Methods and sensor delivery devices for monitoring a fluid pressure within a vascular structure, the devices including an elongated sheath sized for sliding along a guidewire, a sensor assembly including a fiber optic sensor, a housing surrounding the sensor, a first cavity between the distal end of the sensor and a distal aperture of the housing, a filler extending from at least the distal end of the housing distally and tapering inward toward the outer surface of the sheath, a second cavity in the filler with an opening at the outer surface of the filler and adjoining the distal aperture of the housing, and an optical fiber. The sensor delivery device may also include an outer layer that partially covers the second cavity with an aperture over the opening of the second cavity.