Ultrasonic principle

Technology
Piezoceramic transducer systems are used to generate the ultrasonic vibrations. These are bonded to the oscillating tank and contain a piezoceramic disk that expands or contracts when a voltage is applied. To generate ultrasound, a high-frequency generator is used to convert the mains frequency from 50 Hz to at least 20 kHz. The resonant frequency of an ultrasonic vibration system is defined by the assembly and cannot be changed after manufacture.

Frequencies
Frequencies from 20 kHz to 400 kHz are commonly used. Low frequencies around 20 kHz produce bubbles of larger diameter with more powerful penetration, whereas higher frequencies from 35 kHz are more suitable for intensive and gentle cleaning of surfaces. The current common frequency for particles with a diameter larger than 1 µm is about 200 kHz. The frequency range above 400 kHz up to 1.2 MHz is also referred to as megasonic in the technical literature. Here, particles with a diameter smaller than 1 µm are optimally detached. The required frequency is determined by the smallest opening or structure that can be cleaned by the sound.

Application
The use of solvents, acids or alkalis significantly supports the cleaning effect of ultrasonic baths. In particular, the surface tension of the water must be reduced with a suitable cleaning preparation for effective ultrasonic cleaning. No flammable liquids are used as cleaning agents, since ultrasound always causes heat to be introduced into the liquid, which could lead to ignition of the bath liquid if the ignition temperature is exceeded. Ultrasonic baths are also used in sample preparation to cleave, i.e. fragment, biological substances in the analytical laboratory. Ultrasonic baths are used for cell disruption or to shear DNA.

Ultrasonic equipment
Ultrasonic cleaning equipment is often used to clean small, complex and finely structured components. For example, jewelers, goldsmiths, dental technicians, watchmakers and opticians use ultrasonic cleaning baths to clean their products. In the medical industry, high-performance ultrasonic baths are used for the reprocessing of surgical instruments. Blood and tissue residues can be effectively dissolved from the joints of the instruments. In addition, ultrasound shortens the necessary exposure time for disinfectants. In the automotive industry, ultrasonic baths are used for cleaning carburetors, spark plugs and injection nozzles. Larger, industrial ultrasonic cleaning systems are used for cleaning raw parts, e.g. for surface finishing (electroplating).

Structure
The system consists of a liquid-filled, possibly heatable tank, one or more ultrasonic transducers or ultrasonic oscillators and a generator that supplies the transducer(s) with high-frequency electrical energy. Sound transducers can be bonded both directly to the outside of the walls and under the bottom of the tank. With this design, the ultrasound is then introduced directly into the liquid via the walls and/or floor. Another variant is the so-called submersible transducer, which is suspended in the liquid or attached to racks in the tank. Finally, there are plate transducers that are flanged to a corresponding opening in the tank wall. The devices are usually made of stainless steel. The arrangement and distribution of the sound transducers must be such that a uniformly strong, non-static sound field is created. In some cases, the frequency is modulated to prevent standing waves. A function for degassing the cleaning liquid is also not uncommon. Here, the output power of the device is pulsed to allow any gas bubbles to rise to the surface. The background to this is the poor formation of cavitation bubbles in the presence of gas bubbles, as these would absorb the power. The minimum sound power required for a good cleaning result is about 10-20 watts per liter of cleaning liquid. The sound generators require a minimum distance of 1 – 2 cm to form a homogeneous sound field. Contact of persons with an active ultrasonic bath leads to an attack on the cell structure and must be avoided.