A pulsometer is a scale, by which in a simple way the current heart beat per minute can be determined. With the aid of a pulsometer the pulse must no longer be counted for 1 minute or multiplied after 15 seconds.
Pulsometers may be components of chronographs then only the chrono second needs to be started and the number of pulses read on the scale after 15 beats.
With conventional three-hand watches you have to wait, however, until the seconds hand passes the starting point at 60 seconds (at 12 o'clock). Now you start with the count of the heartbeat. So after 15 heartbeats the pulse number can also be read on the scale.