A History of Ultrasonic Record Cleaning

Too low a frequency for too long; if the record doesn't rotate, the cavitation effect focused in one spot could cause damage. I think the literature on ultrasonic cleaning for industrial purposes (non-LP) could be a starting place to look at the risks, which might also include water temperature. I haven't really thought too much about how to damage but it's the flip side of not damaging; lewm raises a fair point about potential damage; we've all engaged in a leap of faith based on manufacturer statements and user anecdotal experience. That's not going to satisfy someone who wants "proof." If I had time, I could ask or explore more; there is a point where experience in cleaning metal parts doesn't translate, but I suppose if it damages metal it is bound to damage vinyl. 

Although,I wasn't trying, but left 2 different but brand new lps in my 40hz cleaner @ 45C for 45 minutes with my spinner off. When I realized it after those complete cycles, I put them through the exact same cycle with the spinner on, then listened. I heard nothing that concerned me.

I see. I think, I wouldn't clean more than once valuable to me records, just in case. It appears that there is no guarantee that there can be no damage even if used correctly.

Interesting read here. I believe a lot has been "discovered" or at least refined on ultrasonic cleaners science in last few years which will help settle your mind on ultrasonics, to a point. I use both a Kirmuss and an Audio Desk. Kirmuss posts a LOT of useful information online and is a must read before you purchase an ultrasonic whether his or others. I am happy with both but believe the Kirmuss does a better job without a doubt, though it typically takes a lot more time and elbow grease - tho worth it to me especially on favorite records. Audio Desk is my go to for mass cleanings or first clean just to hear if a record is an outstanding pressing worth putting the extra time in with the Kirmuss to bring it to the best possible level of sonics. I believe both use ~ 35Hz to avoid damaging records, the science as I know it being is the higher the Hz, the faster and more powerful the microscopic "explosions" of the ultrasonic bubbles on the surface of the records are, thus, in theory if powerful enough could cause damage, and 35Hz is "safe." I’ve run records multiple cycles of up to 30 minutes on both with no problems at all - and I have a VERY revealing system so would likely hear issues if there were any. It’s apparently all in the cycle rate (35hz)and the surfacant that ATTRACTS water to the surface and most importantly INTO the grooves of the record were the hard to get out and sound altering dirt etc lies. Water alone will not do as good a job at removing the dirt, dust, release residue etc, that is in the grooves.