kraken is an open source garmin app for the forerunner 235. It was created to show and store more detailed 24x7 heart rate data. The main idea is to query the sensor only ones every so and so many seconds (user choice)and visualize the last ~90 minutes of heart rate data always on screen.
To be battery efficient the screen is only updated every minute. The sensor is queried always for 15 seconds every minute e.g. 1.22 - 1.43 or 10.22-10.43. The heart rate which is visualized is the mean heart rate obtained from this measurement period.
IMPORTANT: The sensor is queried every second right now, due to a bug in the Garmin SDK. Therefore, the battery lifetime is a little bit more than a DAY. As soon as this is fixed, the battery lifetime should increase significantly.
Right now all parameters are hard coded and not accessible by the watch keys. This has to be changed.
The current SDK supports turning of the sensor, but according to forum.garmin.com there is a bug in the implementation which is why the sensor is taking measurements every second and cannot be turned off ones activated. As soon as Garmin will fix this issue the battery lifetime should significantly increase when using this app.
If you like this app donations are always welcome.