Abstract: The apparatus comprises a constant level flow receptacle and an immersion type freeness tester having two chambers, namely a filtration chamber provided with a bottom screen portion, and a measurement chamber. A sensing element is placed inside the measurement chamber for determining the level of the liquid passed through the bottom screen portion, and connected to a circuit for measuring this level and for recording the freeness according to this level. The dimensions of the filtration chamber and those of the measurement chamber are so selected that, during the immersion of the freeness tester into the pulp, water is supplied into the filtration chamber at a constant pressure differential between the level of the upper edge of the filtration chamber and the level of the edge of the flow receptacle.