In the operation of pump oxygenators during cardiopulmonary bypass it is essential that arterial return to the patient be equal to the venous inflow from the patient. During bypass using a disc oxygenator with gravity inflow, this is usually accomplished by maintaining a constant blood level in the oxygenator chamber, adjusting the speed of the arterial pump as necessary. While not difficult, this requires the undivided attention of the pump operator who may be required to carry out other tasks such as adding blood to the circuit, checking intracardiac drainage systems, drawing samples from the oxygenator, adjusting water temperatures of the heat exchanger, and observing and recording several temperatures. It is possible that sudden changes in the oxygenator level secondary to a variety of causes may go unobserved, at least momentarily. In addition, with the use of the large oxygenator chamber for adults, substantial changes within the oxygenator may be