — gland or pot, rated to twice the depth.
What do you put on the order?
Put the depth, the medium and the disconnect plan on the drawing in one line: 'continuous immersion to 20 ft [6 m] fresh water, 316L wall, entries published to 40 ft [12 m], no underwater mating.'
| Line | What to write | Example for the worked-example housing |
|---|---|---|
| Wall | Material and grade | 316L stainless |
| Depth and medium | Depth, water type, continuous or occasional | 20 ft [6 m], fresh water, continuous |
| Entries | Type per cable and its published depth | Potted penetrators published to 40 ft [12 m] |
| Gland size | Clamping range covers jacket OD | M20 for a 0.35 in [9 mm] jacket |
| Layout | Hole positions and spacing | Export from the cutout designer |
| Disconnect plan | Mate in air only, or wet-mate | Mate in air only |
| Proof | Bench pressure test before launch | Hold 13 psi (1.5 × 8.7 psi) for 30 min |
A custom 316L body with factory-cut entries is delivered in 10 business days, counted from your approval of the drawing; catalog sizes count from the order. Prove the entries before the housing goes near water: pressurize the inside to 13 psi — 1.5 × 8.7 psi for the example depth — hold it 30 minutes, and watch the gauge. Inside-out is the harder direction for every seal, so a steady bench gauge buys the dive. Run the test with the housing empty, no boards and no batteries, so a leak costs you a re-seal instead of a rebuild.
Questions
Is an IP66-rated enclosure safe to submerge?
No. IP66 is the powerful-jet rating, tested with a 12.5 mm nozzle at 100 L/min, and it carries no immersion claim; immersion starts at digit 7 — 1 m [3.3 ft] for 30 minutes — and digit 8 covers deeper, longer duty on conditions agreed with the maker. Our enclosures are rated NEMA 4X / IP66 by the manufacturer; not UL listed, so a submersible build is a custom order with the depth and time written down.
How much does 100 ft of water add?
About 43 psi in fresh water: 100 ft × 0.433 psi/ft = 43.3 psi, and seawater gives 44.5 psi — roughly 3 bar across every joint. That is wet-mate connector territory or a fully potted, welded build agreed with the manufacturer. The rule stays simple: every 10 m [33 ft] of depth adds 1 bar [14.5 psi].
Do I still need strain relief on a potted entry?
Yes, and the pot is not the place for it: keep a service loop outside the housing and clamp the cable within about 6 in [150 mm] of the wall, so a tug or a snagged line loads the clamp, not the bond. The 5-jacket-diameter pot holds the seal; the external clamp holds the mechanical load.
Can I reuse a gland or penetrator after it has been under water?
Treat a gland as a single-campaign part: the elastomer that gripped the jacket takes a set after months at 9 psi, and a second dive can find the path. Replace the gland seal or re-pot with fresh compound, then pressure-test again — a bench hold at 13 psi for the example depth finds the mistake on land, not at depth.
What rating does a housing need if it only gets splashed?
Splash and hose-down need no underwater parts: a Type 4X / IP66 enclosure with IP66-rated glands holds rain and washdown, and the money you save goes into gasket and entry quality. Reach for IP67 or IP68 parts the moment the housing can sit under water, even 1 ft [0.3 m] of a flooded pit.
