A method of detecting the fit of a patient interface device (8) includes determining a plurality of capacitance values when the patient interface device is donned by a user. The patient interface device has a contacting element (14) having a user contacting surface (49), and each of the capacitance values is associated with one of a plurality of locations along at least a portion of the contacting surface, wherein each of the locations has a number of electrode elements (54) associated therewith that are used to determine the capacitance values. The method further includes classifying each of the capacitance values into one of a plurality of predetermined categories, and using the classified capacitance values to identify a number of predetermined fit conditions along the contacting element. Also, a respiratory therapy system implementing the method.
