The case is the part of a dive watch that does the actual diving. The dial gets the attention and the bezel gets the marketing, but the case is what holds back the water, resists the pressure, and either can or cannot be opened and resealed for the next fifty years. Dive-watch cases come in a small number of distinct construction types, each with a different answer to the same question: how do you get inside to service the movement while keeping the water reliably out?
The three ways a case is sealed and opened
Screw-back cases
The most common construction. The case back is a separate disc that threads into the case body against a gasket, like a jar lid. A watchmaker opens it with a wrench, services the movement, fits a fresh gasket, and threads it home. The advantage is serviceability: getting in and out is straightforward, so gaskets can be renewed routinely and water resistance restored. Most modern dive watches, including the Aquastar Model 60, use a screw-back.
Monobloc (monocoque) cases
A monobloc case has no separate back at all. The case is machined from a single block of metal, and the movement is fitted from the front, through the crystal opening. To service the watch, the crystal and bezel come off and the movement lifts out the top. The advantage is one fewer sealing surface to leak, the back cannot flood because there is no back. The trade-off is that servicing is more involved, and the crystal gasket does double duty as the primary seal. Several celebrated professional dive watches of the 1960s and 1970s used monobloc construction for exactly this reason.
Compressor cases
The Super-Compressor, patented by the case-maker Ervin Piquerez S.A. in the 1950s, is a clever piece of engineering. Its case back is a spring-loaded disc: as external water pressure rises with depth, the water presses the back more firmly against its gasket, so the seal actually tightens the deeper you go. Compressor cases are often paired with an internal rotating bezel operated by a second crown, which keeps the timing bezel behind the crystal where nothing can knock it. For the bezel side of this, see how dive bezels work.
Case materials
- Stainless steel. The default for dive watches since the 1950s. Grade 316L is the common marine-grade alloy: strong, corrosion-resistant, and easy to finish and refinish. The vast majority of dive watches are steel.
- Titanium. Lighter than steel and highly corrosion-resistant, favoured for larger professional dive watches where weight matters on the wrist. It scratches more readily than steel and has a distinct grey tone.
- DLC and PVD coatings. Thin, hard surface coatings applied over steel to produce a black or dark finish that also resists scratching. The Aquastar Airstar DLC is an example. The coating is a surface treatment, not a different material underneath.
- Bronze. Used by some makers for its warm colour and the patina it develops. Less common on serious dive watches because it reacts with skin and salt water, developing a living finish that some owners prize and others avoid.
The crystal is part of the case
The crystal seals the front of the case and takes the same pressure as the metal around it. Three materials are used. Acrylic (Plexiglas) was standard through the 1960s: cheap, shatterproof, and easy to polish, but soft and easily scratched. Mineral glass is harder but can shatter. Sapphire, the modern standard, is second only to diamond in scratch resistance and is what almost every current dive watch uses, including every modern Aquastar. On deep-rated watches the crystal is thick and often domed to resist pressure without distorting. A vintage acrylic crystal can be polished back to clarity; a scratched sapphire generally cannot, so it is replaced.
Depth rating and case construction
The deeper the rating, the more the case construction has to do. A 200-metre recreational case needs sound gaskets and a screw-down crown. A 500-metre case needs a thicker crystal, more substantial gaskets, and sometimes a helium escape valve for saturation divers who spend days in a pressurised habitat. Aquastar's Benthos 500 was the first Swiss dive watch rated to 500 metres, and its case was engineered specifically to be serviceable at that depth rating rather than sealed for life. Serviceability is the point: a case that cannot be opened and resealed has a finite life, no matter how deep it is rated. On an Aquastar the interval is every 5 to 10 years, with no annual service needed even with daily diving.
Frequently asked questions
What are the main types of dive-watch case?
There are three main constructions. A screw-back case has a separate threaded back that opens for service. A monobloc case is machined from a single block with no back, so the movement is fitted through the front. A compressor case has a spring-loaded back whose seal tightens as water pressure increases with depth.
What is a Super-Compressor case?
A Super-Compressor is a case design patented by Ervin Piquerez S.A. in the 1950s. Its spring-loaded case back presses harder against its gasket as external water pressure rises, so the seal tightens the deeper the watch goes. It is often paired with an internal rotating bezel operated by a second crown.
What metal are dive watches made from?
Most dive watches use marine-grade 316L stainless steel for its strength and corrosion resistance. Titanium is used on larger professional watches for its lighter weight. DLC and PVD are hard surface coatings over steel for a black finish, and bronze is used occasionally for its patina, though it reacts with salt water.
Is a monobloc case more water-resistant than a screw-back?
A monobloc case removes one sealing surface, the case back, so it has one fewer place to leak. The trade-off is that servicing is more involved because the movement must be removed through the front. A well-maintained screw-back with fresh gaskets is entirely reliable; the difference is about construction philosophy, not a large practical gap in water resistance.
Why does serviceability matter in a dive-watch case?
Gaskets age and lose their seal, so a dive watch must be opened periodically to renew them and restore water resistance. A case that can be opened and resealed can be maintained indefinitely; one that cannot be opened has a finite life. This is why Aquastar engineers its cases, including the 500-metre Benthos, to be serviced every 5 to 10 years rather than sealed for life.
Related reading: Waterproof versus water-resistant · The six features of a professional diver · The Aquastar Benthos 500 · Dive-watch glossary
