More user friendly than the Secchi disk, the portable instrument APW - VB Ponsel, gives you a direct reading of the sludge percentage.
The sensor uses the principle of optical absorption of the infrared light. A measurement of 100 % correspond to a ultra-clear water (without any suspended solid) and a measurement of 0% to a fully opaque sludge (concentrated sludge). So it becomes very easy to locate the interface of sludge blanket.

Range : APW-VB20 : for sludge concentration 0-3 g/L ; APW-VB50 : for sludge concentration 0-50 g/l.