[andrewjshults]

Even the standalone optical HRMs (e.g., Scosche RHYTHM+) aren't good enough to do HRV[1] even though their accuracy compares favorably with the chest strap models[2]

[1] http://www.dcrainmaker.com/2014/05/scosche-antbluetooth-opti... about 10% down the page [2] http://www.dcrainmaker.com/2014/05/scosche-antbluetooth-opti...