A sensing system for determining a position of a plunger within a fluid container. The sensing system includes a light source, a light detector and a controller. The light source is configured to emit light into a barrel wall of the fluid container so that the barrel wall serves as a waveguide to guide the light to travel therein in an axial direction. The light detector is positioned to detect reflected light that was emitted by the light source, traveled through the barrel wall serving as the waveguide, and then reflected off a surface of the plunger. The controller is in communication with the light detector to determine an axial position of the plunger surface based on data from the light detector of the detected reflected light.
