Basic measuring circuit

Basic electronic circuits employed in the sensor area will be discussed with a particular emphasis on the noise aspects, which are important for achieving high resolution values in those contexts where measurement of the lowest concentration values of chemicals is the main objective. 

In order to understand electrochemical impedance spectroscopy (EIS), we first need to learn and understand the principles of electronics. In this chapter, we will introduce the basic electric circuit theories, including the behaviours of circuit elements in direct current (DC) and alternating current (AC) circuits, complex algebra, electrical impedance, as well as network analysis. These electric circuit theories lay a solid foundation for understanding and practising EIS measurements and data analysis. 

A state-of-the-art PEMFC and steady-state current-potential measurements are illustrated in Figure 3.18, which shows a schematic view of the PEMFC geometry, the basic electric circuit of the membrane electrode assembly and the gas diffusion layers at both anode and cathode. 

Fig. 5.11 The basic electrical circuit of the Hartshorn and Ward (1936) resonance method of dielectric measurement.

Figure 6. Block diagram of the experimental. RB system, vhich consists of three basic components a laser system capable of producing tunable ultraviolet radiation, a magnetic sector mass spectrometer with a suitably modified thermal atomization source, and a detection and measurement circuit capable of quantifying the pulsed ion currents produced in the experiment.

The basic measurement technique for intensity-modulated photovoltage spectroscopy (IMVS) is the same as for IMPS. In principle, IMVS measurements can be made for any constant current condition, but in practice it is usual to make measurements under conditions where the net current is zero. In the case of a photoelectrochemical solar cell, this corresponds to the open-circuit condition, and a high impedance voltage amplifier is used to ensure that a negligible current is drawn from the illuminated device. The output of the voltage amplifier is fed to the FRA, and the remainder of the set up is the same as for IMPS (cf. Fig. 12.26). 

Consider a basic thermocouple circuit with one measuring junction, as shown in Fig. 16.17. The two thermoelements A and B are joined at point c to form the measuring junction at temperature T. The thermoelements are connected to wires C at points b and d, both immersed in an ice bath (liquid water and ice in equilibrium) at T0. The two wires C are connected to the input of an EMF measuring device. The input ports, a and e, are maintained at temperature 7j. Applying Eq. 16.18 over the various legs of the circuit gives... 

The basic electric circuit in flow-through voltammetry and coulometry is the same in principle. The indicating electrode is polarized against a reference electrode to a constant potential. In the close vicinity of the indicating electrode a constant convection is maintained e.g. by a stirrer. The sample flows through the cell and the electric current proportional to the concentration of the species to be measured is recorded. The basic difference between the two methods is in the physical meaning of the current. 

The basic EPR circuit is similar to that for NMR though the technique of EPR measurements differs significantly from that of NMR. 

Figure 11 -7 shows the basic circuit diagram for a tank with two domes. The protection current flows via the two interconnected openers of the cover grounding switch to the cathode connection. If one of the covers is opened, the protection current circuit is broken and the tank grounded via the closing contact. The unconnected cable connection of the tank is without current and can be used for measuring potential. By this method, only one tank at a time is separated from the protection system while the other parts of the installations are still supplied with protection current. 

By the use of suitable shunts, the basic moving-coil movement can be adapted to measure m almost unlimited range of currents. Figure 10.46 illustrates a direct-indicating instrument with shunt, to measure current up to 5 A d.c. To ensure that the resistance of the circuit is not materially altered by the insertion of an ammeter, it is usual to install either a shunt or the meter itself (usually a moving-coil meter with internal shunt) permanently in the circuit. Ammeter shunts are normally of the four-terminal type, to avoid contact resistance errors, i.e. two current terminals and two potential terminals, as shown, in fig. J0.46. 

In making measurements of current flowing within a structure, it is extremely important that additional resistance, as for example a shunt, is not introduced into the circuit, as otherwise erroneous results will be obtained. One method is to use a tong test meter. Such instruments are, however, not particularly accurate, especially at low currents, and are obviously jmpracticablein thecaseof, say, a 750 mm diameter pipeline. A far moreaccurate method and onethat can beapplied to ail structures, isthe zero-resistance ammeter or, as it is sometimes called, the zero-current ammeter method. The basic circuit of such an instrument is shown in Fig. 10.47. 

Measurement of resistivity The most usual method of measuring soil resistivity is by the four-electrode Wenner method. Figure 10.48 indicates the basic circuit. The mean resistivity is given by... 

FIG. 17 Schematic illustration of the setup for a tip-dip experiment. First glycerol dialkyl nonitol tetraether lipid (GDNT) monolayers are compressed to the desired surface pressure (measured by a Wilhehny plate system). Subsequently a small patch of the monolayer is clamped by a glass micropipette and the S-layer protein is recrystallized. The lower picture shows the S-layer/GDNT membrane on the tip of the glass micropipette in more detail. The basic circuit for measurement of the electric features of the membrane and the current mediated by a hypothetical ion carrier is shown in the upper part of the schematic drawing. 

The impedance data have been usually interpreted in terms of the Randles-type equivalent circuit, which consists of the parallel combination of the capacitance Zq of the ITIES and the faradaic impedances of the charge transfer reactions, with the solution resistance in series [15], cf. Fig. 6. While this is a convenient model in many cases, its limitations have to be always considered. First, it is necessary to justify the validity of the basic model assumption that the charging and faradaic currents are additive. Second, the conditions have to be analyzed, under which the measured impedance of the electrochemical cell can represent the impedance of the ITIES. 

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