266 KNOWLEDGE What is ultrasound Oscillations with frequencies above 18 kHz (18,000 waves per second) are called ultrasound. These oscillations lead to the generation of millions of small vacuum bubbles in all liquids during the expansion phase. These bubbles implode during the pressure phase and generate highly effective pressure impulses. This process is called cavitation. Low frequencies around 20 kHz, which are for example used for cell disruption, generate bubbles of bigger diameters with strong pressure impulses as opposed to higher frequencies around 35 kHz, which are preferably used for intensive and gentle cleaning. For generating the ultrasound, a ultrasonic generator converts the power frequency to the respective frequency of the device and passes it on to the wave systems for generating mechanical waves in the liquids. Advantages of ultrasonic cleaning The cavitation has the effect that residue dirt and infectious agents are divulsed from the parts in the liquid, even from cavities and drillings - electronic brushing. Ultrasound cleans within a few minutes and excels in cleanliness compared to manual cleaning. At the same time, it works gently, because mechanical damages such as scratches are avoided. Advantages in chemical engineering and sono-chemistry Cavitation is used for many applications. For example, an emulsion made of oil and water will be finer and more stable under the influence of ultrasound than with any other techniques. For sonochemical processes in an ultrasonic bath, the reaction vessel should have a thin-walled bottom. Then, the ultrasonic energy radiating from the bottom of the tank will be effectively transmitted to the reaction vessel. Which ultrasonic bath do I choose? Ultrasonic baths work with 35kHz as cleaning-intensive high frequency. The size and type of the cleaning objects and sample vessels defines the size of the tank and thus the required device type. When selecting the device, the basket dimensions have to be considered. In order to avoid overloading the device, it is recommended to choose a slightly bigger device. This also provides tolerance for other applications which might be added for future use. The diversity of SONOREX ultrasonic baths, ultrasonic special baths and ultrasonic baths for built-in shows that every user can find the bath that is right for him. Does an ultrasonic bath have to have a heating? Heated cleaning liquids shorten the cleaning time. Stains can be removed quicker in heated liquids. In a laboratory, devices with heatings are usually used for cleaning tasks. Disinfection liquids must not be heated, because this can result in a protein coagulation at 40°C , which makes cleaning and disinfecting more dicult. For this task, devices without heating are used. Which accessories are necessary? The cleaning objects and reaction vessels must never stand on the bottom of the tank. The basket avoids scratches at the cleaning object and the bottom of the tank. Containers for cleaning smaller objects or for working with aggressive solutions are hooked in the hole lid. Which liquids can be used? Direct cleaning: In ultrasonic baths, special substances are used, which have been developed especially for use in ultrasonic baths. Water without cleaners does not clean. Do not use household cleaners or pure desalinated water. Indirect cleaning: Indirect cleaning is carried out in containers or tanks. To transmit the ultrasound in the container, a contract liquid which contains tenside, e.g. water with TICKOPUR, is filled in the tank. To work with acids or if acid residues have to be removed, a plastic tank is to be used. It is not allowed to fill solvents like gasoline, alcohol, acetone etc. directly into the ultrasonic bath because of the flammability and explosion hazard. Checking the cleaning performance of ultrasound devices by means of foil tests It is often of interest to evaluate the performance of an ultrasonic bath. In the standard IEC/TR 60886 (1987-03), procedures for proving the cavitation are explained. Cavitation strength and distribution in a liquid volume can be determined by means of the so-called foil test. A thin aluminum foil is tightened on a frame and then inserted in an ultrasonic bath and exposed to the waves. The emerging erosions of the foil within a defined time (max. 3 min) provide a measurement for the eect of the cavitation. The foils can be stored easily. NOTES ON CLEANING IN ULTRASONIC BATH www.folientest.info
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