With its distinct form factor, there is no doubting the iconic status of Seiko’s third historic diver’s watch – the 52mm diameter Professional 600M, unveiled in June 1975. Together with its metallic bezel, this unique monocoque or one-piece diver’s watch is , which has an opening only from the front, is compared to canned tuna. Such an uncanny resemblance, no pun intended, led collectors to affectionately call the Professional 600M Dive watch “the tin can”.
Seiko Tuna has become a classic; almost half a century later, evolved variants constantly ensure the model’s place in the brand’s modern collection, simply because they remain bestsellers. The Seiko Tuna has a separate outer conical body known as the shroud that acts as a protective layer and is precisely fitted over the inner case. Its large overall diameter of around 52mm is a key feature as smaller 43mm diameter versions are known as Baby Tuna.
The SLA041 “The 1975 Professional Diver’s 600m Re-creation” and the SLA042, a 2020 Grand Prix d’Horlogerie de Genève (GPHG) nominee, are reincarnations of the tune found in Seiko’s Prospex collection of sports watches that meet the brand’s exacting quality standards. Prospex is an apt portmanteau, derived from the words “professional” and “specifications”.
In March 2020, Seiko claimed the world’s first use in the watch industry of what it terms “Ever-Brilliant Steel” for the cases of its SLA037 and SLA039 diving watches – both of which are reissues of their first and second generation diving watches, the Professional 150M respectively from 1965 and the Professional 300M from 1968. “Ever-Brilliant Steel” has a PREN, the acronym for Pitting Resistance Equivalent Number – the industry measure of corrosion resistance – that’s 1.7 times more than stainless steel grades used in most luxury watches. This durable material has a brilliant white hue and is widely used in the marine industry, especially on surfaces, liners, bolts and several components of marine structures and vessels.
1975 Professional Diver’s 600m © Seiko
For the SLA041, the reissue of its third-generation 600m diver’s watch, “Ever-Brilliant Steel” is only used on its bezel, as Seiko has remained true to its original titanium body. One must not forget that back in 1975 the use of titanium for the case of a diving watch was a world first. Titanium is strong, corrosion resistant and light.
While there was a three-year interlude between the launch of Seiko’s first-ever diver’s watch and the second-generation model, there was a seven-year wait before the Professional 600M saw the light of day, where the brand would meet the demands of professional divers.
The catalyst that sparked the Professional 600M project was a written complaint by Yo Oshima, a diver in Kure City, Hiroshima and employed by Nihon Kaiyo Sangyo, now known as Sumitomo Marine Development. Oshima’s critical feedback from 1968 was the stark proclamation that the Professional 300M was “unfit for use”.
This was literally a bolt of lightning out of the deep blue ocean. The second generation Professional 300M was about a year old, and its water resistance to 300 meters or about 1,000 feet was twice that of its predecessor, the Professional 150M, due to an innovative one-piece construction method not even witnessed before in Switzerland. made designs, as well as the use of tempered inorganic glass.
Seiko’s developers sought constructive feedback from Oshima, a professional saturation diver from a drillship who made descents and ascents in a pressurized diving bell to work at depths of about 350 meters. Seiko’s development team members even visited the drill ship Oshima worked from to better understand the environment and the problems divers faced with their watches. Problems included magnetized watches when electrical welding was done under water, poor legibility in dim conditions, decompression chamber malfunctions, as well as detached crowns due to accidental impacts.
This explains the use of an anti-magnetic sheet between the case base and the movement, large luminescent circular hour markers, a triangular hour marker at 12 o’clock as a reference, wide hour and minute hands, a luminescent circular indicator on the sweep seconds hand, the brand’s most reliable movement of the time, which was the automatic Caliber 6159, the placement of the crown at 4 o’clock, and “black thermal spraying”—the application of hard coating material to the outer case at o’clock. high temperatures for extra surface protection.
Seiko has stuck to the war against magnetism by using a pure iron disc on the SLA041 to increase the resistance to 40,000 amps per meter.
Saturation divers working at great depths typically breathe a mixture of helium and oxygen. Helium will inevitably enter their watch cases. During ascent, the accumulation of helium gas in the case, which has not escaped, can lead to overpressure and thereby cause the glass to crack or undergo violent displacement. While Rolex created the helium escape valve to solve this, Seiko found its own foundation for its Professional 600M by creating a case that not only resists pressure and decompression, but is impervious to helium. This involved, among other things, the development of Seiko’s own L-shaped gaskets to seal the glass and crown. This explains why Seiko Tuna diving watches, including the SLA041 rated to 1,000 meters, do not require the helium escape valve.
The Japanese proverb, 全生は力なり— pronounced keizoku wa chikara nari and meaning “continuation is power” — personifies Seiko’s unwavering pursuit of robust and reliable deep diving watches. While deep sea divers are indeed a small segment, they are by no means unimportant as their invaluable input has allowed Seiko to win even the hearts of the much larger and financially consistent worldwide “desktop diver” market.
This year, GMT Magazine and WorldTempus have embarked on the ambitious project of summarizing the diving watch since 2000 in The Millennium Watch Book – Divers watch, a large, beautifully laid out coffee table book. This article is an excerpt. Millennium Watch Book – Divers watch is available in both French and English here: