Some Samsung smartphones have a built-in sensor that lets users measure heart rate variability. It's more accurate than a phone camera, so use it if you have one.
How to connect the Samsung sensor
Allow the app to get data from the sensor. There are two ways to do this:
Use the pop-up window that asks for access to Body Sensors
Go to Menu → Settings → Measurement Device, find Samsung HRM, tap it and enable access
After you see Connected next to Samsung, you can start your measurement.
If the app can't find a sensor, it means your Samsung model does not have an HRM sensor. Use a camera or another heart rate monitor.
Put your finger over the sensor located either under the camera or to the camera's side. Don't press down too hard.
Tap the Measurement button to start. Don't talk or move during the measurement.