Instantaneous dead time

Also of consideration is the instantaneous dead time. This is an effect that results when the area analyzed via a rastered primary ion beam is gated, whether electronically or optically (gating and its effects are discussed further in Section 5.3.4.2). This will induce a more severe effect than described by Relation 4.12 as the signal... 

Rastered mode describes scanning of a focnsed primary ion beam over some predefined area over the sample. In this mode, the beam spot size must be substantially smaller than the analyzed area and instantaneous dead-time effects associated with primary beam rastering must be considered (see Section 4.2.3.3.3.2). 

For noninteracting control loops with zero dead time, the integral setting (minutes per repeat) is about 50% and the derivative, about 18% of the period of oscillation (P). As dead time rises, these percentages drop. If the dead time reaches 50% of the time constant, I = 40%, D = 16%, and if dead time equals the time constant, I = 33% and D = 13%. When tuning the feedforward control loops, one has to separately consider the steady-state portion of the heat transfer process (flow times temperature difference) and its dynamic compensation. The dynamic compensation of the steady-state model by a lead/lag element is necessary, because the response is not instantaneous but affected by both the dead time and the time constant of the process. 

The advantage of such a control system is that the feeding of fuel can be adapted to the time curve of the thermal output almost instantaneously. This ensures stable controlling which is necessary for a continuous operation without on/off-operation. On the other hand, a fuel control reacting exclusively to the boiler temperature or the flow temperature, only allows a very slow adjustment of the fuel quantity to the load curve, as a change in load becomes effective only after a certain time. Due to this dead time, there is in general no stable control without disturbance value feed-forward... 

For all the systems we examined in Chapters 10 and 11 and Section 12.1, we have assumed that there is no dead time between an input and the output that is, whenever a change took place in the input variable, its effect was instantaneously observed in the behavior of the output variable. This is not true and contrary to our physical experience. Virtually all physical processes will involve some time delay between the input and the output. 

For a measurement of continuous beam current (i.e. for rates of ion arrival that are too large for ion counting detection as a result of detector dead time, see below), it is appropriate to replace the ion counting time window with a more suitable parameter the appropriate parameter is the time constant (t ) of the detection chain (usually dominated by the RC time constant of the current-to-voltage converter, see Section 7.4). If the 4 value is instantaneously increased to a new value, the time... 

The term loss-free counting refers to systems where there are, in effect, no dead time losses. All the various loss-free counting systems achieve this apparently utopian state by determining the instantaneous count rate through the ADC and, as each pulse is measured, adding additional counts into the spectrum (instead of a single count as in Figure 4.29) to account for dead time losses. 

Beasley s derivation contains, however, as well as some probably trivial mathematical approximations, certain implicit assumptions that make this conclusion doubtful. He did not present a detailed kinetic scheme for the polymerization reaction, but it is implied by his Eq. (2) that the formation of branches is instantaneous after a new radical site has been produced on the dead polymer. That is, no account is taken of the fact that some growing radicals will be removed from the reactor before the growth of the new branch is completed. The mean time for growth of a branch is 1 /akp(M) and the ratio t of the mean residence time V/q to this time, Le. ... 

Under these assumptions, the expressions for rate of polymerization (J p), kinetic chain length (A, the average number of monomer units on a living chain) and instantaneous number average degree of polymerization (DP)," , the average number of monomer units on a dead polymer chain formed at an instant in time) are written as... 

Protection Against Overload Current. In order to protect against overload or excess current, the overloaded circuit, or part of the circuit, must be disconnected quickly. The most simple and common form of overload protection is the fuse. The fuse will offer close protection as it should melt within 4 hours if current of 1 5 times its rating flows (e.g. fuses to BS 88). The fuse should be located in the live side of the circuit only, and not in the earthed neutral side, because if the neutral fuse melts first, the apparatus will cease to operate and will appear dead , while in fact still being dangerously live. Circuit-breakers are also used for overload protection as an alternative to fuses. A circuit-breaker is an automatic switching device, which operates instantaneously when a significant overload current (typically 1.5 times rated current) is detected. Circuit-breakers are easily reset after a trip, but are more expensive than fuses. 

The decreasing Pn of the instantaneously formed product occurs at different points in time resulting in differing dead distributions. The sum of these dead distributions (Flory distributions) in the complete product is a very broad distribution, and is no longer a Flory distribution. 

The assumption of instantaneous momentum exchange brings forward an important limitation of the DSMC approach the real duration of a particle-particle collision must be small relative to the mean free time between two consecutive collisions. Therefore, the method should not be applied to very dense systems in which prolonged particle—particle contacts take place such as in hopper flows or dead zones in fluidized systems. On the other hand, the method is extremely efficient for dilute and semi-dilute systems such as pneumatically conveyed powders, riser flows, and liquid sprays. 

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