Review: The Jaeger LeCoultre Duometre Chronograph

The JLC Duometre caused a sensation among chronograph enthusiasts when it was launched in Geneva this year. Ron DeCorte gives us his technical review of this exciting timepiece

The name Jaeger-LeCoultre takes us back in time to one of Switzerland's oldest and most prestigious watch companies. Founded in 1863 by Antoine LeCoultre in Le Vallée de Joux, Switzerland, it is also one of the only major watch companies that is still located at its original address.

"Chronograph" is the complication of recording short periods of time. In the beginning this short interval was limited to one minute. The recording device, for lack of a better description, was nothing more than a fountain pen at the end of a seconds hand making an ink trace at the perimeter of the watch dial. Needless to say, the pen needed constant refurbishment of ink and the dial wiped clean at the end of each timed event. Terribly inconvenient by modern standards, but it was the start of something that has endured to this day.

Coming back to the Jaeger-LeCoultre Duometre Chronograph, what is "Duometre" and what makes it special? The basic idea was to separate the normal time keeping function of the watch (hours, minutes, and seconds) from the chronograph function.

In most chronograph wristwatches, the time keeping and chronograph functions share a common power source. In simple terms, this means that when the chronograph function is operated, power to the time keeping function is reduced and thus affects the accuracy of time keeping. The Duometre overcame this problem by using two separate power sources (and gear trains) – one for the time keeping function and another for the chronograph function. Sounds simple? Not so. Until now, two separate power sources in a wristwatch have been reserved only for the super complicated sonnerie (clock) watches and are difficult to construct. So the Duometre(dual meter) is akin to having two watches in one.

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The subdial on the left houses the normal hour and minute hands. These hands, together with a center seconds hand that is in continuous motion, and a power reserve hand (lower left), are gold in color to indicate their time keeping functions. In this example above, the time indicated by the golden hands is 1:50 and 22 seconds, and the power reserve is full (+50 hours).

The subdial on the right houses the chronograph minute and hour hands. The elapsed time of the chronograph function is read in the same way as the timekeeping hands – the short hand records the hours and the long hand the minutes. At the 6 o'clock position of this sub-dial is a tiny recording disc that rotates once every ten minutes. Its period of rotation is synchronized with the minute recording hand in order to eradicate any margin of error when reading the minute counter hand. Like the time keeping hands, the chronograph hands, together with the sweeping seconds hand at the center and the power reserve hand, are all of the same color – blue, to indicate their chronograph functions. In the same example, the chronograph has functioned for 2 hours, 20 minutes, 38 seconds. But wait, there's more...

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There is a third sub-dial at the center lower portion of the main dial. This sub-dial houses the flying-seconds (foudroyante) hand that makes one revolution every second, giving a precise fractional indication of time accurate to 1/6th of a second!

Therefore, the chronograph shows an elapsed time of 2 hours, 20 minutes, 38 and 1/6th seconds.

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The Duometre employs bi-directional winding since there are two power sources – one for the normal time function and another for the chronograph function. While you might not use the chronograph regularly, the normal time function is in constant motion and therefore power will need to be replenished at least every two days. Turning the crown in a clockwise direction winds the normal time function barrel (marked: HEURES/MINUTES above).

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On the other hand, winding the crown in a counter-clockwise direction replenishes power to the chronograph function barrel (marked: CHRONOGRAPHE above) when needed. Notice that the same wheels are used to wind both the hours/minutes barrel and the chronograph barrel. You might ask why both barrels are not being wound when the crown is turned in either direction. On closer examination, you will see a ratchet wheel and two clicks at the center of each barrel. This allows one barrel to wind when the crown turns in one direction and the other to wind when the crown turns in the opposite direction.

I should point out that the chronograph function will not operate (even though it might be fully wound) unless the normal time function is operating since it is the latter that constitutes the heart beat of both functions as it contains the escapement.

 

Start-Stop-Reset

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Now that we have wound the watch, let's play with the chronograph function and get a better understanding of the mechanism. The Duometre is a monopusher chronograph, a bit of a rarity these days but very classic.

Starting the chronograph via the push piece located just above the winding crown sets the chronograph into motion. In this illustration above (as well as the next two illustrations), the push piece works by pushing the main chronograph-operating lever in the direction of the red arrow, (upper left of the drawings). The hour, minute, and seconds recording hands are immediately set into motion because the hook that was holding the chronograph gear train in place has released the flying-seconds 6-point star wheel (see insert).

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Pressing the push piece a second time stops the chronograph function as the hook latches on to one spoke of the 6-teeth star wheel (see insert). This stops the chronograph and the precise hours, minutes, seconds, and fractional second of an event are recorded.

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Press the push piece again, this time holding it for one second. This action brings the chronograph function back to zero. Perhaps, you have noticed that one spoke of the flying seconds 6-teeth star wheel looks slightly different from the other 5. This "special" tooth is the only one, out of the six, that is capable of engaging the hook when it has been slightly retracted, as shown in the insert.

It is only when the hook has engaged this special tooth that the hammers are allowed to drop to their heart shaped cams located on each of the hour, minute, and seconds hands, thus returning all chronograph recording hands to their zero positions.

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Wrapping Things Up

The Duometre is finished to the exacting high standards of Jaeger-LeCoultre. I particularly like the blue screws, which add a nice contrast to the rhodium-plated bridges and ruby jewels. A sunburst damascene pattern radiating from the center of the balance wheel decorates the bridges in place of the more commonly seen Côte de Genève pattern – a nice touch in my opinion.Duometre is based on a new Jaeger-LeCoultre movement (ebauche), aptly named the Dual Wing. I’m told that the Dual Wing will form the base for further watches in the Jaeger-LeCoultre stable and I am waiting eagerly to see what comes next.

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