Degassing
Water supplied through water mains under pressure
almost always contains dissolved air and other gasses.
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.
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.
Degassing Liquids
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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.
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
Ultrasonic Degassing
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.
Cavitation Bubble
Shrinking in Size
(increased pressure)
Dissolved Gas
Gas Diffusing Out of
Cavitation Bubble
Ultrasonic Degassing
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.
Buoyant Gas Bubble
Floats to the Surface
Dissolved Gas
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.
Completion of Degassing
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.
• 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
Cautions on Degassing
Increasing Temperature
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.
Cautions on Degassing
Spraying, Agitation and Filtration
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.
Cautions on Degassing
Daily Startup
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.