The Bench · N° 54
A dry pressure test can find a leak without wetting the watch
A modern tester changes the air pressure around a closed watch and measures minute case deformation. The watch can fail without a drop reaching the movement.
Hen & Mills Editorial · 4 min read · 25 October 2026
A dry pressure test can detect a leaking watch without exposing it to water. The instrument changes air pressure around the case, measures microscopic deformation and observes whether that deformation holds, returns or leaks away.
The case behaves like a small spring
A sealed watch case is not perfectly rigid. Increase the air pressure outside it and the crystal or case back deflects inward by a minute amount. Reduce the pressure outside and the trapped air inside pushes outward. The movement is tiny, but a sensitive displacement sensor can follow it.
If the case is sound, its deformation follows a predictable curve and remains stable during the measuring period. If air passes through a failed seal, pressure equalises and the case relaxes differently. The tester translates that change into a pass, a leak rate or a failure. No water is required because air is the test medium and case movement is the evidence.
Vacuum and pressure reveal different behaviour
A dry tester may use negative pressure, positive pressure or both. Under vacuum, the chamber pressure falls and the watch tends to expand. Under positive pressure, the chamber rises above the pressure trapped inside and the watch is compressed. Witschi's ProofMaster family uses controlled vacuum and pressure measurements for this purpose.
Testing in both directions can be valuable because seals do not always behave symmetrically. A gasket may seat more firmly when pressure pushes it one way and open when the load reverses. The instrument also needs limits suited to the watch, since excessive deformation could damage a thin crystal or case back.
Why dry testing is the sensible first step
Putting a leaking watch into water in order to discover that it leaks creates the exact damage the test is meant to prevent. A dry test can reject the case while the movement remains safe. The watchmaker can then inspect gaskets, crown tube, crystal seating and case surfaces before repeating the measurement.
It is also clean and quick enough to use after a battery replacement or routine case opening. The result confirms the assembled condition, not merely that a new gasket was fitted. A pinched seal or a strand of debris can defeat new material immediately.
A pass does not name the weak part
The sensor knows that air is moving, but a simple dry test does not necessarily say where. The leak could be at the crown, case back, crystal, pusher or another opening. Diagnosis may require isolating controls, using a separate localisation method or inspecting components under magnification.
This distinction keeps the machine's role clear. It answers whether the case holds under the test conditions. A watchmaker still interprets the pattern, chooses the next test and decides which parts need attention.
The wet test has a different purpose
Traditional localisation can pressurise a watch in air and then lower it into water while keeping it under pressure. As chamber pressure is released, compressed air inside escapes through the leak and forms bubbles. Because air is leaving, water should not enter during a properly conducted test. The bubbles show the fault's location.
That procedure is useful after a dry failure, but it requires care. Pressurising a case in water from the beginning would allow water to be driven in. Test equipment, sequence and watch limits matter. Owners should not improvise with pumps, jars or kitchen experiments.
Ratings belong to test conditions
A “100 m” mark does not mean a dry tester must imitate a casual journey to exactly 100 metres in the ocean. Water-resistance standards specify controlled pressure, duration and other conditions. Service equipment may test at pressures selected for the case rating and the manufacturer's procedure.
Static laboratory pressure also differs from a day in salt water. Temperature change, movement, impacts and operating a crown introduce variables that a basic chamber test does not reproduce. The result is valuable evidence of sealing at that moment, not a lifetime guarantee against every aquatic decision.
Deformation can complicate the reading
A very stiff case moves less, giving the sensor a smaller signal. A flexible crystal or case back may continue settling under load. Temperature changes can expand trapped air. The instrument compensates for expected behaviour, and the operator chooses supports and test programs suited to the watch.
Large, oddly shaped or exceptionally high-pressure cases may need specialist equipment. Vintage watches with uncertain crystals deserve conservative treatment. A test is safest when the operator knows both the machine and the object being tested.
When testing belongs in ownership
A pressure test is prudent after the case has been opened, after a significant impact, and before returning an older watch to swimming or diving. Regular intervals depend on use, age and manufacturer guidance. Someone who wears a watch daily in the sea has a different risk profile from an owner whose dress watch meets rain twice a year.
The result should be recorded with the test pressure and method. “It passed” is more useful when the next watchmaker can see what it passed. If the watch fails, keeping it dry until repaired is the entire advantage of having tested early.
Water resistance is a current state
Gaskets take a set, lubricant migrates, crowns wear and case surfaces can be damaged. A watch that passed at the factory can fail years later, while a carefully serviced older watch can pass again. The engraving on the back records a design capability. The pressure test records the condition of this assembled case today.
Dry testing is elegant because it turns an invisible seal into measurable motion. The chamber applies air, the case flexes by an amount an eye could never see, and the sensor waits to learn whether the watch keeps that pressure difference to itself.
