Detection of fluid conditions in a section of tube of an administration set. A light source is positioned adjacent to the tube to transmit an infrared light through the tube and any fluid therein. A light sensor senses the infrared light transmitted through the tube and generates an output signal. A frequency of the output signal is a function of an intensity of the light transmitted through the tube. A processor receives and determines the frequency of the output signal, and compares the determined frequency to threshold frequency values to determine whether fluid is in the tube. The processor also monitors the generated output signal to determine if the frequency of the output signal changes over a predetermined period of time, and determines whether fluid is flowing in the tube as a function of the determined change in frequency.