The pressure compensation vessel (PCV) just helps control the liquid level in the pipettes. It has a electrocaloric liquid level sensor that is used as a closed feedback loop for controling the liquid level in the bottle. So the bottle has inside a heater and heat sensor that are positioned in a water proof container. There is an arduino microcontroller that is used for controlling the feedback. More details about how it works is posted here.

The system includes:
    1. 500ml bottle with modified hood for connecting liquid input, output, electrocaloric sensor and air stone filter
    2. Tubing
    3. Bottle clamp for mounting
    4. Waterproof electrocaloric sensor
    5. Optional: peristalic pump
    6. Optional: control electronics, command line software and power supply