Time is money, and that's also the case with industrial cleaning. Surfaces need to be cleaned until they have reached the necessary degree of cleanliness–and not longer. But how can the cleanliness of a surface be measured reliably? We cleaned steel plates of varying lengths in an ultrasonic bath and determined the respective surface free energies (SFE). What can't be seen by the naked eye was able to be made visible in this way: After 120 seconds, the optimum degree of cleaning was reached; any longer waiting time had no additional influence on the SFE of the steel plate samples. Once optimized, the success of the cleaning process could be checked in an instant with one-click measurements with either an “OK” or “not OK” result.