Methods and apparatus for determining leak and respiratory airflow are
disclosed. A pressure sensor (34) and a differential pressure sensor (32)
have connection with a pneumotach (24) to derive instantaneous mask
pressure and airflow respectively. A microcontroller (38) estimates a
non-linear conductance of any leak path occurring at a mask (12) as being
the low pass filtered instantaneous airflow divided by the low pass
filtered square root of the instantaneous pressure. The instantaneous
leak flow is then the conductance multiplied by the square root of the
instantaneous pressure, and the respiratory airflow is calculated as
being the instantaneous airflow minus the instantaneous leak flow. The
time constants for the low pass filtering performed by the
microcontroller (38) can be dynamically adjusted dependent upon sudden
changes in the instantaneous leak flow.