I'd have to figure out where I read a review, but ... yes, somebody
said that the heat *was* an issue for RGP's, and could warp them.

IIRC, though, there are ultrasonic cleaners that have a "regular" and
a "strong" setting, with heat being the difference.  ISTR reading that
the 'regular' setting was ok for lenses.

Might be worth a little Googling....

Agreed :-)

Ultrasonic cleaners can be pretty simple. The prices have always been
inflated by ingenuity and demand, rather than just a function of materials
and labor. As such, over the past 30 years the cost has come down
dramatically.

All that's really necessary is a circuit that generates alternating current
around 20 kHz or more, and a piezoelectric transducer made of lead zirconate
between two layers of tin. The transducer is usually bonded to a resonating
metal container. Better units use multiple transducers, welded instead of
glued, and circuits that generate frequency sweeps to avoid standing waves.
For $30 you probably won't get those features, nor will you get the more
durable "magnetostrictive" transducers. Piezo crystals wear out after a
while.

The cleaning action occurs because the vibration creates tiny cavitation
bubbles. When the bubbles collapse, they generate a tiny "squirt" which
travels about 400 km/hr over a microscopic distance.

Of course the transducers generate heat, but you'd have to place the lens
directly on the transducer without a surrounding bath, and that wouldn't
clean anything.

They work ok, but not that much better than the "gadget" attached to your
hand. Your fingers don't cost any extra and you usually carry them with you.

-MT