POLAR S725 - Manual (Page 53)

C 51
Turning the heart rate zone alarm on/off
Note that you can not use the alarm when Bicycle functions are on.
Note that trip distance is reset by pressing and holding signal/light
button. Therefore Trip should not be displayed in the upper row when
turning zone alarm on/off.
Press and hold the signal/light button.
When appears on the display the zone alarm is on. You will hear a beeping
sound with each heartbeat if your heartbeat is outside the heart rate limit.
If you do not want to use the zone alarm the flashing heart rate reading
indicates if your heartbeat is outside the heart rate limit. This function can
be used if you have set heart rate limits on for Interval Training Set or
BasicSet.
In addition to the alarm you can see:
if you are exercising above heart rate zone.
if you are exercising below heart rate zone.