Reference curves resistive circuits

Overlooking the reference curves for resistive circuits (Fig. 6.162) it is obvious that relatively high currents are permissible for low voltages. More detailed information is given in Tables A.1 (resistive circuits, Groups IIA/B/C) and A.2 (capacitive circuits, Groups IIA/B/C) of EN 50020 or IEC 60079-11 respectively. (Table 6.39 is an abstract of Tables A.l and A.2 as far as resistive and capacitive circuits are concerned.)... 

Figure 6.232 Detailed representation of IIB and IIC reference curves for low voltages (resistive circuits). The safety factor of X1.5 is incorporated.

The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R") and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to... 

In the differential-temperature loop, signals representing the sample and reference temperatures, as measured by the platinum-resistance thermometers, are fed to the differential-temperature amplifier via a comparator circuit, which determines whether the reference or the sample temperature is greater. The differential-temperature-amplifier output then adjusts the differential-power increment put into the reference and sample heaters in the direction and magnitude necessary to correct any temperature difference between them. A signal proportional to the differential power is also transmitted to the pen of a recorder, giving a curve of differential power versus time (temperature). The area under a peak, then, is directly proportional to the heat energy absorbed or liberated in the transition,... 

Rates of corrosion can also be measured using an electrochemical technique known as potentiodynamic polarization. The potential of the test metal electrode relative to a reference electrode (commonly the saturated calomel electrode SCE) is varied at a controlled rate using a potentiostat. The resultant current density which flows in the cell via an auxiliary electrode, typically platinum, is recorded as a function of potential. The schematic curve in fig. 2 is typical of data obtained from such a test. These data can provide various parameters in addition to corrosion rate, all of which are suitable for describing corrosion resistance. The corrosion potential F corr is nominally the open circuit or rest potential of the metal in solution. At this potential, the anodic and cathodic processes occurring on the surface are in equilibrium. When the sample is polarized to potentials more positive than Scon the anodic processes, such as metal dissolution, dominate (Anodic Polarization Curve). With polarization to potentials more negative than Scorr the cathodic processes involved in the corrosion reaction such as oxygen reduction and hydrogen evolution dominate (Cathodic Polarization Curve). These separate halves of the total polarization curve may provide information about the rates of anodic and cathodic processes. The current density at any particular potential is a measure of the... 

This is a well established technique applied to determine the corrosion rate of steel in concrete. A full theoretical description of this technique is given in Chapter 3. The specimen is polarized to within 25 mV of the corrosion potential (Ecorr) as the dependence of current with potential in the vicinity of corrosion potential is linear. The open circuit potential (E oii) of the reinforcement is measured against a reference electrode. Starting from this potential, the steel reinforcement is polarized to 25 mV from Ecorr at a sweep rate of 5-lOmV/min and a plot of A vs At is constructed. The polarization resistance relates the slope of the polarization curve in the vicinity of corrosion potential to the corrosion current. This technique is based on the Stern Geary equation described in Chapter 3, and given below ... 

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