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August 2007

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TechNet E-Mail Forum <[log in to unmask]>, Hfjord <[log in to unmask]>
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Thu, 23 Aug 2007 22:33:46 +0200
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In India, that would be called a punjab..
Inge


-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Whittaker, Dewey (EHCOE)
Skickat: den 23 augusti 2007 16:47
Till: [log in to unmask]
Ämne: Re: [TN] U/S vs liquids vs temp vs cavity efficiensy

Bev used a pun. I'm shocked! It must be a sine of the times.
Dewey

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: Thursday, August 23, 2007 7:42 AM
To: [log in to unmask]
Subject: Re: [TN] U/S vs liquids vs temp vs cavity efficiensy

Brian,
Tank you (pun intended). Other than the last paragraph, I did not know any
of this stuff before reading your e-mail.
Regards,
Bev
RIM

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: Thursday, August 23, 2007 9:52 AM
To: [log in to unmask]
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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