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The examination of Fig. 65 shows that the signal relating to detector 1 ‘falls’ very quickly at theend of a day approximately while that relating to detector D 2 is very correctly adjusted by exponentialdecreasing of time-constant equal to 6.8 days.These observations make it possible to estimate that the tracer probably does not have time tomix radially in the top of the digester but which it has rather tendency ‘to fall’ towards the bottom. Anexplanation could lie in the fact that the tracer was ‘taken’ in a particular mud cluster. At half heighthowever, the tracer can be considered as good mixed in the vicinity of the volume of detection ofdetector D 2 .The experimental RTD curve obtained near the bottom of the digester by the detector D 3 and itsmodel are shown in Fig. 66. It was not normalized because it was not closed to zero (it is expressed incounts per seconds, cps). The mean residence time of this curve was found of 32.6 days. In the sameFig. 66 is plotted for comparison the RTD obtained by detector D 2 . The green curve shows thedifference between two curves, that means represents the maximal quantity of mud that falls down anddeposits somewhere at the bottom of digester. It represents 80% of mud flow rate circulating at thedigester bottom.The experimental RTD curve obtained from detector D5 (mud exit) is given in Fig. 67. Itindicates that the digester behaviors like an ‘axial dispersion plug flow’ than a perfect mixer.FIG.66. Experimental response for detector D 3 (digester bottom) an dits model.FIG.67. Experimental response of D5 (mud exit) and feed rate fluctuations.64