Chronographs are useful tools for the recording of elapsed time, but in addition to this common and easily understood complication, many are fitted with a scale adorning the bezel. These numerical scales, when used in conjunction with the chronograph seconds hand, are able to perform specific calculations for practical use.
There are several different scales that can appear on a chronograph. What follows is a brief explanation of the most common.
TACHYMETER: Of all scales, this is the most common. The tachymeter scale is calibrated to show the speed of a moving object, such as a vehicle, over a known distance. The standard length on which the calibration is based is always shown on the dial, e.g.1,000, 200 or 100 meters, or in some cases, one mile. As the moving vehicle, for instance, passes the starting-point of the measured course whose length corresponds with that used as the basis of calibration, the observer releases the chronograph hand and stops it as the vehicle passes the finishing-point. The figure indicated by the hand on the tachymeter scale represents the speed of the vehicle in kilometres or miles per hour.
TELEMETER: by using a telemeter it is possible to measure the distance of a phenomenon which is both visible and audible. The chronograph hand is released at the instant the phenomenon is seen; it is stopped when the sound is heard, and its position on the scale shows the distance in kilometres or miles separating the phenomenon from the observer. (Calibration is based on the speed of which sound travels through the air, viz. approximately 340 meters or 1,115 feet per second). This is particularly useful in a thunderstorm.
PULSOMETER: The pulsometer scale shows at a glance the number of pulse beats per minute. The observer releases the chronograph hand when starting to count the beats and stops at the 30th, the 20th or the 15th beat according to the basis of calibration indicated on the dial. The hand shows at a glance the number of pulse beats per minute.
ASTHMOMETER: The asthmometer scale shows at a glance the number of respirations per minute. The observer releases the chronograph hand as the patient begins a respiration and stops it on completion of the 15th, the 20th or the 25th respiration, according to the basis of calibration indicated on the dial. The hand shows at a glance the number of respirations per minute.