are several formulations, dependent on the item being cleaned and the type of contamination (e.g., degreasing of metal, cleaning of printed circuit boards, removing biological material, and so on). Reduction of surface tension increases cavitation, so the solution usually contains a good wetting agent (surfactant). Aqueous cleaning solutions contain detergents, wetting agents and other components, which have a large influence on the cleaning process. Correct composition of the solution is very dependent upon the item cleaned. When working with metals, proteins and greases an alkaline detergent solution may be specifically recommended. Solutions are typically heated, often around , however, in medical applications it is generally accepted that cleaning should be at temperatures below to prevent protein coagulation that can complicate cleaning. Some ultrasonic cleaners are integrated with vapour degreasing machines using hydrocarbon cleaning fluids: Three tanks are used in a cascade. The lower tank containing dirty fluid is heated causing the fluid to evaporate. At the top of the machine there is a refrigeration coil. Fluid condenses on the coil and descends into the upper tank. The upper tank eventually overflows and relatively clean fluid runs into the work tank where the cleaning takes place. Purchase price is higher than simpler machines, but such machines may be more economical in the long run. The same fluid can be reused many times, minimising wastage and pollution. Uses Most hard, non-absorbent materials (metals, plastics, etc.) not chemically attacked by the cleaning fluid are suitable for ultrasonic cleaning. Ideal materials for ultrasonic cleaning include small electronic parts, cables, rods, wires and detailed items, as well as objects made of glass, plastic, aluminium or ceramic. Ultrasonic cleaning does not sterilize the objects being cleaned, because spores and viruses will remain on the objects after cleaning. In medical applications, sterilization normally follows ultrasonic cleaning as a separate step. Industrial ultrasonic cleaners are used in the automotive, sporting, printing, marine, medical, pharmaceutical, electroplating, disk drive components, engineering and weapons industries. Ultrasonic cleaning is used to remove contamination from industrial process equipment such as pipes and heat exchangers. Limitations Ultrasonic cleaning is used widely to remove flux residue from soldered circuit boards. However, some electronic components, notably MEMS devices such as gyroscopes, accelerometers and microphones can become damaged or destroyed by the high intensity vibrations they are subjected to during cleaning. Piezoelectric buzzers can work in reverse and produce voltage, which may pose a danger to their