@ EDCba:
Would a bath within a bath still produce the ‘ultrasonic’ cleaning?
My experience with transducers were mounted directly to the canister thus vibrating at 20 to 45 kHz. The problem lays in the vapors that may ignite with a sparking electrical connection. There is also the flashpoint of the solvent vapor if this does heat up. A good ultrasonic device does not heat up the bath. The Piezo does produce heat but not enough to actually warm up the liquid after some 10 to 30 minutes (my use).