AFTER MATERIAL LEAVES APPARATUS, SUCH AS A WASING MAAFTER MATERIAL LEAVES APPARATUS, SUCH AS A WASHING MACHINE OR MERCERISER, IN WHICH IT IS WET PROCESSED, THE LIQUID RETAINED BY THE MATERIAL IS REMOVED IN TWO STAGES, E.G., BY TWO SEPARATE PAIRS OF NIP ROLLERS. MOST OF THE LIQUID IS REMOVED IN THE FIRST STAGE. LIQUID REMOVED IN SECOND STAGE IS COLLECTED, AND ITS NATURE, E.G., ITS PH, EXAMINED TO GIVE AN INDICATION OF THE CHARACTERISTICS OF THE MATERIAL. SIGNAL DEPENDENT UPON INDICATION MAY BE USED TO ADJUST ACTION OF THE APPARATUS.