Open or detach the diffuser. Detach the measurement and adjustment module by pulling gently from the casing (not from the control spindle or measurement tubes). Wipe the parts with a damp cloth, instead of immersing in water.
The measurement and adjustment module is remounted by pushing the body until it meets the stopper.
The sound attenuation material within the plenum can also be removed, to enable cleaning of the inner side of the plenum. Close or replace the diffuser after cleaning.