Optimizing your Ultrasonic Cleaner

To understand how an ultrasonic cleaner can perform cleaning or sonification, it helps to understand the principles of ultrasound and how they are applied in ultrasonic cleaning. Ultrasound is sound transmitted at frequencies beyond the range of human hearing. Ultrasonic energy generated by piezoelectric transducer at a rate of 40,000 times per second creates cavitation, the mechanism for ultrasonic cleaning.

Cavitation consists of the formation and collapse of countless tiny cavities, or vacuum bubbles, in the liquid. The energy produces alternating high and low pressure waves within the liquid in the tank. The liquid is compressed during the high-pressure phase of the wave cycle, then pulled apart during the low-pressure phase. As the pressure in the liquid is reduced during the low-pressure phase, cavities grow from microscopic nuclei to a maximum critical diameter. During the subsequent high-pressure phase they are compressed and implode. The energy is powerful but safe for parts because it is localized at microscopic scale.

Factors affecting the strength of cavitation are temperature, surface tension, vapor pressure, viscosity, and density. When selecting an ultrasonic benchtop cleaner, look for units that allow as much control as possible of these factors. For example, a microprocessor-based digital thermostatic control allows a constant display of the solution temperature for close monitoring and control. Heat also increases the chemical activity of cleaning solutions. Solution temperature generally should be kept between 120-140 deg. F. If the temperature and time can be controlled, then repeatable cleaning consistency can be achieved.

Adding a wetting agent or surfactant to the bath can reduce surface tension of the liquid. Reduced surface tension will increase cavitation strength. Medium vapor pressure is most conducive to ultrasound activity. Low vapor pressure produces cavitation bubbles that implode with relatively greater force, but results in fewer bubbles and a higher cavitation threshold. High vapor pressure is not very effective - more bubbles are created, but they collapse with less intensity due to a smaller internal/external pressure differential. Low viscosity promotes cavitation. high density creates intense cavitation with a greater implosive force.

Never place parts or receptacles directly on the bottom of the unit. It can cause the unit to fail because the parts will reflect the ultrasonic energy back into the transducer(s). Always allow at least one inch between the tank bottom and the beaker or receptacle for adequate cavitation.

Never use solvents in a small benchtop cleaner. It is neither safe nor environmentally responsible. Solvents vaporize quickly. Vapors of flammable solutions can collect under the unit, where ignition is possible from electrical components.

Keep solution within one inch of the top of the unit when the beaker or tray in place. In the liquid versus cavitational activity relationship, it does not follow that less solution will intensify the activity. Units operate at optimum efficiency when filled to within one inch of the top.

Wait 5 to 10 minutes after activating the equipment for fresh solution to degas. This need not be repeated with subsequent use, as degassing is required only after the bath is freshly filled.

If using a tray or basket to lower the parts into the solution, it is better to use a holder that is of open construction, either a mesh basket or an insert tray, that is adequately perforated for drainage. This also permits free access of the sound waves to the parts.

Renew cleaning solutions often to increase ultrasonic cleaning activity. Solutions, as with most chemicals, become spent over time. Solutions can become contaminated with suspended soil particles, which can settle to the tank bottom inhibiting ultrasonic transmission.