Hunter PCB20 Pressure Compensating Bubbler 2.0 GPM Hunter bubblers and bubbler nozzles are pressure-compensating to maintain a constant output of water regardless of the input pressure. This allows for the accurate application of water in both the ½" threaded PCB bubblers and PCN and MSBN bubbler nozzles designed for installation on Pro-Spray® pop-ups. Both bubblers and bubbler nozzles are ideal for tree and large shrub establishment. Bubbler nozzles retract out of sight for a clean look when installed on sprinkler pop-ups. The low-profile design of the PCN and MSBN bubbler nozzles accommodates high-traffic areas and provides the opportunity to convert to larger-radius nozzles as tree root zones mature and grow. With easy installation and ultimate reliability, every plant, shrub, and tree can receive the exact amount of water needed with no excess runoff or waste.