with the drum is provided with the valve segment 27 containing an atmospheric port 28 the segment 27 controls the vacuum cut off and pick up points circumferentially around the drum fig4 when the vacuum is 34 on 34 all the filtrate is drawn through the drum into the valve chamber 21 and the drop leg 17 in some cases where several drum washers are connected together and the filtrate is cycled through all of them the filtrate passing through the valve chamber would include filtrate from all the drum washers filtrate from the tubes 15 corresponding to the sections of the drum 12 still in contact with the pulp mat collects inside segment 27 and drains through the tubes 15 having a downward slope to sample the filtrate in the system requires either taking a sample of the filtrate from the drop leg 17 which as pointed out above is unsatisfactory for sampling and feedback or sampling manually the mat being produced which is likewise unsatisfactory and not applicable to feedback control the sampling system of the present invention comprises conduit 30 connected to the atmospheric port 28 of the valve segment 27 the conduit 30 passes through the valve chamber 21.