Vibrations per hour

Vibrations per hour, usually shortened to vph, is the frequency at which a watch's balance wheel oscillates, counted as the number of half-swings or beats in an hour. It appears on specification sheets as vph, as A/h from the French alternances par heure, or as beats per hour. One hertz equals two vibrations, because a complete oscillation has a tick and a tock.

The arithmetic is worth committing to memory. A movement running at 28,800 vph beats 8 times a second, which is 4 Hz, and 28,800 is simply 8 multiplied by 3600.

One vibration is a real mechanical event, not an abstraction. Each one is a single unlocking of the escapement, one impulse delivered to the balance and one step of the gear train. At 28,800 vph that happens 691,200 times a day and over 250 million times a year. With a 15-tooth escape wheel, which gives 30 vibrations per revolution, the wheel turns 960 times an hour, or 16 times a minute, all of it on a few micrograms of oil. Our guide to how a mechanical watch works follows the chain from barrel to balance.

Why it matters

A faster balance divides time more finely, so any single disturbance is a smaller fraction of a second. It also stores more energy in its swing, which means it recovers faster from a knock and is less easily thrown off by changes in position. That is why 4 Hz became the modern default and why high-beat movements running at 5 Hz have a reputation for stability on the wrist.

The cost is wear and energy. Everything in the escapement happens more often, so lubrication is worked harder and the mainspring is emptied faster. Movements built for long power reserves frequently run at 3 Hz for exactly this reason, trading a little rate stability for a day or two of extra running. The Powermatic 80 and the Seiko 6R35 both sit at 21,600 vph and both quote reserves of 70 hours or more, which a 4 Hz movement of the same size would struggle to match.

Where the numbers come from

For most of the twentieth century 18,000 vph was the standard, a rate that suited the materials and the lubricants of the day. The high-beat era arrived in the second half of the 1960s, when Girard-Perregaux put a 36,000 vph movement into series production, Seiko followed with 36,000 vph calibers of its own, and Zenith launched the El Primero chronograph at that rate in 1969. The claim was better rate stability, and it was largely true; the problem was that oils and escapement parts of the period were not always up to the duty cycle.

Twenty-eight thousand eight hundred settled in as the default with the mass-produced Swiss automatics of the 1970s and 1980s, and it still is: the ETA 2824-2, the Rolex 3235 and the Grand Seiko 9S65 all run at 4 Hz. The recent movement has been the other way, back toward 3 Hz, because buyers now ask for a weekend of power reserves more often than they ask for a fast balance.

How to read the spec

Frequency tells you what the seconds hand will look like. At 4 Hz it steps eight times a second, each step an eighth of a second, which the eye reads as a sweep but a loupe shows as distinct steps. At 2.5 Hz the steps are visible without help, which is part of the character of older movements.

Frequency also matters practically if you time your own watch. A timing machine must be told the correct vph before its readings mean anything, and the lift angle as well before the amplitude figure means anything. Feed it 21,600 for a 28,800 movement and the trace will be nonsense rather than merely wrong. The beat rate converter handles the units, the timegrapher reading explainer covers what the screen is telling you, and timing your own watch works through the procedure.

Common confusions

The unit gets mangled constantly. A movement is 28,800 vph or 4 Hz; it is never 28,800 Hz. A watch listed that way is a listing written from a spec sheet nobody read.

Frequency is also not accuracy. It affects how stable a rate is likely to be, not how close that rate is to zero, which is a matter of regulation and of adjustment in positions. Chronometer certification makes no distinction: a 21,600 vph movement and a 36,000 vph movement are both held to the same tolerance of minus 4 to plus 6 seconds a day, as set out in accuracy and chronometer standards.

Nor does any beat rate give a truly continuous seconds hand. Every mechanical watch steps; higher frequencies only make the steps smaller. The exception is Spring Drive, which has no escapement and no beat rate at all, and whose hand genuinely glides.

Quartz watches are a separate case. Their crystal oscillates at 32,768 Hz, chosen because it is 2 to the power of 15 and so divides down to one pulse a second in fifteen simple halvings. The figure is not comparable with a balance frequency, and the seconds hand still steps once a second.