Dead time measurement

Fig. 5.2. Analysis of (a) dead time, measuring time and time constant and (b) analysis interval.

In fact, even the dead time measured by injection of water is not very reproducible, especially when the data are taken at the maximum of the first perturbation of the baseline. The measurement is better when performed by injection of micellar solutions of the analytes [25]. The experimental work can provide a great number of replicates which can be used for this purpose. 

This works well at low to moderate count rates but is limited at high count rate. Dead time is discussed further in Chapter 14 in this connection. For now, it is sufficient to say that, in general, high dead times are to be avoided. Each laboratory has its own arbitrary limit. My own was about 30 %, but many laboratories have much lower limits. At very high dead times, measurement of hve time may be inaccurate because of differences in shape between the detector pulses and the live time clock pulses, which are not blocked in the same way by the input gate. Inaccurate live time measurement does not prevent nuclide identification but does affect the quality of quantitative nuchde measurements. 

Distance-Velocity Lag (Dead-Time Element) The dead-time element, commonly called a distance-velocity lag, is often encountered in process systems. For example, if a temperature-measuring element is located downstream from a heat exchanger, a time delay occurs before the heated fluid leaving the exchanger arrives at the temperature measurement point. If some element of a system produces a dead-time of 0 time units, then an input to that unit,/(t), will be reproduced at the output a.s f t — 0). The transfer function for a pure dead-time element is shown in Fig. 8-17, and the transient response of the element is shown in Fig. 8-18. 

Time-Delay Compensation Time delays are a common occurrence in the process industries because of the presence of recycle loops, fluid-flow distance lags, and dead time in composition measurements resulting from use of chromatographic analysis. The presence of a time delay in a process severely hmits the performance of a conventional PID control system, reducing the stability margin of the closed-loop control system. Consequently, the controller gain must be reduced below that which could be used for a process without delay. Thus, the response of the closed-loop system will be sluggish compared to that of the system with no time delay. 

Dead time. Probably the best example of a measurement device that exhibits pure dead time is the chromatograph, because the analysis is not available for some time after a sample is injected. Additional dead time results from the transportation lag within the sample... 

The dead time is typically 3-5 ms. so stopped flow is not quite as fast as continuous flow, but it requires less than a milliliter of each solution per run. Methods have been described for measuring the dead time " " these are based upon standard reactions whose kinetic behavior is well known. The error introduced by collecting data before mixing is complete can be corrected." ... 

Integral intensities were obtained after dead-time corrections, background subtraction and normalization to averaged monitor counts. The Lp correction was applied in the usual way. Since the polarization ratio was not measured at BW5 so far, 90% linear horizontally polarized radiation was assumed for all scans. Calculations show that even a change in the beam polarization of 10% would effect the intensities of the highest order reflections of less than 1.5%. 

To determine the expressions for the optimised counting times, we write the expressions (10) and (11) in terms of count-rates and times (count rates are constant quantities for each Bragg reflection). We assume that the incident neutron flux is constant during a flipping ratio measurement, and that no dead-time correction is needed. In these conditions, we have the relations ... 

We now derive the time-domain solutions of first and second order differential equations. It is not that we want to do the inverse transform, but comparing the time-domain solution with its Laplace transform helps our learning process. What we hope to establish is a better feel between pole positions and dynamic characteristics. We also want to see how different parameters affect the time-domain solution. The results are useful in control analysis and in measuring model parameters. At the end of the chapter, dead time, reduced order model, and the effect of zeros will be discussed. 

Empirical tuning with open-loop step test Measure open-loop step response, the so-called process reaction curve. Fit data to first order with dead-time function. 

Roudaut et al. (1999a) used low-frequency pulsed-proton NMR and dielectric dynamic mechanical spectroscopies to study molecular mobility in glassy bread (<9%) as a function of temperature. Based on NMR results, they reported that some (if not all) of the water molecules were much more mobile than the polymer matrix whose relaxation time could not be measured within the 20-p,s dead time of the RF probe. 

Assuming we know the two rate constants ki and fc that allow the computation of C. Assuming further, we only have measured spectra between time = 200 and 1200 (fast reaction with significant dead time of the instrument). The task is to determine the three absorption spectra of the pure compounds A, B and C. All three are not accessible directly in the range of available spectra because of severe overlap. 

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