I actually sell industrial sonic cleaners (among many other things). Main customers are aerospace shops making precision parts and cleaning those after fabrication (remove cutting oil, etc.). I do clean my bike parts at work on demo units we have. They are not in the same league as the Wal-Mart units, but the generally, the principle is the same:
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They will generally come in three range of frequencies: around 25kHz, 45kHz and 130kHz. Lower frequency (25kHz) means much harsher cleaning. Higher frequency means very gentle cleaning (usually for jewelry and electronics). For stainless steel, titanium, aluminum parts, 25 or 45 is fine. If you had anodized aluminum parts (I don’t think I’ve come across those in the bike world), you do not want to put anodized aluminum in an ultrasonic cleaner. Period. Carbon parts would be safer in 45kHz, although they should be safe in 25kHz as well.
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Heat! I doubt at that price the units have a heater, but if you can get one, heat is a must. Ultrasonics by themselves will not do everything. Heat is extremely important. If you can’t get a unit with heat, get some water around 50-60 Celsius and put it in the cleaner.
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Cleaning agent: necessary, especially for grease, oil, dirt. Do not use a solvent in an ultrasonic cleaner unless it is explosion proof - and at 90$, it is not explosion proof. First of all, you do not need a solvent and second, you do not want it to catch fire. For bike parts, a little bit of dishwasher soap (2% in volume at most) will be fine.
But… to be frank, if I didn’t have an easy access to demo units (and often use my cassettes as demo for customers), cleaning the parts by hand will do an adequate job and will probably be quicker.