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The perpetual calendar, Vacheron Constantin reminds us in the press release for the Traditionelle Twin Beat Perpetual Calendar, is the most "useful" of high complications, but its utility comes with one major drawback. If by chance a perpetual calendar should be allowed to run down, re-setting the indications ?which may include the date, as well as the day, month, and year, as well as sometimes the age and phase of the moon, depending on the design ?may prove to be something of a challenge; with many traditional designs, performing the operation improperly means the watch must go in for service to be re-set by a watchmaker. (One of the more grim moments in my life as a watch writer was setting a synchronized perpetual calendar which hadn't run in three years ?as all the calendar indications were set from the crown, this meant turning the hands, via the crown, for three years' worth of 24 hour cycles. This took two days. The fam found it hilarious. There was colorful language.) Vacheron Constantin has come up, in the Traditionelle Twin Beat Perpetual Calendar, with a most intriguing and unusual solution: a watch with a "standby" mode, in which the power reserve is 65 days. Not hours, days ?over two months of running time.

Now, you would expect a watch than can run for two-plus months to be a bit of a behemoth ?to possess a certain (to borrow a phrase) ursine heft, not to put too fine a point on it. This is simple physics: beyond a certain point, an escapement running at a certain frequency, even a leisurely 2.5 hertz (18,000) balance, will require a larger mainspring in order to have a longer power reserve, no matter what else you do to optimize the going train.

The only other option, if you want a longer running time, is to have a slower-frequency oscillator, but this opens another can of worms. The slower the frequency, the easier it is for a watch to wander off its rate due to external disturbances anything from just moving your arm around to outright whacks and bangs will make the oscillator hiccup to some extent cheap replica watches, which is why a relatively fast 28,800 vph is more or less the default choice for modern wristwatch movements. The answer Vacheron came up with was a beautifully simple idea: have two oscillators, in one watch, both powered by a single energy source, and run one at a high beat and one at a low beat; 5 hertz and 1.2 hertz, respectively ?and, when the owner knows the watch may be laid aside for a few days or longer, let the owner choose the lower frequency. The movement, caliber 3610 QP, is moreover only 32mm in diameter and rather unbelievably, only 6mm thick ?getting all this into such limited space is (to steal another phrase) as complicated as performing an appendectomy on a life raft, and thereby hangs a tale.

There have been a number of replica watches with dual oscillators, of course, but without exception these have all had balances oscillating at the same frequency. Having two different frequency oscillators in a single watch, both of which keep time correctly, and between which the user can switch on demand, is (as far as I know) a first in watchmaking, and for a good reason ?it creates a number of technical problems. Add to that the fact that a perpetual calendar, like all complications, requires additional energy from the mainspring, and you have what seems like a neat idea, but a technical impossibility. However, Vacheron has developed some extremely clever and very unexpected technical solutions.

The single mainspring barrel and two gear trains and balances of the Twin Beat. On the right; the active balance; (5Hz, 36,000vph) left, the standby balance (1.2Hz, 8,640vph).

Above is the heart of the watch: the mainspring barrel, and the two trains and two oscillators. The first thing you'll notice is that the barrel is fairly thick ?there is only a single barrel, but it contains two superimposed mainsprings, on the same axis. The switching mechanism that selects which balance is running can be seen as well ?it's basically a stop-seconds lever (on steroids, from both a size and technical perspective) and you can see the the bar for the pusher set into the case at the lower right, as well as the long, curved stop lever, and the two, very delicate brakes, shaped rather like the antennae of a butterfly. In the illustration above, the standby balance is stopped, thanks to the brake pressing against its impulse roller; the active balance is free to oscillate.

Reverse view of the braking system of the Vacheron Constantin Twin Beat, seen in isolation.

The system has been designed so as to interrupt timekeeping as little as possible ?obviously, you want as little lag as possible between one balance stopping and the other starting up, and even under full power a balance does not immediately begin to oscillate at full running amplitude. The brake geometry is attractive aesthetically, but it's also designed to allow the newly selected balance a brief interval to power up before the deselected balance stops completely.

The pusher for switching between active and standby mode is located on the case flank at 7:00.

Like many engineering solutions, this solves one problem but poses another ?if both oscillators and both going trains run off a single mainspring barrel, stopping even one balance ought to stop that balance's gear train from turning. This, yikes, will stop the mainspring barrel from rotating, which will also stop the other balance, even if its brake isn't pressing against it. This rather ruins the point of having two balances, as you can imagine, but here Vacheron has also come up with a solution.

