valves 38 40 and 42 respectively and filter means 44 more particularly the filter means includes an inlet 45 and a pair of filters 46 and 48 connected in parallel to define a output 50 that is connected with waste via flow meter 52 and back pressure adjusting valve 54 a central cross connection between the filters includes another output 56 connected with the fluid input of the uv analyzer which analyzer has an output connected with waste via waste sample valve 58 electrical control signals that correspond with the measured uv absorbance characteristic of the lignin dissolved in the cooking liquor are supplied to the digester temperature and cooking length time controls 18a and 18b via conductors 20a and 20b respectively and to the disk spacing controls of primary refiners 8 and 10 via conductors 22a and 22b respectively electrical power is supplied to the relay control means and to the uv analyzer from power supply 60 in accordance with an important feature of the invention a uniform flow of the cooking liquor through the filter means and through the uv analyzer is achieved by the regulation of the back pressure adjusting valve 54 as a function of the outlet pressure of the uv analyzer more particularly pressure air from the air source 64 is supplied to the control chamber of the diaphragm operated back pressure adjusting valve 54 via manually operable valve 66 pneumatic pressure transducer 68 and pressure transmitter 70 the.