Tham khảo tài liệu 'process control systems episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Analysis of Some Common Loops 83 FIG . Agitation reduces the effective dead time while increasing the time constant. Time sort of response could be represented by dead time plus a single capacity. Thus the simple model just postulated is quite valid if imperfect. The Analyzer Dynamics associated with the analysis play an important role in the performance of the loop. The foremost limitation in the speed of analysis is generally of the sample to the detector. Fortunately some composition measurements can be made without withdrawing a electrolytic density and are notable examples. But any analysis requiring the withdrawal of a sample particularly if that sample must undergo a certain amount of preparation results in a significant of dead time see the example cited at the close of Chap. 2 . Naturally any effort spent in minimizing the sampling time will be rewarded by tighter control and faster response. Some analyzers are They produce only one analysis in a given time interval. This characteristic is worthy of much more attention because it periodically interrupts control loop. Process are the principal but not sole constituents of this group. The response of this kind of control loop will be given extensive in Chap. and methods for coping with it will be presented. A few analyzers exhibit a time lag in addition to the dead associated with sample Sormally this property is of little conse- quence except when the process itself consists of nothing but the volume of a pipeline whose time may be less than that of the analyzer. Those measurements which are fast are by the same subject to noise. Conductivity and are usually in this category because they are fast enough to react to an incompletely mixed solution or particles of an immiscible phase. Dead time in sample lines is understandably constant. Dead in a pipe carrying the main varies with flow. Dead within a stirred tank is slightly affected by flow to the extent of F F in most systems variation would not be .
