An acoustic sensor detects presence and/or absence of fluid in a fluid delivery medium. The acoustic sensor detects fluid absence based on the difference of the speed of sound between air and a fluid. For example, the acoustic sensor may detect fluid absence based on a phase shift between acoustic signals transmitted through the fluid delivery medium when fluid is present as compared to acoustic signals transmitted through the fluid delivery medium when fluid is absent, e.g., when air or bubbles are present in the fluid delivery medium.