Seiko [SixOneThreeNine]-SevenOneZeroZero 自动 Chronograph. I have a soft spot for 自动 chronographs and Seikos are what I could afford at the current moment. Swiss 自动 chronographs are a hefty price to pay for. This mint NOS example has a helmet shaped case, very retro feel to it. I love it! Production date: Jan, 1976Cal [SixOneThreeNine] was introduced by Seiko in 1969 and used in the 1970s Seiko 自动 Chronographs. [SixOneThreeNine] has 17 or 21 jewels and dimensions of [SixOneThreeNine] are 27.4 x 6.5 mm, beating at 21,600 bph. Watches with [SixOneThreeNine] movement have one subdial only (30 minutes counter). In terms of accuracy, Seiko promised that both [SixOneThreeEight] and [SixOneThreeNine] movements will keep time within +35 to -20 sec per day.
The beats per hour (BPH) are the number of half swings the balance wheel makes in an hour (i.e. from fully counter clockwise rotation to fully clockwise, or vice versa). The "standard" beat movement is considered to be 18,000 bph, or five beats per second. The true fast beat is 36,000 bph, or 10 beats per second. The advantage of a fast beat movement is that for a given percentage change in torque or power input, there is a small percentage change in balance amplitude (i.e. the "length" of the swing. For chronographs, fast beat movements allow the measure of smaller increments of time (at least theoretically).
A fast beat balance dissipates more energy and thus requires more torque in the form of a thicker spring. The thicker spring, however, has less even torque output over its wind. This offsets some of the advantage of the fast beat. The superiority of the fast beat movements is attributed partly to a smaller drop in amplitude when moving the watch from an edge position (e.g. crown right) to dial up or down. In moving the watch from an edge position to dial up the following changes in amplitude were found. Fully wound: 33 and 20 degrees (21.6 and 36.0 respectively). 24 hours down: 26 and 17 degrees.
In actual use on the wrist, error introduced by rapid arm movement (inertial forces) are lower in the fast beat movement. So, although a fast beat movement may not need more frequent servicing than a slower beat movement, it will be less tolerant of lubrication failure (due to aging, dissipation, etc).
