Abstract: A wearable apparatus selects at least two signal channels corresponding to an arterial signal, the channels associated with corresponding optical sensors of the wearable apparatus. Data is obtained from signal channels over a predetermined time. A function is applied to the data to transform the data to a frequency domain. Phase values are determined for frequency components of the data in the frequency domain. A phase difference value is determined between the phase values. A time shift value is determined between the data based on the phase difference value. A modified pulse transmit time is determined, based on the time shift value, representing a transit time for a pressure wavefront to travel between optical sensors. A pulse wave velocity is determined based on the modified pulse transit time. A blood pressure value is calculated based on the pulse wave velocity. A message is provided based on the blood pressure value.