A CIP cleaning process takes place in several successive steps. First, the product remaining in the plant is ejected. In the second step, organic trace elements are eliminated by means of lye and mineral deposits are removed by the use of acid. Finally, the system is rinsed with fresh water and prepared for the next production step. The liquid returning in the CIP return is monitored by a conductivity meter (e.g., SLI03). The clarification of the cleaning liquid is carried out on the basis of the measured data. But in order to detect the turbidity (impurities by residual particles) in the returning cleaning liquid, turbidity measuring devices are increasingly used. As a result, contamination can be detected early and eliminated. Especially for dairy products, the detection is very fast and efficien