Times time/current curves

Figure 9.6 Fuse and cable time versus current curves.

Since 1976 many relays have generally followed the recommendations of the IEC255-4, Clause 3.5.2, regarding the shape of their time-current curves. The general function recommended has the form -... 

The principal difficulty in calculating overlapping peak areas is that the time-voltage or time-current curve produced by the detector cannot be described by a simple mathematic model and — worse — that the curve is not reproducible from run to run. It varies with the amount of the substance the skill of the technician in injecting, the temperature... 

FIGURE 5.153 Comparative time-current curves of a typical induction overcurrent relay plus instantaneous trip. 

To ascertain Zs, the designer needs to know Ze, which is the earth loop impedance at the supply intake. The earth loop impedance between the intake and the relevant item of equipment can then be added. Ip calculated, and the suitability of the protective device ascertained from its time/current curves. 

Minimizing Electrolysis Time The current-time curve for controlled-potential coulometry in Figure 11.20 shows that the current decreases continuously throughout electrolysis. An exhaustive electrolysis, therefore, may require a long time. Since time is an important consideration in choosing and designing analytical methods, the factors that determine the analysis time need to be considered. 

Thiese can be drawn tor temperature versus time or current versus time as desired, depending upon the calibration of the device, such as a motor protection relay. Below we provide a brief procedure to draw these curves. 

To make a proper selection of HRC fuses it is essential that the current-time characteristic curves for Ihe releases of the breaker and the fuses are available from their manufacturers. 

Figure 5.22. Thermal desorption spectra after electrochemical O2 supply on Pt deposited on YSZ4,s at 673 K. The different curves correspond to various times of current application I=+I5 pA. Desorption was performed with linear heating rate, p=l K/s 2FNg/I=2570 s.4,7 Reprinted from ref. 7 with permission from Academic Press.

The depression in the drop time-potential curves measured under the condition of constant Ucs+ was more conspicuous with an increase of acsTPhs demonstrated in Figs. 4(a)-(d), whereas the depression was indifferent to aQs+ as seen in Figs. 4(c), (e), and (f). These results suggest that the adsorbed species responsible for the maximum current is the ion pair, Cs" TPhB. ... 

It is clear, that the various modes of LSV at the dme require an integrated and coherent regulation of sweep time, current sampling and drop knocking, preferably by an electronic device and on with computer guidance. A disadvantage of LSV. at the dme, in contrast to normal DC polarography, is that for mixtures of components the latter yields a simple evaluation by curve extrapolation on the basis of additivity [see Fig. 3.34(a)], whereas the former suffers from an uncertain evaluation [see Fig. 3.34(b)],... 

The case of the prescribed material flux at the phase boundary, described in Section 2.5.1, corresponds to the constant current density at the electrode. The concentration of the oxidized form is given directly by Eq. (2.5.11), where K = —j/nF. The concentration of the reduced form at the electrode surface can be calculated from Eq. (5.4.6). The expressions for the concentration are then substituted into Eq. (5.2.24) or (5.4.5), yielding the equation for the dependence of the electrode potential on time (a chronopotentiometric curve). For a reversible electrode process, it follows from the definition of the transition time r (Eq. 2.5.13) for identical diffusion coefficients of the oxidized and reduced forms that... 

Various types of controlled-potential pulsing are shown in Fig. 5.18. The simplest case is the single-pulse potentiostatic method (Fig. 5.18A). The current-time (I-t) curves obtained by this method have already been described in Section 5.4.1, Eq. (5.4.10) and (5.4.14). 

Investigations on mass-transfer rates along planar electrodes (F2, H3) in which the rate of increase of current, or of cell voltage, was varied systematically from one measurement to the other revealed that the time taken for attaining the limiting current influenced the limiting-current curve. This unsteady-state effect was noticeable both in the quality of definition of the... 

The effect is most prominent in free convection. Limiting-current curves recorded by Hickman (H3) at a horizontal cathode facing upward in free convection are shown in Fig. 6. The apparent limiting-current value is definitely dependent on the time necessary to reach the limiting current an 80% increase in this value is noted as the rate at which the current rises varies from 0.25 to 20 mA cm-2 min -. ... 

Deposition of mercury at boron-doped diamond (BDD) and platinum electrodes has also been studied [33]. Deposition and oxidation of mercury was performed by cyclic voltammetry from the solution of 1 mM Hg2 ( 104)2 in 1 M Na l04. In order to learn more about this deposition, it was carried out also under chronoamperometric conditions. The results obtained are shown in Fig. 2 in the form of dimensionless current-time transients. Experimental curves obtained at two different overpotentials were compared with the theoretical curves calculated for instantaneous and progressive nucleation. A good agreement of experimental plots with the instantaneous nucleation mechanism was... 

There is a technique (called Tast) that consists of measuring the current only for a very short period of time, late in the lifetime of the mercury droplet. This method eliminates the saw-toothed spikes from the current curve. However, it leads to a lower sensitivity because the diffusion current decreases as the analyte flux goes from the bulk of the solution to the surface of the droplet (Fig. 19.3). 

Kinetic studies of ECE processes (sometimes called a DISP mechanism when the second electron transfer occurs in bulk solution) [3] are often best performed using a constant-potential technique such as chronoamperometry. The advantages of this method include (1) relative freedom from double-layer and uncompensated iR effects, and (2) a new value of the rate constant each time the current is sampled. However, unlike certain large-amplitude relaxation techniques, an accurately known, diffusion-controlled value of it1/2/CA is required of each solution before a determination of the rate constant can be made. In the present case, diffusion-controlled values of it1/2/CA corresponding to n = 2 and n = 4 are obtained in strongly acidic media (i.e., when kt can be made small) and in solutions of intermediate pH (i.e., when kt can be made large), respectively. The experimental rate constant is then determined from a dimensionless working curve for the proposed reaction scheme in which the apparent value of n (napp) is plotted as a function of kt. 

Some additional experiments relevant to the data in Figure k are suggested by the preceding discussion. In particular, if the corrosion sites also act as the recombination centers that control current onset in the absence of sulfide ions (as discussed earlier) then there are no oxidized recombination centers before exposure to light. In that case a CdS electrode biased at a voltage below the saturated portion of curve 1 in Figure k would show a higher initial current than indicated in curve 1 and then decay in time to the curve 1 value. This situation can be analyzed by ... 

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