source 14 of medicinal fluid b are both coupled in fluid communication with a proximal end 16 of a cassette 15 the flow of medicinal fluid a into the cassette is selectively controlled by a supply valve 20 and the flow of medicinal fluid b is selectively controlled by a supply valve 18 if cassette 15 is to be used to pump only one of these two medicinal fluid at a time only the appropriate supply valve 18 or 20 is opened to select the medicinal fluid to be pumped the selected medicinal fluid or fluids then flow s through an air sensor 22 and into a mixing chamber 26 the air sensor and mixing chamber are common features of cassette type infusion pumps the purpose of the air sensor is to detect air bubbles that may be entrained in medicinal fluid a and or b before the fluid is passed on into the pumping chamber and to the patient excess air bubbles entering a patient 39 s bloodstream can cause an air embolism with potentially harmful consequences a proximal or inlet pressure sensor 24 is disposed within mixing chamber 26 the selected medicinal fluid or fluids exit the mixing chamber through an inlet valve 28 when the inlet valve is in its open position and into a.