This work compares the time series obtained by measuring simultaneously the RR time series with a Polar Sensor and the Pulse to Pulse (PP) time series by using the camera of smartphones. The measured differences between time series are modulated by the breathing rate and have little influence from the measuring device. The standard deviation of the differences between time series depends mostly on the subject under measurement and can change from values of around 10 ms order to values near 100 ms. These differences affect the quantification of HRV. On average, a breathing rate around 0.2 Hz is advised to minimize these errors.