In some circumstances, ultrasonic cleaners can be used with plain water, but in most cases, a cleaning solution is used. This solution is designed to maximise the effectiveness of ultrasonic cleaning. The primary solvent may be water or a hydrocarbon (historically, toxic solvents such as carbon tetrachloride and 1,1,1-Trichloroethane were used industrially, but have been phased out[9][10]). There 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. The 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 50–65 °C (122–149 °F), however, in medical applications, it is generally accepted that cleaning should be at temperatures below 45 °C (113 °F) 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. The 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.