A fluid cassette for a blood processing system includes a cassette housing and a rigid structure. The cassette housing defines the structure of the cassette and has a fluid path at least partially extending through it. The fluid path is configured to allow a fluid to pass through the housing. The rigid structure defines a cavity that is in fluid communication with the fluid path. The rigid structure also has an interface for interfacing and/or connecting with a pressure monitoring device. The interface allows the pressure monitoring device to measure the pressure within the fluid path. The cavity has a volume of air located between the fluid path and the interface.
