In the first part of our guide to chronographs we covered the mechanism and its operation, different kind of mechanisms and different chronographs available.
In this second part, we’ll see how to operate it and how to read the different scales we can find in these watches.
The two most iconic models easily are the Rolex Daytona and the Omega Speedmaster.
For their glorious history and characteristics, they are the most known chronographs ever and, even though powered by different movements, their functioning is the same.
The pusher at two activates and stops the chronographic measurement, while the second pusher at four is used to reset the measurement when the chrono is stopped.
A technical difference between the two watches is that modern Daytonas feature screw-down pump-pushers to ensure higher water resistance, with the need to be unscrewed before the use.
To ensure a precise measurement of timespans over one minute we can rely on sub dials that, in these two specific watches, allow us to record up to 12 hours.
These have a thirty minutes counter for the minutes one and, every two rounds, we’ll see the hours register moving one hour forward on the hours subdial, divided in twelve parts.
The third counter, with a sixty-seconds scale, is for the running seconds, independent from the chronograph and always moving.
Both come with a tachymetric scale on the bezel and that’s our first step to explain the different uses of a chronograph.
It’s surely the one that made this complication famous.
We use it to measure the mean speed over a predetermined length (usually one kilometer or a mile)
To do so we find, on the bezel or on the dial, a scale of mean speeds of the measured object. The more time it takes, the lower the reading is.
Practically, the measurement is started when the object crosses the beginning of the reference distance and stopped when the end is reached.
The average speed is displayed on the scale by the seconds hand.
If the speed is under 60 uph (units per hour) there are some scales (not featured in the Daytona or the Speedmaster) that allow the reading of lower speeds on a “spiral scale” (also known as “snail”).
The telemetric scale is used to determine the distance of a visible and audible event.
It is said that was used during the war to determine cannons placement to hit targets.
The measurement was started when the cannon flash occurred and stopped when the cannonball hit the ground.
This allows for the precise calculation of a phenomenon’s distance, first seen and then heard.
For example, today it can be used to measure the distance of a lightning, based on flash and thunder.
This is achieved thanks to the difference between the speed of sound (around 345 meters per second) and the speed of light (300.000 kilometers per second).
Pulsometric scales are prefect for doctors or everyone who needs to quickly measure someone’s heart rate.
On the dial we usually find the number of beats to record after the beginning of our measurement: in the picture we can see “gradué pour 30 pulsations”, French for “calibrated for 30 pulsations”.
When the chronograph is activated, the beats are recorded (usually 15 or 30) and then stopped. The seconds hand will tell the beats per minutes of the patient.
Usually, professional pulsometric watches are produced as monopusher chronographs to ease the use to who, besides holding the chronograph, needs to measure the patient’s beats and thus has only one free hand.
Less precise therefore less common, the asthmometric scale works exactly as the pulsometric.
Instead of recording the heartrate, it measures the patient’s breaths per minute.
This peculiar scale allows to divide a minute in one hundred parts, easily converting time into decimals.
Decimal time conversion was used in scientific and industrial fields to ease the calculation of production costs and assembling times.
Very unusual and particular scale, can be found on rare models by Universal Geneve and Heuer from the 50s.
Specifically speaking about the Universal Geneve Film-Compax, the double colored scales on the dial are used (…were used, may we say) to calculate how much photographic film was consumed in feet per minute.
The red scale refers to 16mm film while the black refers to 35mm film, according to the era “typical” 24 photograms per second.
Furthermore, the sub dial at three shows both films increments in feet per second.
ALL IN ONE
To satisfy customers that may need to measure different phenomena on a single watch, it’s possible to find timepieces that showcase three scales on the same dial.
On the famous Patek Philippe ref. 5975 dial, apparently complicated to read, we find a telemetric scale, a 15-pulses pulsometric scale and a spiral tachymetric scale capable to track speeds as low as 20 km/h.
And now, what’s missing? You have to wait for Part 3 to know…
Translated by Lorenzo Spolaor (@itsdoc_oclock)