The Casio F91W is a digital watch that has remained virtually unchanged since its introduction in 1989. The watch is famed for its affordability, reliability, and its plain, simplistic design. However, experiments and mods have pushed the watch far beyond its intended capabilities, particularly with a hydro-modification.
The hydro-mod process involves filling the watch case with mineral oil. This effectively eliminates any air pockets inside the case. This mod drastically steps up pressure resistance by counteracting external water pressure — a method usually seen in high-end dive watches. Unlike standard watches that rely on specialized case construction and seals to handle depth pressure, the hydro-mod can enable a simple digital watch like the Casio F91W to survive extreme depths.
A test from 2023 involved strapping a hydro-modded Casio F91W to the manipulator arm of a remotely operated vehicle (ROV), which was then submerged into the ocean. The test was designed to push the limits of the watch’s endurance, with the ROV descending to a crazy depth of 4,950 meters (16,240 ft). Under normal circumstances, the unmodified F91W would likely fail due to pressure-induced case deformation, but the hydro-mod process allowed the watch to stay functional throughout the descent.
At nearly 5,000 meters, the pressure exerted on the watch was over 7,200 pounds per square inch — far beyond what any unmodified Casio is rated for. What makes the hydro-mod particularly appealing is its simplicity. Filling a watch with oil prevents moisture intrusion, equalizes pressure, and improves legibility/readability underwater by reducing internal reflections. The method is quite inexpensive as well, requiring only basic tools and a steady hand. You can read more about the method here, or watch a video below.
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Please note that this mod does not turn the F91W into a full-fledged dive watch. It still lacks an ISO dive certification, luminous markers, and a depth gauge — however, it does demonstrate the extreme durability of Casio’s design when paired with basic engineering principles. Tests like these are a solid reminder of how an unremarkable, everyday digital timepiece like the Casio F91W can defy odds.