There are several non-invasive ways to tell if a fluid-filled tube has an air bubble. One way is with optoelectronics which use light to detect changes in viscosity or movement of fluid in the tubing.

Optoelectronics have the capability to sense the presence of the tube, bubbles, and in some cases the type of fluid in the tube. Under controlled conditions this can be a simple process using a microprocessor or comparator to interpret output signals from an optoelectronic sensor.

Optoelectronic devices consist of a light-emitting diode (ultraviolet, visible, or near infrared) and photosensor (photodiode, phototransistor, or photologic device). The LED emits light at a specified wavelength that is sent through a translucent tube that contains air or another gas. The internal walls of the tube reflect some light, reducing the amount of light received by the photosensor. With fluid in the tube, the amount of photons picked up by the sensor changes, changing the output of the photosensor.

Several factors affect optoelectronic performance. Here are a few tips to get the most out of them.

• LED power may differ depending on the production lot. This can be compensated for by adjusting the current through the LED.
• Sensitivity of the phototransistor may also vary depending on production lot. This can be controlled by adjusting the output power of the LED or load resistor.
• As temperature rises, the output power of the LED increases while the sensitivity of the phototransistor decreases. This keeps the overall signal constant.
• During initialization, a designer can adjust the LED's current drive to get an expected output. Normally this is done without a tube. The initial output becomes a reference point for all other states. To ensure the proper tube material is used, use the clear-tube condition as the reference point.
• An LED driven at 20 mA can last over 100,000 hours of continuous operation, with an expected degradation of between 5 and 20%. This is over 10 years of continual use. Lowering the LED drive current reduces system degradation and provides an extremely stable configuration.