The following list gives the most common features generally expected to be found on a basic heart rate monitor.
Continuous Heart Rate Monitoring
Heart Rate Training Zone with Alarms
Duration of Exercise Session
Average Heart Rate of Exercise Session
Maximum Heart Rate of Exercise Session
Basic Time Keeping
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Continuous Heart Rate Monitoring
With this feature you can see what your current heart rate is on your wrist watch and how it fluctuates while you are exercising. The word continuous is the key. Wireless heart rate monitors that come with a chest strap transmitter are able to give continuous monitoring, whereas strapless heart rate monitors usually cannot do so. With continuous monitoring, you don't have to stop doing your current activity in order to look up your heart rate.
Heart Rate Training Zone with Alarms
With this, you can manually set your heart rate training zone (lower and upper heart rate limits) in which you will be doing your exercise. It usually comes with an audio or visual alarm that alerts you when you go above or below your heart rate training zone. In addition some models are able to calculate your heart rate zone limits automatically using your age (and sometimes sex) as input. This feature can usually be switched off i.e. you can train without having set a heart rate training zone beforehand.
Duration of Exercise Session
You use your heart rate monitor's stop watch to start and stop the training session. Once you have stopped, the watch tells you how long you have been exercising.
Average Heart Rate of Exercise Session
This information is available once you have finished your session. It tells you how hard on average you have been training. It is a measure of the average intensity of your training session and is a very important piece of information. And if you have preset a heart rate training zone, you can check if you have been training within your intended heart rate limits.
Maximum Heart Rate of Exercise Session
This is again only available once you have finished your session. It gives you an indication of what your maximum training effort was during your exercise session. This information is normally not as useful as the average heart rate of exercise session.
Basic Time Keeping
One would expect to be able to find basic time keeping features on all heart rate monitors. However, this is not the case. Some basic heart rate monitors, for example, won't let you set the date and a wake up alarm. So don't automatically assume that you can do them.
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This can be important if you, for example, expect to do some road running at night in dimly-lit streets.