What an O-ring is actually doing
Why a squeezed rubber ring seals, and how pressure helps it.
An O-ring seals by being squeezed into a slightly smaller space than its relaxed cross-section wants to occupy, so that the rubber pushes back against the surfaces on either side with a constant, built-in force before any water pressure is applied at all. That squeeze is what closes the gap a seal exists to close; the ring's shape and material only make the squeeze possible.
An enclosure rated for depth leaked at the cable entry, nowhere near the lid, which forced a proper look at what an O-ring was doing at every sealed joint in that housing. Until then, a ring dropped into a groove had been trusted to do its job simply by being an O-ring.
Squeeze, and why rubber keeps pushing back
A ring's cross-section, the fat circular profile most people picture, is deliberately made slightly larger than the gap it will occupy once the two mating surfaces are brought together and clamped. That compression, called squeeze, forces the rubber outward against both surfaces at once, filling every microscopic irregularity in a way a loosely fitted ring never could. Rubber's elasticity is what makes this work. A compressed ring keeps pushing back for as long as the compression is maintained, whereas a rigid material, once deformed, would simply stay deformed and stop pressing on anything.
Without that compression an O-ring does very little. A rubber ring resting loosely in a channel with nothing pressing on it will let water straight past, and the whole sealing action depends on the ring being held smaller than it wants to be for as long as the joint stays assembled. The O-ring tends to get the credit for sealing, but the job is really a partnership. The ring supplies a flexible, elastic material and the surrounding assembly supplies the squeeze, and neither achieves anything on its own.
Pressure then adds to the squeeze. Water pushing from one side shoves the ring across its groove against the far wall, and the rubber, behaving much like a very thick liquid, passes that push outward onto the surfaces it touches. The contact force rises with the pressure being sealed against, so a correctly squeezed ring grips harder the harder the water pushes, up to the point where the rubber starts to be forced into the thin clearance gap between the two parts.
The bath plug
A rubber bath plug seals a drain by being squeezed slightly, its flexible rubber flaring outward against the sides of the hole under its own weight and the water pressing down on it, filling whatever small gaps sit between the plug and the drain's real, imperfect shape. An exact match to the hole's shape is neither possible nor needed. An O-ring does the same job at a smaller, more controlled scale, oversized relative to its groove so that it too presses outward against both surrounding surfaces, closing gaps that no rigid seal machined to a theoretically perfect fit could close as reliably against the small imperfections real parts carry.
A plug sized exactly to the drain's nominal diameter, with no flare to spare, lets water seep steadily past its edges however carefully it was made. An O-ring given a perfect, unsqueezed fit in its groove seals no better than that plug.
Compressed by less than a millimetre
For a static seal, an O-ring is typically squeezed by somewhere around 15 to 30 percent of its relaxed cross-section once installed, enough to feel firm resistance when pressing the ring into its groove by hand. Too little squeeze leaves gaps the rubber cannot bridge, and too much brings its own problems, the subject of the third article in this set. Cross-sections are often only a few millimetres across, so the compression being aimed for is small in absolute terms: a 3 mm ring squeezed by a quarter is compressed by about three quarters of a millimetre.
A seal made of two parts
Because sealing depends on squeeze, sizing the groove matters at least as much as choosing the ring. A groove cut too deep leaves too little compression however well the ring is made, and a groove cut too shallow over-compresses it. An O-ring seal is a system of two parts, the ring and the groove that squeezes it, and judging either without reference to the other misses the mechanism doing the sealing.
The leaking cable entry delivered exactly this lesson. Every O-ring surface in the enclosure had been checked, while the cable entry relied on a different sealing method that nobody had scrutinised with the same care, because it did not look like the kind of joint that needed attention. Its unfamiliar appearance is precisely why it slipped through, and a sealing method that is harder to picture can be every bit as demanding to get right.
Squeeze is also insufficient when the surfaces the ring presses against are damaged, scratched or contaminated. A ring squeezed exactly right can still fail against a scratch deep enough to give water a path around the rubber, so the finish on the mating faces matters as much as the compression. A well-squeezed ring on a poor surface and a poorly squeezed ring on an excellent one both fail for reasons the squeeze figure alone cannot explain. The rest of this set returns to this two-part system from different angles: the groove next, then the danger of over-compression, and finally the cable entry that started this line of questioning.