The Delta’s optical heart-rate sensor calculates how many times your heart beats per minute (bpm).

In heart rate mode, the device will start measuring your heart rate automatically after a few seconds and then continue checking in 15 minute intervals and you can view the heart rate on the device directly.

The device also continuously records your heart rate when you are in exercise mode.

By default, the Delta will also periodically record your heart rate throughout the day. You can disable this in the app settings. This is used to inform the resting heart rate and VO2max Traffic Lights in the app. 

Your physical characteristics, the fit of the device and the type and intensity of physical activity can affect the heart rate reading.