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Editorial - Watch Myths: Rethinking The Power Reserve

Editorial Watch Myths: Rethinking The Power Reserve

You've definitely heard this one before…

There are things you pick up over the years, when you love watches and immerse yourself in this endlessly fascinating subject. This sort of knowledge comes in a few forms, mostly either your own personal empirical observations or received wisdom, the stuff that supposedly everyone knows and no one questions.

Here’s the thing. I ask questions. I ask a lot of questions, and I don’t stop asking until I’m satisfied that there’s nothing more to be uncovered. And while this combination of curiosity and persistence was not particularly appreciated by my parents and educators when I was a child, it’s these very qualities that led me to journalism. Hurrah! So I’ve decided to write a little series about these entrenched myths that we need to stop repeating, fallacies which get in the way of our maturation in watch knowledge.

First up: The Mystery Of The Diminishing Power Reserve.

Raise your hand if you’ve been told that the more you use your complication, the less power reserve you will have. I’ve been told this so many times, especially in context of chronographs, which is more or less the only on-demand complication that draws its energy from the same barrel as the oscillator. (The minute repeater is also an on-demand complication, but it has its own separate barrel.)

If we extend this statement to its logical conclusion, we can arrive at the following scenarios:

  1. You have a chronograph watch, and you never activate the chronograph. Your power reserve remains at its maximum.
  2. You have a chronograph watch, and you rarely activate the chronograph. Your power reserve drops slightly.
  3. You have a chronograph watch, and you often activate the chronograph. Your power reserve drops significantly, enough that you notice it, enough that it might bother you, if you’re the kind of person who expects your watch to perform at the technical specifications as defined by the brand.
  4. You have a chronograph watch, and you keep the chronograph running all the time. Your power reserve drops by 20% or 30% or something equally horrific.

Question: How many of the above four statements are true?
Answer: Only the first one.*

The power reserve does not, I repeat, does not, drain faster when you run your complication. It does not disperse more energy simply because a complication calls for it. It is not like that box of chocolates that lasts for a whole week in your desk drawer but disappears in half an hour when you leave it out in the office break room. The only variable that affects the energy released from the barrel is the torque of the mainspring as it unwinds. The only variable that affects the frequency at which this energy leaves the barrel is the balance frequency.

Why, then do people still continue to perpetuate the belief that activating a complication will diminish your power reserve? It’s because under one very specific set of conditions, it’s actually true.

Let’s take, as an example, a 4Hz (28,800vph) chronograph movement, armed with a barrel of standard diminishing torque. This means that the barrel “pushes” harder when the mainspring is fully wound and more weakly as the mainspring progressively unwinds. The barrel makes one such push — or energy release — eight times a second, as determined by the 4Hz regulating organ. What does the movement do with the energy it gets from each push? Most of the time, it goes straight to the balance, which makes the balance very happy and allows him to have a great time swinging around at his optimal amplitude.

When the chronograph is activated, it forces the balance to share the energy that was previously all just for him. (The barrel doesn’t care, he gives out the same amount of energy as usual, how that energy is spent throughout the movement is none of his business.) The balance is less happy that he’s not getting the full share of energy, and it really shows. He oscillates at a lower amplitude. It’s fine, as long as he still gets his minimum level of energy.  He’s still dancing, though not as enthusiastically as before. When the chronograph is stopped, it’s business as usual for the balance. Full share of energy, full amplitude, full performance.

Towards the end of the power reserve, however, things get a little dangerous. Remember, at this point, the barrel is “pushing” quite weakly because the mainspring is almost totally unwound. At this point, if the chronograph is activated and starts pulling energy away from the balance (like those annoying people who like to hog all the blankets in bed), then the energy that’s left for the barrel might be below his minimum requirement. The balance stops dancing, he goes to bed, the watch stops ticking.

Then, and only then, using the complication does make your power reserve drop slightly. If you stop the chronograph, allowing the full share of energy to go back to the balance, you might get him to start up again, but he’s a sensitive guy. He might not feel like getting out of bed, and you’ll have to wind the mainspring again.

Use your chronograph as much as you want. As long as the mainspring is still pushing out high levels of torque, you don’t have to worry about a thing. Your power reserve is safe. On the other hand, you could leave your chronograph inactive for 99% of the power reserve, but if you start using it then, you’re probably going to lose that last 1% of power reserve.

To those of you who prefer reading about watch movements in purely mechanical terms, I apologise unreservedly for the whole dancing metaphor. For the rest of you, who (like me) find it amusing to think about watch movements having a good time and twirling about with boundless energy, high five! I hope you all understand now why using your complication does not affect your power reserve. Go spread the word.

Next up: Tourbillons. (Don’t freak out.)

* A pedantic friend of mine requested that I specify this statement is in reference to traditional lateral-clutch chronographs. And that statement 4 could also be about maybe 5% true, sometimes.