I use an ultrasonic cleaner daily in my work. I understand the fire danger is from sonicating for a long time (greater than 10-20 minutes) and filling the bath only an inch or two, instead of full. It takes a long time for an ultrasonic bath to get hot. That's why many of them are sold with timers.

But then again, in my practise, I rarely fill the bath with solvent. Instead, it is filled with water, and a beaker filled with solvent is placed in the bath for the part.

For jewelry, I use water and detergent.

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95 E320 Cabriolet, 169K