A system includes a cleaning device and a concentration measuring device. The cleaning device includes a first pipe and a first pump. The first pump is configured to move first liquid in the first pipe. Two terminals of the first pump are respectively coupled to the first pipe. The concentration measuring device includes a tube, a cooler, a concentration meter and a second pump. The tube is coupled to the first pipe, and is configured to retrieve the first liquid. The cooler covers the tube to cool the first liquid. The concentration meter is configured to measure a concentration of the first liquid cooled by the cooler. The second pump is coupled to the tube, and is configured to move the first liquid according to the concentration.