Whereas most ventilators use expensive flow sensors, servo motors and other specialized components to push air in and out of the lungs, Callaghan started from scratch with a basic pressure sensor, typically used in devices like blood-pressure meters, that cost about $10. As an air compressor forces air into the chest, the sensor, connected to a tube inserted into the mouth, measures the airway pressure. Software uses the pressure sensor's data to calculate the volume of breath in the lungs—this allows the ventilator to sense when the patient needs to take another breath, at which point the software directs the compressor to supply the air through a valve system. If the patient is beginning to be able to breathe on his own, the software instructs the compressor to supply less air, a feature that helps recovering patients gradually begin to breathe independently again. Other low-cost compressors don't have this kind of fine-tuned ability to adjust to individual needs.