1) Wet the cotton swab with alcohol.
2) Put the cotton swab in parallel position to the sensor and brush softly by turning the cotton side (Fig. 1). Never brush the sensor by dragging the cotton over it (Fig. 2).
Fig. 1 - Ceaning the transducer. This is correct.
Fig. 2 - This is wrong.
3) Wait until the alcohol has evaporated and apply talcum powder with another cotton swab by rotating softly in parallel position to the sensor again (Fig. 3).
Fig. 3 - Applying talcum powder to the transducer.