The BE-A Series from Panasonic is an optical air bubble sensor that simply and easily connects to small diameter transparent tubes and monitors presence of liquid in the tube. When an air gap (bubble) is present the sensors' outputs will change state. There are 3 different versions available for 2mm, 3mm, and 4mm outside diameter tubing. The high speed output is perfect for rapidly moving liquid in the line.
The BE-A series is great for use in liquid dispensing instruments, clinical laboratory instruments, or anywhere precise liquid dispensing is needed.