[Jewelry making - Article 00605] - Re: [Orchid] Ultrasonic cleaner and damage to hands

[Danny Mitchell]

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    Hi folks, All sound travels as a series of successive compressions
    and rarefactions of the conducting medium. This diagram may help
    visualise this: 

    |   |   |   |  | ||| |  |   |     |      |     |    |   |  | ||| |  | 

     No sound    compression         rarefaction             compression 
                    <-- direction of travel 
 

    (Optional math: If we plot the pressure against time, we will get a
    superposition of sinusoids.) The frequency of the sound is simply
    the number of compressions per second, or hertz (which means
    compressions per second.) For audible sound, the frequency is
    between about 20 and 20000 hertz (= 20 kilohertz, or kHz.) Anything
    above 20kHz is ultrasonic. The energy of the wave (or the amplitude)
    is the magnitude of the compressiond or rarefactions; for a wave (in
    air) of low energy, like voice, variations in pressure of 35
    millionths of atmospheric pressure are fairly normal (the human ear
    is sensitive enough to detect a bee's wing dropped onto it from a
    few cm.) The speed of the sound depends on the medium it's
    travelling through; for air it's about 334 m/sec, though it's very
    much higher in liquids and solids, where atoms are very much closer
    together (in water it's about 1500 m/sec). Frequencies higher than
    about 50kHz in air are attenuated so heavily that they are
    effectively not transmitted, though in liquids and solids,
    frequencies of several million hertz (megahertz, MHz) are common
    (ultrasound imaging equipment uses about 5 MHz, though the energy
    level is very low. It uses the fact that sound travels at different
    speeds through different materials, and the different absorbtion
    rates of tissues, to build its pictures. The minimum distances it
    can measure are of the order of one wavelength (wavelength is the
    distance between successive compressions - so it's the speed of the
    sound divided by the frequency) so with these frequencies accurate
    measurements of less than half a mm are possible.) It's easier, from
    an engineering viewpoint, to produce ultrasonic frequencies, because
    smaller transducers are required. The boiling point of water at sea
    level (one standard atmosphere) is 100 deg. celcius. As the pressure
    drops, so does the boiling point, so that it's about 35 deg. on top
    of Mount Everest. Because sound consists of compressions followed by
    rarefactions of pressure, high energy sound can yield high vacuums,
    followed very quickly by high pressure. Consequently, if living
    tissue is exposed to high energy sound (of any frequency) water in
    the cells can boil, leading to areas of very low pressure, which are
    then re-compressed, giving rise to gas bubbles. Gas bubbles in
    living tissues can cause many different problems: in blood vessels,
    it's called an embolism, and can block veins and arteries, killing
    dependant cells; in nerves, it can trigger random firing and
    twitches as the nerves are cut off from the brain; in joints, it's
    called the bends - yes, it's the same thing careless divers get -
    and can, if untreated (in bad cases, even if treated), stiffen the
    joints. It may also contribute to arthritis. Since it leads to cell
    death, long-term exposure is cumulative. High energy ultrasonic
    devices are being used in medical experiments to perform
    non-invasive surgery (see, for instance,
    http://www.icr.ac.uk/education/studentships/terHaar1.html) So, Tina,
    in answer to your question, holding the cast tree in the ultrasonic
    cleaner can lead, in the long-term, to tissue damage. The risks are
    reduced by hanging the tree on a hook and holding the hook, but to
    eliminate the risks, it should not be touched while it's in contact
    with the cleaner. Danny Mitchell 


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