Measurement of Processes

The Ziegler and Nichols closed-loop method requires forcing the loop to cycle uniformly under proportional control. The natural period of the cycle—the proportional controller contributes no phase shift to alter it—is used to set the optimum integral and derivative time constants. The optimum proportional band is set relative to the undamped proportional band P , which produced the uniform oscillation. Table 8-4 lists the tuning rules for a lag-dominant process. A uniform cycle can also be forced using on/off control to cycle the manipulated variable between two limits. The period of the cycle will be close to if the cycle is symmetrical the peak-to-peak amphtude of the controlled variable divided by the difference between the output limits A, is a measure of process gain at that period and is therefore related to for the proportional cycle ... 

The most popular and widely used technique for determining the best controller pairing is the relative gain array (RGA) method (Bristol, On a New Measure of Process Interaction, IEEE Trans. Auto. Control, AC-11, 133, 1966). The RGA method provides two important items of information ... 

Several measures of process capability have evolved and are presented as indices Cp, Cpi, and P i. These are defined in Appendix A. 

If we had a completely accurate model of a process and accurate measurements of process disturbances at their inception, then corrective action could be taken directly without the need to measure the output streams from the process after the disturbance has propagated through it. But because we generally do not have adequate models, the output streams of processes must be measured for the purpose of feedback control. 

For accurate dynamic measurement of process temperature, both X2 and X should be small compared with the time constant of the actual process. 

Table 8.47 shows the available options for the analysis of polymer processing aids, namely combustion and instrumental methods. The best method is dependent on PPA type, the level to be measured, and the available equipment (see also Section 8.2.1.2). Fluoropolymer processing aid concentrations can be determined by WDXRF configured to measure either fluorine or a tracer, and by EDXRF to analyse a tracer [29]. Calibration curves are required. At present, EDXRF or benchtop XRF units cannot directly measure fluorine. For resin or masterbatch producers who prefer to make on-line XRF measurements of processing aid concentrations (to letdown levels of 50-100 ppm), processing aids that contain a tracer (usually BaS04) are available. The analysis time is less than two minutes. 

In Other words, the assurance of quality by measurement of process impurities in the end product has been replaced by assurance of quality by the removal of variance in the process (by continuous monitoring of a continuous process). Naturally, whether online process analysis is being used as a surrogate for an alternative off-line technique to measure specific analytes or as a monitor to reduce process variance it needs calibration and validation. These stages require measurement of process analytes by a reference off-line technique, usually HPLC, and subsequent demonstration that the resulting calibration model has reliable predictive power. 

The torque, screen force, specific energy and power could be regarded as end-point metrics and, therefore, a measure of process conformance and product quality. Additional examples are shown in Table 6. 

These data are utilized for process control, improvement of product quality, the saving of raw material and energy, and an assurance of safety. In this section, we focus on the measurement of process variables as the basis of bioprocess control. 

A process variable is a parameter of the current status of a process under operation, and, therefore, the measurement of process variables is a key to understanding what is happening in a bioreactor and to being able to control the process. Most of the sensors used for the measurements of process variables are inserted into the culture broth of a bioreactor. These must have qualities that will allow them to be repeatedly sterihzed (autoclavable) in order to prevent microbial contamination in bioreactors. Naturally, all the sensors must be calibrated before sterihzation. 

Mention was made above that oscillating die curemeter based instruments are now widely used for obtaining measures of processability. In fact, the other types of curemeter considered in Section 4, oscillating disc,... 

ASTM D5099, 1993. Measurement of processing properties using capillary rheometry. 

Radioisotopes are widely used in die measurement of process variables, including the level of liquids and solids in tanks, silos, and other vessels, the density and specific gravity of fluids and solids, the thickness of sheets and coatings, the moisture content of soils and other solids, the mass flow of materials in pipelines or on belts, and the determination of chemical composition of raw materials, in-process materials, and end-products. Representative examples of these applications are given in Table 6. 

KDC Technology has developed a cost-effective microwave sensor technique for monitoring constituents and moisture in a wide variety of products including foods (KDC 1993). The KDC sensor is adaptable to measurement of process parameters of products contained in tubes, chutes, bins, vessels as well as moving on conveyer lines. 

Time is to be understood, in a qualitative way, as a measure of process or ordered change in the model universe. 

In the present case, it turns out that the isotropies already imposed on the system conspire to provide an automatic resolution of the problem that is consistent with the already assumed interpretation of time as a measure of ordered change in the model universe. To be specific, it turns out that the elapsed time associated with any given particle displacement is proportional, via a scalar field, to the invariant spatial measure attached to that displacement. Thus, physical time is defined directly in terms of the invariant measures of process with the model universe. 

A number of risk indices have been developed over the years as chemical process loss prevention and risk management tools. Many of these are based on the inherent characteristics of the processes, and they can be used as measures of process inherent safety. In general, these indices measure a single aspect of inherent safety, and it is necessary to use several indices to obtain a full understanding of the overall process characteristics. 

Rigorous modeling is the desired ideal. This means the ability to develop a complete process model with no direct measurement of process values. This kind of modeling generally relies on balance equations, kinematic approaches (e. g., for transference of energy by thermal conduction) and material data. 

A key problem in bioreactor control is the difficulty in obtaining reliable sensors and consequently of reliable on-line process information. Demands for product consistency and process productivity produce requirements for more process information.13 Especially in the case of fermentors, rapid, accurate on-line measurement of process variables is often a complex task. As a result, much research effort has focused on methods for quantitatively estimating compositions within reactors and on using model-based control techniques. 

In the process of preparing compounds for screeners, uncertainty can be unintentionally introduced in several areas. For example, a wrong compound may be delivered an empty container may be delivered if a delivery tip is clogged. Some measures of process quality are ... 

If a is some (standard deviation type) estimate of the baseline variation inherent in an industrial process (obtained, e.g., from a calculation such as (5-10) or from data taken from the process after eliminating all physical sources of assignable variation), it essentially specifies what is possible in terms of consistency of process output. There are, however, several common ways of using such an estimate to produce related measures of process capability. 

The exergy efficiency 17 is a universal measure of process efficiency that accounts for the first and second law principles. 

The ease with which plasticiser is combined with PVC is a measure of processing characteristics critical in the dry blend mixing operation of S-PVC or the gelation stage of plastisols. Predictive equations have been developed (349). 

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