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Mouthpiece and filter used for the bench study. The test mouthpiece was a 15-mm angled, rigid plastic mouthpiece (Philips Respironics) with a heat-and-moisture exchange filter (Hygroflux 1, Vygon). The graph represents the resistance (flow-pressure slope) of the mouthpiece. Pressure drop was measured as pressure at the proximal end of the mouthpiece, the tip of which was open to the atmosphere.
