A system and a method for detecting the presence of air bubbles in an intravenous (IV) line supplying a medicinal liquid to a patient. An air bubble sensor includes an ultrasonic transmitter acoustically coupled to an ultrasonic receiver to detect the presence of a gas (e.g., air) in a portion of a tube comprising the IV line. The transmitter and receiver are mounted on pivoting transducers that are disposed on opposite sides of the tube. A spring biases the transducers inwardly toward each other so that the transmitter and receiver contact opposite sides of the tubing. This assembly automatically accommodates different sizes of tubing and tubing of a relatively wide range of stiffness. The tube is connected to a disposable pumping cassette that is engaged in a pump chassis on which the transducers are pivotally mounted. A user actuated plunger on the pump chassis is depressed to cause the transmitter and receiver to move away from the tube when the pumping cassette is removed from or inserted into the interior of the pump chassis. A controller precisely monitors the flow of medicinal liquid through the tubing to detect the size of gas bubbles and total volume of gas infused. The controller automatically compensates for minor contamination of the exterior surface of the tube, e.g., if the surface is wet with a liquid.
