Better late than always...
Rainer,
Freon was too dense, acetone prohibited, Isopropyl alcohol flammable and
clear water not tolerable...I don't get it..what worked then?
/Inge
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Blomberg, Rainer (FL51)
Skickat: den 30 augusti 2007 21:50
Till: [log in to unmask]
Ämne: Re: [TN] U/S vs liquids vs temp vs cavity efficiency
Getting into this late, It has been my experience and understanding from
precision cleaning the last 35 years that vapor pressure and density of the
liquid are the prime driver of efficiency. We ran experiments and
determined that our Freon u/s cleaning was best when the solvent was at RT
in a glass beaker supported approx. 1/2 "off the bottom of the U/S tank that
was filled with water at least as high as the solvent in the beaker.
Obviously we are cleaning small parts but the principle would be the same
for larger applications. Water as the transmission fluid was better than
simply putting Freon in the tank since we use several solvents and can
simply switch beakers instead of replacing the solvent in the tank each
time. Freon is very dense but it's vapor pressure being very high made it
difficult to clean items. We used to use acetone in U/S once but a spark
from the electronics and a fire placed a prohibition on acetone. Isopropyl
alcohol works well too but has the flammability issue as risk. Most of the
items we clean can not tolerate water exposure so we have to resort to the
"double boiler" set-up with different solvents, but none are heated.
> Rainer G. Blomberg
> Honeywell -Space Systems Clearwater
> Staff Production Engineer
(727) 539-5534
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Thursday, August 23, 2007 10:25 AM
To: [log in to unmask]
Subject: Re: [TN] U/S vs liquids vs temp vs cavity efficiency
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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