With its distinctive shape, the iconic status of Seiko’s third historic diver’s watch, the 52mm Professional 600M launched in June 1975, is undeniable. Coupled with a metal bezel, this unique monocoque or one-piece case construction dive watch has an opening that opens only from the front similar to a tuna can. Such a striking resemblance, an unintentional pun, has led collectors to affectionately nickname the Professional 600M Dive the “Tuna Can.”
The Seiko tuna has become a classic; nearly half a century later, evolving variants continue to secure the model’s place in the brand’s contemporary collections, simply because they remain bestsellers. Seiko tunas have a separate outer tapered body, called a shroud, that acts as a protective layer and fits precisely on the inner shell. It has an overall diameter of approximately 52mm, a key feature as the smaller 43mm diameter version is known as a small tuna.
SLA041 “The 1975 Professional Diver’s 600m Recreation” and SLA042, nominees for the 2020 Grand Prix de la Horlogerie de Genève (GPHG), are replicas of the tuna in Seiko’s Prospex line of sports watches that meet the brand’s exacting quality standards. Prospex is an apt compound word, derived from the words “professional” and “regular”.
In March 2020, Seiko claimed the world’s first use of what it called “Ever-Brilliant Steel” in the watch industry for the cases of its SLA037 and SLA039 diver’s watches — both of which were its first and second On behalf of the diving watch, they were the Professional 150M in 1965 and the Professional 300M in 1968. “Ever-Brilliant Steel”‘s PREN, an acronym for Pitting Resistance Equivalent—an industry measure of corrosion resistance—is 1.7 times stronger than the stainless steel grades used in most luxury watches. Featuring a bright white hue, this durable material is widely used in the marine industry, especially for surfaces, linings, bolts and multiple components of marine structures and vessels.
1975 Professional Diver 600m © Seiko
For the SLA041, the reissue of its third-generation 600-meter diver’s watch, “Ever-Brilliant Steel” was used only for its bezel, as Seiko kept its original titanium body. Don’t forget that as early as 1975, the case of a diving watch was made of titanium, which was the first time in the world. Titanium is strong, corrosion resistant and lightweight.
With a three-year gap between the launch of Seiko’s first dive watch and its second-generation model, and a seven-year wait for the Professional 600M, the brand hopes to satisfy professional divers in demand.
The catalyst for the Professional 600M project was a written complaint by Yo Oshima, a diver in Kure, Hiroshima, employed by Nihon Kaiyo Sangyo, today Sumitomo Marine Development. Oshima’s critical feedback in 1968 was a stern statement that the Professional 300M was “unfit for use”.
It was a bolt from the blue. Launched about a year ago, the second-generation Professional 300M is twice as water-resistant to 300 meters (approximately 1,000 feet) as its predecessor, the Professional 150M, thanks to an innovative one-piece construction method never before seen in Switzerland— design, as well as the use of hardened inorganic glass.
Seiko’s developers sought constructive feedback from Oshima, a professional saturation diver from a drillship who descends and ascends in a pressurized diving bell, working at depths of approximately 350 meters. Members of Seiko’s development team even visited the drillship where Oshima worked to better understand the environment and the issues saturation divers face when wearing the watch. These problems included magnetized watches because welding was performed underwater, poor legibility in dim conditions, movement failures in decompression chambers, and crowns coming off from accidental impacts.
This explains the use of a magnetic shield between the bottom of the case and the movement, the large luminous round hour markers, the triangular hour markers at 12 o’clock as a reference, the wide hour and minute hands, the indicator on the luminous round seconds hand, the brand at the time The most reliable movement is the self-winding Caliber 6159, with the crown at 4 o’clock, and “black thermal spray” – the application of high temperatures to the hard-coated material on the outer case for added surface protection.
Seiko insists on anti-magnetic, and uses a pure iron dial on the SLA041 to increase the anti-magnetic ability to 40,000 amperes per meter.
Working at great depths, saturation divers typically inhale a mixture of helium and oxygen. Helium inevitably finds its way into their case. During the ascent, the accumulation of unescaped helium inside the case can lead to overpressure, which can cause the glass to break or violently displace. Rolex invented the helium escape valve to solve this problem, and Seiko found its footing for its Professional 600M by making a case that not only withstands pressurization and decompression, but is also helium-tight point. Among other things, this required the development of Seiko’s own L-shaped gasket to seal the glass and crown. This explains why Seiko Tuna diving watches, including the SLA041 rated to 1,000 meters depth, do not require a helium escape valve.
The Japanese proverb 継続は力なり – pronounced keizoku wa chikara nari, meaning “continuity is strength” – embodies Seiko’s unwavering quest for rugged and reliable deep dive watches. While deep-sea divers are indeed a minority, they are by no means insignificant, as their invaluable input has earned Seiko the hearts and minds of an even larger and economically more important global “tabletop diver” market.
This year, GMT Magazine and WorldTempus embarked on the ambitious project of summarizing dive watches since 2000 in The Millennium Watch Handbook – Dive Watches, a beautifully laid out coffee table book. This article is an excerpt. Millennium Watch Book – Divers watches available in French and English: