Thanks for the lifting from a lower to a higher level without understanding. OK, when I fall back, I may send a flash of light to somebody else. I searched in my 'Cleaning Bible' but did not find anything about various liquid's efficiency. The bubble that is pumped can be seen on page 2 from the below link from Blue Wave Ultrasonics.
http://www.bluewaveinc.com/reprint.htm
We have a big U/S cleaner, with hundreds of liters of alcoholic (don't know which one)+ DI rinsers. Detectors feel the fuel/air build-up and break everything if the level approaches dangerous levels. This machine is not the object of interest, but all the small tanks that we use around. These are probably used without knowing about the danger of misuse, so I will instruct our responsible person about this fact. Most of the small tanks have tank and generator in same case, which is not allowed when using flammables. And we warm up the solvents to some +60 Centigrades! I think that even if the tank is placed under a evacuation hood, it may be risky.
By the way, the 'Cleaning Bible' should be used by all processing engineers. There is absolutely no personal connection with or pressure brought on me from the author when I recommend this book:
"Cleaning and Contamination of Electronics Components and Assemblies" Electrochemical Publications Ltd 1986 ISBN 0 901150 20 7. The author is well known. Ought to be able to buy still.
Inge
-----Original Message-----
From: Brian Ellis [mailto:[log in to unmask]]
Sent: den 23 augusti 2007 15:52
To: TechNet E-Mail Forum; Hernefjord Ingemar
Subject: Re: [TN] U/S vs liquids vs temp vs cavity efficiensy
Ow, there a hundred ifs and buts to this one. Generally speaking, all liquids cavitate better cold than hot, although I would hesitate taking water down to -40°C (no doubt, Dewey will make an icy remark!). The reason is simple: the cavities are "filled" with vapour and the lower the temperature, the less the mass of fluid. As the cavity collapses, the shock wave that is generated depends on the degree of collapse. Too much vapour and there is a cushioning effect that slows the collapse as the vapour condenses with the increased pressure and the mechanical energy transferred to the liquid phase is lowered. This has to be studied in relation to the physical characteristics of the liquid (density, surface tension, viscosity etc.) to find out the optimal conditions at a given frequency.
For the same reason, the liquid must be degassed because the cavities will form around the nucleus of the gas molecules, rather than where the contaminant is, so it's a double whammy: distance of cavitation from the article being cleaned AND cushioning of the collapsing cavity by the presence of gas. This is particularly important with water and aqueous solutions, because many gases dissolve readily in water. Strangely, some perfluorocarbons also need degassing, as they may have a high affinity for oxygen.
Increasing transducer power is not necessarily advantageous because it increases the frequency of cavitations and, in a properly functioning system, this will occur farther and farther away from the part being cleaned and the energy thus uselessly dissipated only heats the fluid.
As some cavitation will inevitably occur at the liquid/air interface, atomisation will occur and the mist will evaporate readily. With acetone (or other solvents with a low flash point), this will increase the concentration of vapour in the vicinity of the tank, increasing the risk of explosion, should an article with a high electrostatic charge be introduced. Unlike halogenated solvents, acetone vapour is not significantly denser than air and cannot be well-contained as in a vapour-phase degreaser.
There are many misconceptions about ultrasonics: do you know what causes the shock wave, for example? Indirectly, it is the cavity collapsing in the range of pico- to nano-seconds, but the sudden compression within the cavity causes the few molecules of vapour to produce an adiabatic temperature rise of, wait for it, of thousands of degrees and it is the instantaneous dissipation of the heat that produces it. If you like, cavitation is an energy pump: a few hundred cycles of energy from the transducer is used to create a cavity which gets larger with each cycle, this ever-increasing energy being stored within the cavity. When it collapses, the energy is released. To get the maximum effect, the objective is to have conditions whereby the cavity is as large as possible before the collapse. This requires fine tuning for a given set of conditions.
Brian
Hernefjord Ingemar wrote:
>
> Hi all,
> we have ultrasonic cleaned with hot Acetone for years, without knowing
> that this liquid has its highest cavity efficiency at -40 Centigrades.
> Nor did we realize, that freons are ineffective compared with water
> (only 5%). Water with some detergent or tenside addition is seemingly
> superiour to most solvents in terms of cavity efficiency.
> We have observed, that our Acetone cleaning was not so effective, now
> we know why. Of course, you can increase the transducer power, but not
> all parts like that.
>
> Just thought someone more liked to see the interesting chart. Asking
> Steve to put it on his wall.
>
> Inge
>
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