We mentioned that though there is a single mainspring barrel, there are two mainsprings. These ensure that there's enough gas in the tank for either four days, or 65 days, depending on the selected beat rate.

The internal differential, which transfers power from the mainsprings to the barrel. On the right is another, spherical differential which carries the pinion driving the hour and minute hands; on the left, the differential system for the power reserve.

Even if one going train is immobilized, you still need to be able to power the other. The internal differential allows both mainsprings to continue to unwind even if one of the trains is stopped. It also facilitates winding the mainsprings, and reduces the amount of torque being delivered to the standby train. The standby train takes power from the topmost of the stacked gears of the barrel, while the active train takes power from the lowest (both gold, in the illustrations). This creates one further set of problems: the power reserve has to be able to receive input from both the uppermost and lowermost (standby and active) driving gears, and output the power reserve to a single indicator.

To solve this, natch, we a double differential system for the power reserve, which is visible to left in the drawing above. And of course, the watch hands need to be able to receive input from two sources as well, and output the correct time. In this case, the two input sources are not the barrel, but rather, the first gear in each going train. (This gearing is visible clearly in the first drawing up top; the differential on the pivot driving the hands is visible in isolation in the image directly above.) That's a total of four differentials: the internal one inside the mainspring barrel; the two responsible for the power reserve; and the one for taking input from the first gear in either the standby or the active train, and outputting the hours and minutes.

Making the perpetual calendar more efficient was also critical to the success of the Twin Beat, all the more so because all the date, month, and year indications are all instantaneous jumping indications. Normally, jumping indications use a snail cam system ?a cam shaped like a snail's shell rotates ?in the case of the date indication, for instance, once every twenty four hours ?and as the cam rotates, the beak of a spring - loaded lever presses on its edge. At the end of a full rotation, the beak of the lever snaps off the highest point of the cam and drops onto the lowest, triggering the jump.

The problem with this system is that there's a lot of friction at the point of contact of the lever with the cam, so for the Twin Beat, Vacheron Constantin developed a system which uses a spiral spring. The jump occurs when a wheel with no gear teeth along part of its circumference, jumps forward across the area with absent teeth, under the influence of the spiral spring, as the gears driving the system turn. The whole system uses about four times less energy than would be the case if a standard system had been used ?this was essential for the long power reserve, since even in standby mode, all the indications still jump when the day, month, and year change over.

The high-efficiency jumping system for the perpetual calendar indications; the wheel with no teeth along part of its circumference is visible, sandwiched between two other wheels, at lower right.

There is something of a tendency for highly complicated replica watches to also be, if not actually unwearable, certainly something you'd hesitate to put in your daily rotation (short of a supervillain lifestyle that involves selecting today's quintuple axis tourbillon grand sonnerie in a Vibranium case before murmuring silkily, "How do you like my sharks, Mr. Bond?") However Vacheron seems to have understood that the point of making a watch intended to increase the convenience of the perpetual calendar, is lost unless you follow through on your obligation to make it something that is also convenient to wear, and visually compelling to boot. The Twin Beat is beautifully and lavishly finished both front and back, and I suspect owners will find the temptation to switch between the two oscillators ?not out of any practical necessity but instead, just for the fun of it ?irresistible.

In recent years, Vacheron Constantin has increasingly shown a capacity for making very complex timepieces that are also plausible as pieces you'd actually wear with some frequency (the Celestia astronomical watch is another excellent case in point). At this level of watchmaking you want it all ?fresh ideas, something to spark animated conversation, and a level of impeccable craft throughout that seamlessly connects the past and present, as well as giving a sense of possibility about the future. We'll be looking more closely at the Twin Beat Perpetual Calendar in the coming weeks, but for now it's one of the most impressive technical and creative debuts from a major fine watchmaking maison in some time.

The Vacheron Constantin Traditionelle Twin Beat Perpetual Calendar: Case, platinum, 42.mm x 12.3mm; dial, hand-guilloché slate colored gold and transparent sapphire with applied 18k gold markers. Movement, Vacheron Constantin caliber 3610 QP, hand wound perpetual calendar with high-efficiency instantaneous jumping indications; Twin Beat dual oscillator system with user-selectable balances vibrating at 5Hz (36,000 vibrations/hour), 1.2Hz (8,640 vibrations/hour) in active or standby mode respectively; 480 components; dimensions 32mm x 6mm, running in 64 jewels; Geneva Hallmark. Price, $199,000. More at Vacheron-Constantin.com.