Eyewear Care & Ultrasonic Cleaning
Several Pairs of Glasses in the Same Tank: What You Need to Know About Changing the Liquid
If the same water-and-concentrate fill is used for multiple pairs of glasses in a row, grease, skin particles and dissolved dirt increasingly build up in the liquid. There's no exact 'change after X pairs of glasses' rule – visible and detectable signs matter more.
What technically happens if the liquid isn't changed
Every cleaning cycle lifts grease, skin particles, dust and sweat salts off the glasses – these remain in the liquid until it's changed. Cavitation itself keeps working even in slightly cloudy liquid, but the cleaning effect gradually weakens as the liquid itself increasingly contains dirt particles that can redeposit.
How to tell when a change is needed
There's no scientifically defined number for 'change after X pairs of glasses' – the level of soiling varies too much from one pair to the next. Practical signs to look for instead of a fixed number:
- Visible cloudiness: the liquid is no longer clear, but milky or greasy-streaked.
- Visible particles: small flecks of dirt or grease droplets floating on the surface.
- Smell: an unpleasant, soapy or greasy odour is a clear signal.
A fixed rhythm for professional use
In professional settings (e.g. at a salon or practice reception desk), it's also worth establishing a fixed rhythm – for example, changing the liquid at the start of business and again whenever visible soiling appears during the day.
Is this hygienically fine when used on multiple people?
The Furtwangen studies (see How many bacteria live on a pair of glasses?) didn't specifically examine reusing the same cleaning liquid across multiple people – we're not aware of specific research on this point. The contact-free cavitation effect itself demonstrably reduces germs; a regular, visibility-based liquid change (as above) is nonetheless the sensible precaution for professional multi-person use.
Author: Moritz Weinert, Gründer NURO Labs · Last updated on 8 September 2026