Ikuo Tokunaga joined Seiko as an engineer in 1970, working from the company’s factory in Suwa on special projects. The professional dive watch project came across his desk, and for the next half decade it became his obsession. Tokunaga isn’t a diver, but he approached the problems described in the diver’s letter with an engineer’s knack for problem solving, tackling each issue separately: a 350m working depth; pitch-black, cold conditions; high-impact work and the helium-saturated living environment which was presenting a unique problem to wristwatches. He responded with over 20 patented innovations.
To provide high pressure resistance capable of standing up to the tremendous depths it would face, the Professional Diver’s was made with a one-piece case, which eliminated the screw-on back entirely and necessitated loading the movement from the top of the watch. The case was made from titanium, the first dive watch to be crafted from the metal, which is notoriously difficult to machine. Titanium provided strength, light weight and resistance to corrosion and magnetism. The gasket used to seal the top-loaded crystal in place was L-shaped, the first of its kind in the industry, which provided a better seal than a deforming round gasket. This helped the watch achieve its rated 600m water resistance, but it also solved another unique problem that plagues commercial divers’ watches: helium intrusion.
But this wasn’t a watch designed for desk divers; it is perhaps the most purpose-built, uncompromising timepiece ever developed.
Living and working at depth requires divers to breathe a gas mixture that replaces the nitrogen and much of the oxygen in normal air with helium, to eliminate the narcotic effects of nitrogen and the toxicity of oxygen at high pressure. This causes the voice change in the divers, and it has a more insidious effect on wristwatches: The helium atoms are small enough that they can sneak past the conventional crown, case back and crystal seals to penetrate the watch case. This alone isn’t a problem, but when the diving bell is slowly decompressed to return the divers to sea-level pressure, those atoms expand in size and the rise in pressure inside the watch causes it to explode with catastrophic effect. Other watch companies responded to this problem by fitting the case with a one-way gas-relief valve, but Seiko’s L-shaped gasket obviated that need, as the watch was impenetrable.
To enhance the watch’s shock resistance, Tokunaga’s team introduced an outer titanium shroud that covered much of the upper case. The shroud was secured to the case with four Phillips-head screws and had two well-placed cutouts for access to the rotating timing bezel. The watch sat high on the wrist, not unlike other wrist instruments like a compass or depth gauge, and with the shroud it gave the appearance of a hockey puck or a can of tuna, which led to the watch being dubbed the “Tuna Can,” a name that has stuck ever since.
To address the low-light legibility issue, Seiko added broad arrow-shaped hands and huge distinctive dial markings, coating them all with prodigious amounts of its proprietary LumiBrite paint which, when exposed to light for a short length of time, glows with nuclear intensity for hours, a trait for which Seiko is now legendary.
Finally, Seiko introduced a special strap for the Professional Diver’s watch, an extremely long rubber one that had accordion-like “vents” in it. This allowed the strap to be pulled tight around the wrist over a dive-suit sleeve and as the suit compressed under water pressure, the strap could take up slack and remain tight on the arm. This style of strap is now an industry standard on diving watches, but it was a patented innovation by Seiko in 1975.
Over the years since its introduction, the Seiko Professional Diver’s watch has evolved. In 1978, it was fitted with a quartz calibre, the first dive watch to have such a movement, and today, there are several “Tuna Can” divers in Seiko’s Prospex (Professional Specifications) lineup, including those with Kinetic, quartz, Spring Drive and automatic movements. The top grade Marinemaster Automatic 1000M is the heir to the Professional Diver’s throne; its shroud is now ceramic and its crystal sapphire while water resistance has been boosted to a full kilometer. Despite these improvements, the aesthetic remains polarizing to watch enthusiasts. On the one hand, the tall profile, sloping shroud and lug-less design are awkward and don’t lend themselves to daily wear topside. But this wasn’t a watch designed for desk divers; it is perhaps the most purpose-built, uncompromising timepiece ever developed, and its impact on dive-watch design can not be understated.
In 1975, seven years after that fateful letter arrived at Seiko, Ikuo Tokunaga proudly presented his work to the diver who wrote it. He approved.