Abstract: A connector is formed to connect with the outlet of a humidification chamber. A calibrated tube extends through the connector to be retained in use in at least a substantially vertical direction. A bias flow outlet also extends through the connector. In use the connector of the measuring device is fitted to the outer port of a humidification water chamber with the calibrated tube extending into the reservoir of water within the chamber. With an air supply to the chamber activated the subsequent water level in the tube indicates the operating pressure of the blower under a flow condition simulating losses through leakage in the breathing circuit and at the patient connection.