Everyone is familiar with Zenith's high-frequency El Primero automatic chronograph movement, released in 1969. However, not everybody is familiar with the world's first high-frequency diver. The laurels, in this case, go to Longines with its 1968 Ultra-Chron ref. 7970 with a 5Hz frequency and a depth rating of 200m. Reintroduced in 2022, the Ultra-Chron returns in 2025 in a carbon fibre case.No stranger to high-frequency calibres, in 1914, Longines produced a 5Hz handheld stopwatch with a split-seconds hand capable of measuring 1/10th of a second. In 1916, it unveiled its 50Hz, 1/100th of a second stopwatch. Ten years before Zenith's El Primero, Longines produced the first wristwatch with a 5Hz movement (calibre 360) to compete in observatory chronometry competitions (it was manual winding). In 1968, Longines introduced its 5Hz calibre 431 inside a diver's watch, converting the Ultra-Chron into the world's first high-frequency diver with an accuracy of one minute per month. If you'd like a more in-depth analysis, don't miss Brice's coverage here. Ad - Scroll to continue with article In 2022, Longines rescued the 1968 Ultra-Chron, capturing its retro style but with contemporary proportions and a modern movement. Sporting a cushion-shaped case with a 43mm diameter and 14mm thickness, the latest Ultra-Chron marks Longines' first experience with carbon, giving the watch a stealthier, darker vibe. Carbon is widely used in aeronautics, and the Ultra-Chron case is made from strips of unidirectional carbon fibre and epoxy resin. The materials are placed in a mould, heated to a high temperature, compressed under high pressure and cold cut to replicate the cushion-shaped case of the Ultra-Chron. The random patterns produced by the carbon fibres make each case unique and contribute to the featherlight 80g weight of the watch (including the strap).Unlike the earlier edition with a unidirectional rotating bezel with a sapphire insert and luminescent markings, this model has a fixed titanium bezel with a matte black aluminium insert and silver numerals. It’s a shame that the bezel is fixed because it deprives the dive watch of a key function. The titanium crown and caseback are screwed-down to ensure the 300m water-resistance.The sandblasted anthracite dial has applied rectangular indices with a central strip of Super-LumiNova, matching that applied to the grey PVD rectangular hour and minute hands. Other differences here regard the all-important minutes hand that is not picked out in a different colour and the application of SLN on all the indices. Applied above the 6 o'clock marker is the original high-frequency Ultra-Chron logo. Underscoring its vintage provenance, the dial is protected by a box-shaped sapphire crystal.Hidden under the sealed caseback is calibre L836.6, based on a heavily modified ETA with a frequency of 5Hz, an anti-magnetic silicon balance spring and a 52-hour power reserve. Classified as a chronometer following ISO 3159 standards, the certification is provided by TimeLab.The Ultra-Chron Carbon comes with a black technical fabric strap and a titanium buckle. It retails for USD 4,900. More information at Longines.com.