A depth gauge is a watch complication that measures how far below a reference surface, such as the water’s surface, the wearer is. It lets a diver read their depth on the wrist. Watches use three designs: an open capillary tube, a flexible metal membrane or a curved Bourdon tube.
How does a capillary depth gauge work?
The capillary gauge, also called the Boyle-Mariotte gauge, is a clear tube open at one end. As the diver goes down, water enters the open end and squeezes the air trapped inside. The more the air is squeezed, the deeper the diver. The gauge reads quite accurately down to about 10 metres. Its weakness is temperature: when the trapped air bubble warms or cools, the readings go badly wrong. Oris’s patented Aquis Depth Gauge works on the same principle. Water fills a channel cut into the sapphire crystal and squeezes the air trapped inside.
How do membrane and Bourdon tube gauges work?
A membrane gauge, also called an aneroid gauge, uses a metal capsule with a flexible end. Water pushes on the capsule, and the end bends further as the pressure rises. In a watch, this metal diaphragm is squeezed as the diver goes down and springs back on the way up. A Bourdon tube gauge uses a curved tube of springy metal. Rising pressure straightens the tube. Falling pressure lets it curl back. The amount the tube straightens shows how deep the diver is.
Where it comes from
The Bourdon tube is named after Eugène Bourdon, a French scientist, and dates from 1849. IWC launched its Deep One in 1999, and its depth gauge uses a Bourdon tube. As the diver goes down, rising pressure straightens the tube. On the way back up, the tube curls back again. The Deep One’s gauge stops at 45 metres. That limit is set by how the Bourdon tube behaves.
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