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Gronefeld One Hertz

The World’s First Independent Deadbeat Seconds Series Wristwatch

Dutch watchmakers Tim and Bart Grönefeld present “One Hertz”, the world’s first and only production wristwatch with independent deadbeat seconds (secondes morte in French). While rare, the deadbeat seconds complication does exist, but to date has been derived from other mechanisms, usually a constant force device or remontoir d’égalité.

The Grönefeld One Hertz features a completely original in-house developed movement indicating hours and minutes on a sub-dial at 2 o’clock, a large sub-dial for the deadbeat seconds filling the majority of the dial with a power reserve indicator at 12 o’clock, and a setting-winding indicator at 3 o’clock. Setting-winding is ergonomically selected by pushing the crown instead of pulling it out.

Deadbeat seconds – where the second hand advances in full steps of one second instead of an apparently smooth sweeping action – was a very respected complication until the 1980s; however, its popularity died with the dominance of quartz movements, which also stepped in full seconds. A smooth sweeping second hand came to differentiate mechanical from quartz.

With their new One Hertz, with its unique secondary gear train, Tim and Bart Grönefeld have resurrected this long neglected complication and re-positioned it where the complication originated, i.e. on the pedestal of high precision.

The quartz-like movement of the large second hand of the One Hertz subtly signals its unique mechanism, invisible to most but obvious to haute horlogerie aficionados who will appreciate the flawless fine-finishing of the in-house developed calibre G-02.
The One Hertz launches with a subscription-only limited edition of 12 pieces in steel called the “One Hertz 1912” –1912 was the year Tim and Bart’s grandfather qualified as a watchmaker.

History of the Deadbeat Seconds
With the introduction of the pendulum in the 17th century, clocks finally became accurate enough to measure seconds. It was then not long before a hand indicating seconds on a long clock’s dial signified a precision timepiece. A pendulum with a period – the time to swing forward and back – of two seconds (the most common) resulted in a single tick per second.

The invention of the balance spring, which replaced the pendulum, enabled miniaturization. As portable pocket watches became more accurate, watchmakers naturally thought to copy the one-second steps of the second hand, which signified a precision timepiece; however, the rapidly oscillating balance meant that it could not be directly driven from the oscillator as with the pendulum, so it either necessitated a new mechanism if independent, or it had to be driven from another complication e.g., a constant force device. While pocket watches have featured independent deadbeat seconds in the past, the Grönefeld One Hertz is the first wristwatch featuring independent deadbeat seconds.

The Grönefeld One Hertz is unique among wristwatches in that its deadbeat seconds are powered by a secondary gear train that is independent of the gear train for the hour and minute indications.

One Hertz
The hertz (symbol: Hz) is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. This SI unit is named after Heinrich Hertz. One Hertz simply means “one cycle per second”. The second (SI symbol: s), sometimes abbreviated sec., is the international base unit of time. To highlight the SI seconds, the One Hertz displays deadbeat seconds with a large second hand in its own dial. Hours and minutes are non-SI units of time because they do not use the decimal system so are displayed separately. Mechanical wristwatch movements often have balance frequencies of
2.5 to 5 hertz, which results in the second hand making tiny steps of five to 10 steps each second and looking like a smooth movement.

The “One Hertz” independent deadbeat seconds complication
Displaying deadbeat seconds in a wristwatch without a constant force device is no easy task. The friction of the mechanism has to be absolutely minimal so it does not interfere with the escapement.

Tim and Bart Grönefeld used an independent deadbeat seconds mechanism that is driven from its own secondary gear train with its own power supply. The seconds are driven from one mainspring barrel and the hours and minutes from another. Friction with this system is guaranteed to an absolute minimum and the complication has no adverse influence on the escapement and freesprung balance.

The two mainspring barrels are wound simultaneously from the crown, which features an innovative “push to set, push to wind” function, with the mode selected indicated on the dial at 3 o’clock. A power reserve indicator at the top of the deadbeat seconds dial keeps track of the 60 hours of autonomy.


Technical Specifications: “One Hertz” 1912
Calibre G-02, mechanical hand winding, independent deadbeat seconds, power reserve indicator and setting indicator

Calibre G-02 – Movement dimensions: 35mm x 9.4mm – Parts: 254 – Jewels: 39, set in gold chatons. Power reserve: 60 hours – Barrels: Two barrels, one for the going gear train and one for the independent deadbeat seconds mechanism; both barrels are wound at the same speed and in the same direction. Balance wheel: 9.12mm free-sprung variable inertia balance wheel. Balance frequency: 21’600 bph/2.5Hz
Balance spring: Phillips terminal overcoil curve, triangle-style stud
Main Plate: Spotted and snailed rhodium plated nickel silver
Bridges: Stainless steel, hand-bevelled, micro-blasted centre and the underneath spotted, circular grain on the top, relief engraved on micro-blasted surface
Gearing: Two independent gear trains each with their own energy source

Deadbeat seconds mechanism: Independent mechanism, cam with 30 teeth on the going gear train on the seconds wheel, escape wheel on the seconds wheel of the independent gear train, double lever with jewelled pallets

Winding-setting mechanism: Push function crown for winding or setting
Power reserve mechanism: Classic Breguet style by means of a cone moving up and down on the threaded barrel arbour. Indications: Hours and minutes in sub-dial at 2 o’clock, large seconds at 7 o’clock, power reserve, setting-winding indicator at 3 o’clock

Case: stainless steel, limited edition of 12 pieces, gold security screws, polished bezel and centre band with hand-finished straight graining. Case dimensions: 43mm x 12.5mm.
Sapphire crystals: top domed with anti-reflective treatment both sides, display back with anti-reflective treatment inside

Water resistance: 3atm/30m/100feet – Crown: Stainless steel with engraved “G” logo

Dial: Hour and minutes sub-dial, seconds sub-dial, power reserve indicator, setting-winding indicator, Grönefeld logo and model name on individual screwed-down nameplates. Hands: Hours and minutes, long thin counter-poised seconds, power reserve and setting-winding. Strap and buckle: Hand-sewn matte black, alligator leather with stainless steel engraved tang buckle


Biography: Tim and Bart Grönefeld

The name Grönefeld and the art of watchmaking have a family history spanning nearly one hundred years, originating in the ancient town of Oldenzaal in the Netherlands. There, in a shop facing the ancient basilica church from 1240, Johan Grönefeld, Tim and Bart’s grandfather, began his career as a watchmaker in 1912, marking the beginning of the highly talented dynasty of Grönefeld watchmakers that continues today.

Tim and Bart’s workshop is located in Johan Grönefeld’s original building, representing a continuous, unbroken watchmaking family history that is exceptionally rare to find anywhere in the world today.

Tim and Bart underwent extensive training in Switzerland, and within a relatively short span of time proved themselves world specialists in the production of the most coveted and exquisite horological complications of all: the tourbillon and the minute repeater wristwatch. In 2008, after working anonymously behind the scenes for prestigious Swiss brands, they presented the first watch bearing their own name, the GTM-06 Tourbillon Minute Repeater.

In June 2010 Tim and Bart presented their second watch, the One Hertz, the world’s first wristwatch with independent deadbeat seconds and featuring a completely new in-house movement.

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Written by kevin

June 23, 2010 at 3:04 pm

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