Sensors and systems for determining a physiological parameter such as blood pressure using external pressure are disclosed. Such sensors and/or systems may be embodied in a wearable user device. The wearable user device may include a cuff configured to apply a pressure to a portion of a user at one or more substantially constant pressure levels; a photoacoustic sensor configured to obtain photoacoustic signals generated from light incident on a blood vessel; and a wearable structure including the cuff and the photoacoustic sensor. Techniques involving the wearable user device may include: obtaining photoacoustic signals from a portion of the user; determining a curve associated with the user, the curve including a plurality of spatial measurements of the blood vessel as a function of a plurality of discrete pressures, the curve enabling determination of a characteristic of the blood vessel; and determining the physiological parameter based at least on the characteristic.