This is a fantastic app that works flawlessly on those Android 6.0 devices I have tried, namely Samsung S6 Edge and Sony Xperia Z4 Tablet. I am recording the signal with a Bluetooth based Suunto Smart Sensor ("Movesense") breast strap. There is nothing in the app universe that comes even close because it records raw data and it gives you direct access to your raw data. The data are written to a txt file and you can export them from there to a PC or archive them. You can download and install Kubios HRV Standard analysis software for free and conduct your own analysis of your data. Other apps give you a single number to "summarise" your HRV. Reducing the complexity of an HRV signal over several minutes or hours to a single number is complete nonsense. To use Alan Watkins's phrase (see his book "Coherence" page 50), it is like listening to a Chopin piano concerto and saying that the average key is 'F sharp'. It is a complete mystery to me why this app is not the top selling HRV app. It puts you in control. Here, YOU are in the driver's seat. All other apps take you for a ride. All this for the price of a cup of coffee. The steal of the century, if you are taking your health seriously.
Works: Polar H7, Samsung Note 3 Records text file of R-R data as promised, for use with the marvelous Kubios free software package developed by researchers in Finland.