Standard 22mm tubing for CPAP and BiPAP. The 18" is typically only used to connect your machine to an external humidifier such as the F&P HC150.
All Tubing is smooth bore designed. Smooth bores have no ridges inside the hose for air pressure to resist against. The supportive wire frame is located on the outside of the hose. This design provides consistent pressure delivery and ensures a quieter performing hose.