Vacheron Constantin Launches An Updated Version Of The Record-Setting Twin Beat Perpetual Calendar
A watch from 2019 with a record-setting 65 day power reserve, is now part of the regular collection.
The perpetual calendar solves one problem but potentially raises others. The solved problem is the calendar, in which there are months with either 31 or 30 days, and also the month of February, which has 28 days but which gets an extra day once every four years (in a Leap Year). Watches with a date window require the owner to manually correct the date for all 30 day months at the end of the month, and at the end of February. The perpetual calendar switches the date at the end of the month automatically, even at the end of February for a leap year.
Setting The Perpetual Calendar
The downside to this has always been setting the watch. The traditional method uses pusher correctors set into the side of the case which you press with a small tool (or a toothpick, in case you have lost the brand provided tool. (I read a report years ago on a forum in which a collector said he had lost his correcting stylus, and when he called his repair center for a new one, there was a long, judgmental silence on the other end of the line). These do the job, but can be fiddly to manipulate and you have to make sure you don’t try and change an indication when the calendar mechanism is switching over, or you might break something. Another option is to design a calendar mechanism in which you can change all the indications from the crown – no in-case correctors. This however may mean, depending on the design, that the watch can only be set forwards or backwards one day at a time.
In 2019, Vacheron Constantin released a watch which offered a third solution, which basically was to create a watch with such a long power reserve that even if months go by, it will still be running and therefore will not need to be set.
That watch was the Twin Beat. The Twin Beat has two balances. One controls the rate of the watch during daily wear, and runs at the high frequency of 36,000 vph/5Hz. However, if you want to put the watch down for more than a few days, there is a second balance, beating at just 8,640 vph/1.2Hz. A switch on the side of the case lets you disengage the 5Hz balance and engage the 1.2Hz balance. Normally, the power reserve is four days, but with the 1.2Hz balance running, the power reserve is more than two months.
The Twin Beat: A Perpetual Calendar With A 70 Day Power Reserve, And How It Works
Today, Vacheron Constantin has announced that the Twin Beat, which was produced in only a few examples in 2019 and which was something of a concept watch, will become part of the permanent collection.
The 2019 release officially had a 65 day power reserve with the 1.2Hz balance running; for 2026, some updates to the movement have extended that to 70 days.
The Twin Beat is a bit on the large size judged simply as a perpetual calendar, at 42mm x 12.3mm, in a platinum case however, in that space there is a large double stacked mainspring barrel, several differentials, two distinct and separate going trains for Active (5Hz) and Standby (1.2Hz) modes, and the mechanism dedicated to switching between Active and Standby modes. And, of course, there’s the perpetual calendar as well. Despite the complexity of the watch, the movement, caliber 3610 QP, is quite thin. Overall the movement, with 480 parts, is 32mm in diameter and just 6mm thick (for comparison, the automatic caliber SW 200-1, a widely used third party automatic movement, is 4.6mm thick).
Seen from the back, the two balances are easily visible. The 5Hz balance is on the left, and the 1.2Hz is on the right. There are some interesting differences and similarities. On the left, the high beat balance has a standard steel escape wheel. However, on the right, the 1.2Hz balance has a silicon escape wheel, whose pivots are protected by antishock springs. The antishock springs are one of the updates to the movement since 2019. At that time, they weren’t on the pivots of the silicon escape wheel, and their introduction may reflect some of the refinements which allowed the power reserve to increase from the official 2019 claim of a 65 day power reserve, to the 2026 claim of a 70 day power reserve.
You’ll also notice that the new version is 3610/2, while the first version is 3610. One other notable feature, as well, although this is a historical rather than a technical feature, is the presence of the Geneva Seal, or Poinçon de Genève, on the balance cock of the 1.2Hz balance. At one time, the most elite Genevan watch brands were proud to have the stamp on their movements, which is the quality seal of the Geneva watchmaking school. The Geneva Seal has a number of requirements, some functional and some with respect to finishing, and it is administered by a third party. Vacheron Constantin still adheres to the Geneva Seal standard, which, in a time when many brands have switched to using their own internal standards, is a rare example of an external standard for both performance and craft in use.
Between the two balances, you can see a Y-shaped lever which, when you press the pusher at 8:00, stops one balance and allows the other to start.
