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August 2007

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TechNet E-Mail Forum <[log in to unmask]>, "Whittaker, Dewey (EHCOE)" <[log in to unmask]>
Date:
Thu, 23 Aug 2007 18:08:17 +0300
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TechNet E-Mail Forum <[log in to unmask]>, Brian Ellis <[log in to unmask]>
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Brian Ellis <[log in to unmask]>
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Isn't this going off at a tangent, or do you have some other angle?

Brian

Whittaker, Dewey (EHCOE) wrote:
> 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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