We recommend Polar FT60, Polar M400, Polar A300
Polar Ft 60
User Manual
https://support.polar.com/e_manuals/FT60/Polar_FT60_user_manual_English/ch01.html
Youtube Tutorial
The Polar M 400
User Manual
https://support.polar.com/e_manuals/M400/Polar_M400_user_manual_English/Content/Introduction.htm
Youtube Tutorial
Polar A300
User Manual
https://support.polar.com/e_manuals/A300/Polar_A300_user_manual_English/Content/Introduction.htm
Youtube Tutorial
Polar M200
User Manual
https://support.polar.com/e_manuals/M200/Polar_M200_user_manual_English/manual.pdf
https://support.polar.com/e_manuals/M200/Polar_M200_user_manual_English/manual.pdf
Youtube Tutorial
Apple Watch Series 3 and 5
Youtube tutorial Apple 3
User Manual
https://www.applewatch3manual.com/2017/02/apple-watch-series-3-manual.html
5 User Manual