The ionograph is a system to measure the level of contamination of the surfaces of printed circuit boards. The printed circuit board or the populated assembly is completely immersed in a vessel containing a rinsing fluid, which is being continuously circulated. Integrated into this circulation are both a measuring cell to measure the conductivity and an ion exchanger. The conductivity of the rinsing fluid is determined by the ions which are being removed (dissolved?) from the surface of the board or assembly. After passing through the measuring cell, the ions are neutralized in the ion exchanger. The level of conductivity is arrived at over time, and the measured value is expressed in ion concentration pee unit area. The systems are calibrated with NaCl, and the measured value corresponds then to the equivalent of X mg NaCl per unit area.