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Horology 101 - b.p.h. & accuracy

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Published on 10-25-2008 10:34 PM

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The term 'beats per hour' (bph) refers to the number of times a balance wheel rotates within a 60 minute span. A single-direction rotation is also called a vibration. The first wristwatches in the early 1900's were converted pocket watches, generally made from smaller size movements found in ladies pendant watches which ran at 18,000 bph.

1908 Elgin wire-lug conversion:
Brown Watch Analog watch Wood Watch accessory

When better lubricants were developed, faster bph rates could be used since friction was reduced. Starting in the late 1940's several brands put out higher beat calibers that vibrated at 19,800 and 21,600 bph. As the quality of lubricants evolved, correspondingly higher rates were introduced. Rolex often uses a 28,800 bph rate, Omega's co-axial vibrates at 25,200 bph, and brands such as Zenith beat as high as 36,000 bph.

5 beats per second = 18,000 bph = 2.5 hz

5.5 beats per second = 19,800 bph = 2.75 hz

6 beats per second = 21,600 bph = 3 hz

7 beats per second = 25,200 bph = 3.5 hz

8 beats per second = 28,800 bph = 4 hz

10 beats per second = 36,000 bph = 5 hz

There are many factors in a watch that can affect it's accuracy. If all these other variables are equal, a 36,000 bph movement has twice as many increments as an 18,000 bph movement so it can be fine-tuned in a shorter period of time. With the price of labor factored in, it's easy to see that a high beat watch is less costly to regulate than a lower beat version. Another advantage to a higher beat count is that it will resist fluctuations from external disturbances. This has to do with the frequency of the every day motion a watch is subjected to while being worn. Without getting into the math, the normal rotational activity of our arm motions usually occurs at a rate very close to the frequency of a lower bph movement. The closer the motion frequencies are to each other, the more it influences the balance assembly's oscillations and the more it will have an adverse effect on accuracy. This partly accounts for why your watch keeps different time on your wrist as compared to on your winder.

There are drawbacks to a high beat count of course. All other aspects being equal, a 36,000 bph watch requires servicing more frequently than an 18,000 bph watch. It does move twice as fast which creates more friction and more potential for wear. Lubricants can be flung off the pallet jewels, so specific ones were developed for these calibers. Also, the faster oscillations of the balance wheel dictate a smaller diameter due to the physics of rotational mass. A smaller balance wheel can't oscillate as consistently as a larger one which can negatively affect the timekeeping. It turns out to be a trade off.

Some gratuitous pictures (because we love pictures)

Omega Co-axial 2500:
Watch Analog watch Clock Watch accessory Jewellery

Rolex 3035:
White Rim Auto part Circle Pattern

photo copied from the Rolex Reference Page with permission of John Holbrook / JBHII

and 3 gorgeous Zenith shots:
Brown Watch Analog watch Clock Font

Watch Analog watch Clock Font Watch accessory

Watch Analog watch Light Clock Watch accessory

photos courtesy of Rob / donut

For comparison purposes, a quartz watch typically vibrates at a frequency of over 32.7 k hz. That's not a typo - hz, not bph. That's equivalent to over 235,000,000 bph so you can see how quartz watches can achieve great accuracy.

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Horology 101 - b.p.h. & accuracy
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