Deadtime

Delay time, transportation lag, or deadtime is frequently encountered in chemical engineering systems since we did not earn our reputation as underpaid plumbers for nothing  

Suppose a process stream is flowing through a pipe in essentially plug flow and that it takes D minutes for any individual element of fluid to flow from the entrance to the exit of the pipe. Then the pipe represents a deadtime element. 

If a certain dynamic variablesuch as temperature or composition, enters the front end of the pipe, it will emerge from the other end D minutes later with exactly the same shape, as shown in Fig. 9.S. 

Let us see what happens when we Laplace-transform a functionthat has been delayed by a deadtime. Laplace transformation is defined in Eq. (9.1). 

The variable t in the above equation is just a dummy variable of integration. It is integrated out, leaving a function of only s. Thus we can write Eq. (9.51) in a completely equivalent mathematical form  

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Another problem in many NMR spectrometers is that the start of the FID is corrupted due to various instrumental deadtimes that lead to intensity problems in the spectrum. The spectrometer deadtime is made up of a number of sources that can be apportioned to either the probe or the electronics. The loss of the initial part of the FID is manifest in a spectrum as a rolling baseline and the preferential loss of broad components of... 

Advantages. The experiment can be carried out with a conventional fast-spuming MAS probe so that it is straightforward to implement. For recording the satellite transition lineshapes it offers better signal-to-noise and is less susceptible to deadtime effects than static measurements. As the effects differ for each value, a single satellite transition experiment is effectively the same as carrying out multiple field experiments on the central transition. 

The low MW power levels conuuonly employed in TREPR spectroscopy do not require any precautions to avoid detector overload and, therefore, the fiill time development of the transient magnetization is obtained undiminished by any MW detection deadtime. (3) Standard CW EPR equipment can be used for TREPR requiring only moderate efforts to adapt the MW detection part of the spectrometer for the observation of the transient response to a pulsed light excitation with high time resolution. (4) TREPR spectroscopy proved to be a suitable teclmique for observing a variety of spin coherence phenomena, such as transient nutations [16], quantum beats [17] and nuclear modulations [18], that have been usefi.il to interpret EPR data on light-mduced spm-correlated radical pairs. 

FID does not die away before the deadtime has elapsed. In die case of inliomogeneously broadened EPR lines (as typical for free radicals in solids) the dephasing of the magnetizations of the individual spin packets (which all possess slightly different resonance frequencies) will be complete within the detection deadtime and, therefore, the FID signal will usually be undetectable. 

In electron-spin-echo-detected EPR spectroscopy, spectral infomiation may, in principle, be obtained from a Fourier transfomiation of the second half of the echo shape, since it represents the FID of the refocused magnetizations, however, now recorded with much reduced deadtime problems. For the inhomogeneously broadened EPR lines considered here, however, the FID and therefore also the spin echo, show little structure. For this reason, the amplitude of tire echo is used as the main source of infomiation in ESE experiments. Recording the intensity of the two-pulse or tliree-pulse echo amplitude as a function of the external magnetic field defines electron-spm-echo- (ESE-)... 

The curve (a) traces the outline of the peak obtained directly from the number of events recorded (Figure 31.5). The second curve (b) traces the outline of the peak obtained after correcting for coincidental events (dead time, shown by the shaded area). The centroids of peaks a and b are shown, and it can be seen that they occur at the same m/z value. Thus the deadtime correction alters only the abundances and not the m/z values of the ions. 

The other major benefit of RF heating was in reduced presstimes. A typical steam-heated MDF press was operated at about 163°C. Presstimes, not including deadtime, for 19-mm (3/4 in.) board would be about 7 min. With RF, this time could be reduced to about 5 min. It will be noted that these presstimes, even with the use of RF, are longer than those requited for particleboards and this, in addition to the more costiy base fiber and the higher resin requirements, explains much of the manufacturing cost differential between MDF and particleboard. 

