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To use a tachymeter, time a known distance or one completed task with your watch’s chronograph, then read the number on the tachymeter scale where the stopwatch seconds hand points. For example, timing one kilometer in 45 seconds gives an estimated speed of 80 km/h. The exact hand and button layout varies by Seiko model, so check the instructions for your watch’s caliber before operating it.
What a tachymeter measures
A tachymeter converts the time taken for one unit into an hourly rate. If the unit is one kilometer, the reading is kilometers per hour; if it is one product or job, the reading is products or jobs per hour. Seiko’s instructions describe using the scale to determine how many seconds it takes to run one kilometer or complete one job (Seiko 8R46/8R48 tachymeter instructions).
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The scale does not measure distance or identify what counts as a completed task. You must know the distance in advance, or define a consistent unit of work. The result is an estimate based on the timed unit.
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- Identify your caliber. Find the caliber number on the case back and look up its instructions using Seiko’s instruction lookup.
- Check the chronograph controls. In the instructions, identify the stopwatch seconds hand, start/stop control, and reset control. Confirm the chronograph is reset and its seconds hand is at zero before starting.
- Choose a measurable unit. For speed, use a known distance such as one kilometer. For an hourly output rate, choose one clearly defined product or job.
- Start the chronograph as the unit begins, then stop it when the unit is complete—for example, when you reach the end of the measured kilometer.
- Read the tachymeter scale at the position indicated by the stopwatch seconds hand. The number represents the hourly rate for the unit you timed.
- Reset the chronograph according to your caliber’s instructions when you are finished.
Seiko’s 8R46/8R48 manual gives two worked examples: 45 seconds for one kilometer corresponds to 80 km/h, while 20 seconds for one product corresponds to 180 products per hour (Seiko 8R46/8R48 tachymeter instructions). These are examples of how to interpret the scale, not performance claims.
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What if the event takes longer than a minute?
A standard tachymeter scale may not provide a direct reading for every elapsed time. Some Seiko models have a scale designed for measurements longer than one minute. Seiko’s V172/V174/V175/V176 instructions include a 72-second example that reads 50 per hour; use the instructions for your specific scale rather than assuming every Seiko tachymeter works over the same range (Seiko V172/V174/V175/V176 tachymeter instructions).
Why Seiko chronograph layouts differ
Seiko chronographs do not all share one tachymeter or control layout. The scale may appear on the dial or bezel, and the position and function of the hands and buttons depend on the watch’s design and caliber. For example, the cited 8R46/8R48 chronograph instructions specify button A to start and stop and button B to reset. Those controls should not be assumed to apply to other calibers. The same manual warns against pulling the crown to adjust the time or date while the stopwatch is running (Seiko 8R46/8R48 chronograph instructions).
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When checking a particular watch, look for four things in its manual: where the tachymeter scale is printed, which hand records elapsed seconds, which controls start, stop, and reset the chronograph, and whether the scale covers the duration you plan to time.
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