Monday, April 12, 2021

Power reserve, in plain English

"Yesterday, Rolex has released a few more models with the new "72 hours power reserve movement". Why is this such a big news when Panerai, JLC, Omega, and IWC already offer 8 days going mechanical watches?"

This is a very good question. Before we go further: add to that list Seiko Prospex models with 6R35 movements with 70 hours power reserve which could be had for just $799. In other words, 70 hours power reserve itself is not a big deal. However, like with anything in physics, larger power reserve always come with some kind of unwanted trade-off.

Power reserve is simply an amount of energy on disposal to run a system. Here is an example: in theory, a small car battery (60Ah)would power up a 50 Watts LED light for 12 hours. Put two car batteries in parallel, and you'll get 24 hours of light.

In watches, the energy to run the watch is stored in the mainspring. There are number of 'consuming devices' in the system, but one of them, directly responsible for timekeeping, is the balance wheel (oscillator). More mainsprings (barrels) is one way to provide more energy, but this solution comes at the cost of increased watch size (both diameter and thickness).

The other solution is to lower the consumption and increase the efficiency of the oscillator.

There are two 'powers' in the balance wheel system. The first one is the balance power, which is the amount of power presented in the oscillator, calculated as a product of balance inertia, amplitude squared and frequency cubed. For the Rolex calibre 3135, the balance power is 372 micro Watts. The second power is the oscillator maintaining power - the power required to keep the oscillator running. Again, for the Rolex 3135 that is 1.24 micro Watts. Fine tuning the ratio between the two is an engineering challenge because extending the power reserve by reducing the energy consumption of the balance wheel will come with a trade-off: degraded performance and poor timekeeping.

The simple question is this: what do we really want- more power reserve (watch running longer) , or more power allocated to maintain the performance of the balance wheel, and better accuracy?

For the past few years Swiss watchmakers have been trying hard to increase the power reserve while maintaining good timekeeping. The goal is to reduce consumption by making the balance wheel oscillating system as friction free as possible. Increasing the beat (frequency) is one of the ways forward, but there are number of solutions available: changes in escapement geometry, low friction materials, magnetic bearings - or completely new innovations such as the silicon oscillator by Zenith.

Back to the new Rolex: my understanding is that the 72 hours power reserve is largely result of  improvements to geometry of the standard Swiss lever escapement increasing efficiency by 15%.. Rolex calls it "The Chronergy Escapement". It has been around since 2015, but until recently, it was reserved only for and Day-Date model. However, Omega Calibres 8500 and above already benefit from Daniel's coaxial escapement which is even more energy-transfer efficient. In reality, Rolex is once again simply catching up with Omega.                         


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