A Depth Gauge is a small, deeply technical niche within dive watches: a complication that mechanically reads your current water depth on the wrist. The principle is simple, the execution is not. Rising water pressure either compresses or bends an element, and that movement is translated into a hand or a scale on the dial. Two classic methods dominate. The Bourdon tube is a curved tube that straightens as water fills it, while the membrane (or diaphragm) system uses a thin flexible disc whose deflection is amplified by levers and gears. Favre-Leuba was first to put this on the wrist with the 1968 Bathy. Most depth gauges also carry a separate maximum-depth hand that records the deepest point of a dive, so you can read it back at the surface.
A few models define the field today. The Oris Aquis Depth Gauge reads depth through a water channel milled into the sapphire crystal: water enters, compresses the trapped air, and Boyle's law does the rest along a printed scale, with no moving depth parts at all. Its 46mm steel case is rated to 500 metres (50 bar), and at roughly £2,900 to £3,400 it is the category's accessible entry point. The IWC Aquatimer Deep Three links a membrane on the side of the case to a rack-and-pinion driving two hands, one for current depth and one for maximum, to about 50 metres in a titanium case. The most ambitious example is the Blancpain Fifty Fathoms X Fathoms, whose amorphous-metal membrane reads to 90 metres, with a separate 0 to 15 metre scale accurate to around 30 centimetres, a maximum-depth memory and a retrograde decompression counter.
Before you buy, be honest about use: most people choose these as a mechanical achievement and a romantic alternative to a dive computer, not as primary safety gear. Bourdon and membrane systems need calibration and can drift with salinity and temperature, so you still dive with a computer. In practice the Oris is the right starting point for most buyers, thanks to its price, durability and easier servicing. The IWC and Blancpain, with titanium cases, more intricate mechanics and far higher prices, speak to the collector and the engineering enthusiast. On membrane watches, keep the case apertures and diaphragm clean; on the channel-based Oris, keep the crystal channel clear.
Two sub-approaches separate cleanly within the category. On one side sits Oris with its physics-based solution and no moving depth parts; on the other, Favre-Leuba, IWC and Blancpain build full mechanical complications with gears, levers and memory functions. On the vintage side, the original Favre-Leuba Bathy and its later variants are genuine depth gauges that carry collector appeal but demand careful maintenance. Buying new, modern pieces such as the Raider Bathy 120 MemoDepth resist water to 200 metres and hold maximum depth to 120 metres on a counter at three o'clock.