Cheers.

So, if safe medical use (which admittedly does cause a reaction in tissues) allows for as much a 3W/cm2 and we're seeing around .05W/cm3... well, I'd say brief exposures are absolutely harmless. Now, I wouldn't leave my hand in there all day - it is likely that after long enough exposure it could begin to break down live skin and tissues - but I'd have no issues with up to 30 minutes or so... after that all you'd have is very clean hands (not a dead skin cell left on 'em I'd bet) but be none the worse for wear.... well, ok - you might have wrinkly fingers from the water. ;)

Of course all bets are off if you use anyhting harsher than water. Stick your hand in an ultrasonic acetone bath and leave it there for 30 minutes and you'd most assuredly regret the decision.

I hope Ozzie and Sonicare are indeed "overcautious" about the risk of joint injury, though I admit I'll be a bit more hesitant to engage in my own bouts of unconventional infant cleansing.

Actually, I think you just came up with an excellent name and tagline for your blog.

What I meant to say was:

Yes! I flense the flesh from babies' bones! Hahahaha! Death to America!

Anyhoo, if you are a walking skeleton then no, it's probably not a good idea to put your bare bones into even a low powered cleaner, though I presume you're animated by magic and so will probably be OK even then. Although I suppose it'd be just your luck if you'd been reanimated by an absent-minded necromancer who forgot to cure your arthritis.

Normal humans have soft flesh over their bones - a bit of it, even on the hands - and that flesh soaks up ultrasonic energy very quickly. That's why you can attenuate high pitched sounds so effectively by putting your fingers in your ears. I see no reason to believe that enough energy to do damage will make it as far as the bones of even a very small hand, when there's only a few tens of watts being put into the litre or so of liquid in the first place.

Ultrasonic cleaners are also like microwave ovens, in that they more or less distribute their energy through everything in the cleaner, so the more water and/or objects you put in the cleaner, the smaller will be the share of the cavitation energy that each object/unit volume of water receives. We probably could have stung Thomas pretty good if we stuck just his hand in there with a shallow puddle of water. But with a litre or so of water and Mum's hand holding his, he clearly didn't feel anything distressing.

I am, of course, (arguably) a grown-up, but I've had my hands in cheap ultrasonic cleaners hundreds of times, and have never suffered any harm. Heck, these "50 watt" cleaners haven't even had enough power to give me a rash when I've been using water/detergent/alcohol mixtures to clean something really filthy and stuck my hands in then, to turn over the thing being cleaned, or whatever.

You'd think there'd at least be a bit of cavitation around the base of your fingernails that'd make them sting, but I've never noticed even that, even with the stronger solvent mixtures.

And yes, I stood there with my hand in my old cleaner (which was only filled with water...) for a few minutes once, just to see what would happen. The tingly sensation from my current big cleaner isn't any stronger than the sensation I got from my old small one. Actually, I think the new cleaner's less powerful for a given fill level, but the 30 minute timer more than makes up for that as long as I'm not in a hurry.

So I think your concerns, while laudable, are unfounded.

Try the foil test again with the foil wrapped in ham, and see how many holes develop then :-).

(I'd also like to see what a 650 watt cleaner filled only to the "MIN" line can do to, say, a chicken drumstick.)

I just bought one of the little cheapies - 50 watt output and quite effective.

Now - take a look at http://www.soniclean.com.au/FAQ.html#hands #6 question in the FAQ by Soniclean.

Next take a look at http://www.soniclean.com.au/ulc/foiltest.html
I tried this test in my 50 watt cheapie and I was pleasantly impressed to see the dimpling as shown on the site - probably not as pronounced but I was also impressed to see a number of holes in the foil.

Simply put - NO WAY would I put a child's hand into this kind of cavitating sound energy field and I would not publicise the action as you have due to the apparent risk of joint tissue damage in the short and long term - please do remember that the bones are still growing and there is a disk of high growth activity in EACH bone in a child's hand - not something that should be subject to disruptive ultrasonic sound levels.

Ozzie (A long term fan of your column!!)