Fitting Dynamic Models to E erimental Data In developing empirical transfer functions, it is necessary to identify model parameters from experimental data. There are a number of approaches to process identification that have been pubhshed. The simplest approach involves introducing a step test into the process and recording the response of the process, as illustrated in Fig. 8-21. The i s in the figure represent the recorded data. For purposes of illustration, the process under study will be assumed to be first order with deadtime and have the transfer func tion ... 

On/Off Control An on/off controller is used for manipulated variables having only two states. They commonly control temperatures in homes, electric water-heaters and refrigerators, and pressure and liquid level in pumped storage systems. On7off control is satisfac-toiy where slow cychng is acceptable because it always leads to cycling when the load hes between the two states of the manipulated variable. The cycle will be positioned symmetrically about the set point only if the normal value of the load is equidistant between the two states of the manipulated variable. The period of the symmetrical cycle will be approximately 40, where 0 is the deadtime in the loop. If the load is not centered between the states of the manipulated variable, the period will tend to increase, and the cycle follows a sawtooth pattern. 

Good for systems with large time constant or deadtime Cannot cope with unmeasured disturbances... 

Setting the deadtime between the pass unit and the synchronous rectifier MOSFETs... 

One could perform the turn-on and turn-off delay calculations presented in Section 3.7.2 and still have to adjust the value of the deadtime delay-setting resistor (R6) at the breadboard stage. A starting value of 100 nS is good. The typical MOSFET turn-on delay is about 60 nS. The 100 nS will assure that there is no push-through current. 

The IC produces an asymmetrical deadtime delay. From the graph in the datasheet a value of lOOkohms will produce a pass unit turn-on delay of about llOnS and a turn-off delay of 180 nS. 

For a first order function with deadtime, the proportional gain, integral and derivative time constants of an ideal PID controller. Can handle dead-time easily and rigorously. The Nyquist criterion allows the use of open-loop functions in Nyquist or Bode plots to analyze the closed-loop problem. The stability criteria have no use for simple first and second order systems with no positive open-loop zeros. 

At present dedicated TCSPC FLIM boards are commercially available. They are compatible with most LSMS and are easily synchronized with the scanning microscope and pulsed laser. These boards, often plug-in cards for PCs, have a lower deadtime than do the conventional TCSPC electronics intended for use in spectroscopy and the memory bottle neck of the histogram-ming memory has been removed [21, 22], Consequently, these dedicated boards provide higher acquisition speeds. 

After the detection of a single photon, PMTs need a specific recovery time before they are sensitive again. During this deadtime, no photons can be detected. Because of the stochastic nature... 

Reverse recovery shoot-through current spike through Q1 occurs if during the preceding td (deadtime) interval the freewheeling current prefers to go through the body diode of Q2 instead of the paralleled Schottky... 

We will neglect any delay time (deadtime) in the vapor line from the top of the column to the reflux drum and in the reflux line back to the top tray (in industrial-scale columns this is usually a good assumption, but not in small-scale laboratory columns). Notice that y T is not equal, dynamically, to x. The two... 

A 6-inch ID pipe, 300 feet long, connects two process units. The liquid flows through the pipe in essentially plug-flow conditions, so the pipe acts as a pure deadtime. This deadlime varies with the flow rate through the pipe. From time equals zero, the flow rate is 1000 gpm for 2 minutes. Then it drops to 500 gpm and holds constant for 3 minutes. Then it jumps to 2000 gpm for 2 minutes and finally returns to 1000 gpm. Liquid density is 50 lh, ft. ... 

Write a digital computer program that gives the dynamic changes in the temperature of the liquid leaving the pipe, for this variable deadtime process. 

Jfinr.- The easiest way to handle de time in a digital simulation is to set up an array for the variable to be delayed. At each point in time you use the variable at the bottom of the array as the delayed variable. Then each value is moved down one position in the array and the current undelayed value is stuffed into the top of the array. For fixed step sizes and fixed deodtimes, this is easy to program. For variable step sizes and variable deadtimes, the programming is more complex. 

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