This is one of the plan drawings Vacheron provided at the 2019 launch and it shows the mainspring barrel and the going trains and balances. It’s inverted from the photograph just above it; in the drawing the larger 1.2Hz balance is on the left. The switching lever has two stop levers attached to it, each of which makes up one side of the Y. In the drawing, the stop lever on the left is pressing against the roller on the pivot of the 1.2Hz balance, preventing it from running. The stop lever on the right, on the other hand, is lifted off the balance roller, allowing it to run.
You would think that stopping one balance would stop both automatically, as both are turned by the same mainspring barrel; however, a differential ensures that even if one balance is stopped, torque is still transmitted to the other and the mainspring barrel continues to turn. (In the Dufour Simplicity, which has two balances, there is a differential which sends torque to two balances, and Dufour has said that while he thought one ought to continue running even if the other was blocked, that visualizing the operation of the differential was “very difficult” and he wasn’t sure he was right until the movement had been assembled for the first time).
You might be thinking, well, in Active mode the power reserve is four days, and in Standby mode, it’s 70 days, which is much more than the difference between 5Hz and 1.2Hz. The reason the power reserve can be extended so dramatically is thanks to the fact that the 1.2Hz going train has been optimized to reduce energy loss due to friction as much as possible. The power reserve of a watch is just how long it takes before the mainspring becomes too weak to overcome whatever friction is in the going train.
Typically this is when there is not enough energy for the escapement to push the balance hard enough to keep the escapement working. Two of the features of the 1.2Hz side which enhance power reserve, are the use of a light balance (although it’s wider than the 5Hz balance, it’s also quite a bit thinner) and the use of a very low inertia silicon escape wheel, which also, unlike a steel escape wheel, does not need to be oiled. Finally, the balance spring for the 1.2Hz balance is quite light and thin as well (just 0.015mm) and when the watch was announced in 2019, Vacheron also noted that it’s made of a special alloy optimized for a slow beat, light balance. This means that less energy is necessary to push the balance through an amplitude large enough for good accuracy, which also extends the power reserve (in fact, at the 2019 announcement Vacheron said that balance amplitude for most of the then-65 day power reserve of Standby mode, was about 300º).
The other major contributor to a long power reserve, is the switching mechanism for the perpetual calendar. The perpetual calendar indications all switch together, instantaneously, at midnight. The potential problem with this, is that the mainspring barrel has to provide all the energy for switching the indications in one fast burst, which reduces (briefly) the torque going to the balance. This can cause a drop in balance amplitude, which can affect accuracy, but if the mainspring is running down, there may not be enough energy to switch the indications and the watch may stop outright. The classic instant switching system uses a combination of steel levers and straight tempered steel springs. However, the caliber 3610 uses a system in which a spiral spring is gradually loaded until midnight, at which time the spring is released, causing a driving gear to rapidly snap forward.
Vacheron says that the switching mechanism in caliber 3610 is four times more efficient than the conventional instant switching mechanism, which is a big help, especially when the mainspring is getting to the end of its power reserve.
Between the optimized switching mechanism, the lower mass of the balance, the absence of oil on the escape wheel, the low-resistance balance spring, and the lower inertia of the silicon escape wheel, Vacheron has gotten quite a lot more miles out of the tank, so to speak.
How The Twin Beat Ensures Accuracy For 70 Days
Normally, such a low inertia balance running at such a slow frequency, would mean a risk of unsatisfactory rate variations in a watch worn on a daily basis. However, the 1.2Hz train is designed for accuracy when the watch is not being worn, so concerns about (for instance) rate changes due to changes in position, or physical shocks, should not apply (and if the watch is off the wrist and stored at room temperature, changes in rate due to temperature are minimized as well).
At 70 days, or over two months, I think there is an excellent chance that the Twin Beat will remain the world record holder for power reserve, albeit that only applies to Standby mode. However I think that the sheer size of the power reserve while impressive, is not the whole story. The Twin Beat is a very ingenious wristwatch, but it is, in its mechanical solutions, a sort of textbook in miniature of some of watchmaking’s most fundamental challenges – and a summary of some of watchmaking’s most imaginative solutions.
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.