Ultrasonic cleaning baths : field measurements and cleaning efficiency
Ultrasonic cleaning baths are routinely used for cleaning dental and surgical instruments. The importance of efficient cleaning prior to sterilisation is great. Micro-organisms can survive the sterilisation process as debris can act as an insulator. The information available on ultrasonic field distributions in cleaning baths is limited. A thermal technique, employing a thermistor probe coated with an absorbing material measures the temperature rise when the field is switched on compared to no sound temperature. Contour plots of the field of the cleaning bath resulted from point-to-temperature. Contour plots of the field of the cleaning bath resulted from point-to-point measurements. Optical methods were also employed for qualitative measurements of the field. The main optical method used in this study is the Sarvazyan, dye/paper method where dye patterns represent the distribution of the field's cavitation activity at that area. Titanium coated mica was also used for seeding the bath. When the field was switched on, these particles migrated towards the antinodes, the distribution of particles in the field gives qualitative measure for the field. Aluminium surfaces were sonicated and erosion with pitting was evident after 30-60 minutes sonication time. Some plates were examined under an SEM. Cleaning efficiency was directly examined by coating perspex and stainless steel plates with blood. The plates were left to dry for 24 hours then sonicated. Water and Decon-90® detergent gave very similar removal times, with the detergent to achieve cleaning marginally faster compared to water. A disinfectant-detergent was also used, Virkon®. The results with Virkon were disappointing, red cells were caused to lyse up by Virkon and as a result a colloidal mixture was formed. This mixture was not able to be removed ultrasonically, even after an hour or more sonication time. Manual brushing was required in that case. In general all baths tested showed a non-uniform field pattern that can result in insufficient cleaning.