Degassing

1. Degassing Bubbles that form in a glass of water drawn from a tap and left undisturbed for a period of time are dissolved gas that is spontaneously released as the pressure is reduced to that of the atmosphere and the temperature increases. The same effect is responsible for the bubbles that appear in soda and other carbonated liquids as dissolved CO2 is released once the container is opened and the liquid is no longer held under pressure and/or chilled. Water supplied through water mains under pressure almost always contains dissolved air and other gasses.

2. Degassing Ultrasonic cleaning relies on the catastrophic collapse of cavitation bubbles to produce the mechanical cleaning effect. If the cleaning liquid contains gas, the gas diffuses into the cavitation bubble during the growth of the cavitation bubble (when the pressure within the bubble is negative) and prevents its catastrophic collapse or “implosion.” This is because gas within the bubble exerts an outward pressure preventing the bubble’s collapse similar to the effect of trying to compress an inflated basketball or toy balloon. Degassing is a necessary procedure prior to starting any ultrasonic cleaning process.

3. Degassing Liquids Increasing the temperature of a liquid reduces the amount of gas that the liquid can dissolve. Ultrasonic cleaning tanks should be heated to the final operating temperature before use. Increasing temperature also speeds the rate at which dissolved gasses are released from a liquid and enhance the ultrasonic degassing procedure described below. Adding chemistry helps release gas from a liquid. Cleaning chemistry should be added to assist degassing of liquids prior to use. Ultrasonic energy is effective in degassing liquids. Ultrasonic energy should be activated well in advance of any ultrasonic cleaning procedure.

4. Ultrasonic Degassing During the growth phase of a cavitation bubble in a liquid with dissolved gas, the reduced pressure within the bubble due to its increase in size causes dissolved gas to diffuse out of the surrounding liquid and into the growing bubble cavity. Cavitation Bubble Growing in Size (reduced pressure) Dissolved Gas Gas Diffusing into Cavitation Bubble

5. Ultrasonic Degassing Cavitation Bubble Shrinking in Size (increased pressure) Once the cavitation bubble has reached its maximum size and begins to contract, gas trapped within the cavity becomes pressurized and may begin to diffuse back into the surrounding liquid. Dissolved Gas Gas Diffusing Out of Cavitation Bubble

6. Ultrasonic Degassing Buoyant Gas Bubble Floats to the Surface However, since the surface area available for diffusion is shrinking as the pressure is increasing and the diffusion of gas requires a finite amount of time, not all of the gas in the cavity is able to diffuse back into the surrounding liquid and forms a buoyant gas bubble that floats to the surface of the liquid. Dissolved Gas

7. Ultrasonic Degassing This process is called “rectified diffusion” and occurs as the cycle of bubble growth and shrinkage occurs repeatedly. Eventually gas is eliminated from the liquid.

8. Completion of Degassing • Gas bubbles stop rising to the surface of the liquid • The liquid no longer has a cloudy appearance • There is a rippled pattern on the liquid surface resulting from ultrasonic activity • There is a distinctive change in sound There is no absolute indicator of the end of the degassing process of a liquid. An operator experienced in ultrasonic cleaning processes is usually able to tell when the degassing process is complete using one or more of the following indicators.

9. Cautions on Degassing If the temperature of a previously degassed liquid is increased, it may be necessary to again perform the degassing procedure. An increase in temperature reduces the amount of gas that the liquid can dissolve which means that a liquid although totally degassed at one temperature may not be totally degassed if its temperature is increased. Cleaning solutions should be stabilized in temperature and stirred to assure that there is no temperature stratification before proceeding with ultrasonic cleaning. Increasing Temperature

10. Cautions on Degassing In some cases, long periods of filtration without ultrasonics or certain spraying and agitating procedures utilized in parts cleaning may entrain gas in the cleaning solution. For this reason, it is always a good idea to operate energy for a period of time before introducing work into the cleaning process. Two to five minutes of ultrasonic operation is usually adequate to eliminate residual gas from a cleaning solution that has been previously used and totally degassed. Spraying, Agitation and Filtration

11. Cautions on Degassing Cleaning solutions left at temperature overnight need only a few minutes of ultrasonic operation to be restored to a fully degassed state. Cleaning solutions that cool overnight should be heated to operating temperature and fully degassed prior to introducing work into the system. This degassing procedure will usually not require the same time as for a “fresh” solution but will be longer than that for a system left at temperature overnight. Daily Startup