Current flow measurements zero resistance ammeter

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. 

Electrochemical noise measurements may be performed in the potentiostatic mode (current noise is measured), the galvanostatic mode (potential noise is measured), or in the ZRA mode (zero resistance ammeter mode, whereby both current and potential noise are measured under open-circuit conditions). In the ZRA mode, two nominally identical metal samples (electrodes) are used and the ZRA effectively shorts them together while permitting the current flow between them to be measured. At the same time, the potential of the coupled electrodes is measured versus a low-noise reference electrode (or in some cases a third identical electrode). The ZRA mode is commonly used for corrosion monitoring. 

A variety of techniques based on noise exists, but the most common uses two identical working electrodes and a noise-free reference electrode situated between the two working electrodes. The current flowing between the two working electrodes is measured by a zero-resistance ammeter, and their potential is monitored versus the reference electrode. 

Metallic corrosion is usually an electrochemical process. Electrochemical processes require anodes and cathodes in electrical contact, as well as an ionic conduction path through an electrolyte. The electron flow between the anodic and cathodic areas quantifies the rates of the oxidation and reduction reactions. When anodes are physically separated from cathodes, the current can be readily measured by replacing direct electrical contact with a zero resistance ammeter. The conversion of the reaction rate per unit area. 

Some work [23-25] has been done in which an occluded cell condition is set up, with electrically isolated anode and cathode connected through a zero resistance ammeter. The current flow between the electrodes is proportional to the corrosion occurring on the 2inode. This technique provides a measure of the rate of corrosion penetration in occluded areas. With this technique, it is possible to allow the corrosion reaction to start (forming oxide deposits) and then determine the ability of different inhibitors to control the corrosion in the presence of the deposits. 

The remote crevice assembly technique (see Chapter 19) is a research tool that allows one to separate the anode and cathode areas of a crevice corrosion test sample so that the current flowing between them can be measured with a zero-resistance ammeter. This technique is similar to the dual cell method, and it lends itself well to studies of microbial effects on crevice corrosion [7]. It allows direct measurement of microbial effects on both the initiation time and propagation rate for crevice attack, provided again that a suitable control experiment without the microbial influence can be done concurrently. The scime technique of separating the anode and cathode can be used to study the influence of microbes in biofilms on galvanic corrosion [li]. 

The adjustable potential E is measured with a zero-resistance ammeter (ZRA) and an analog-to-digital converter (ADC). The main objective of the potentiostat is to control the potential difference between WE and RE by supplying a current flow through the AE. 

The essential point is to measure the fluctuations of the current flowing between two identical electrodes kept at the same potential by means of a zero resistance ammeter (ZRA), and at the same time measure their voltage fluctuations with respect to a reference electrode (RE). The voltage measurement can be made between the two electrodes connected by the ZRA and a third electrode, which may be identical to the others or be a RE, (see configuration b in Fig. 7-21). The noise resistance is then calculated as the ratio of a second-order statistics of the voltage fluctuations divided by the same quantity relative to the current fluctuations. Often, the quantities chosen are the standard deviations measured over a fixed period of time. 

The zero resistance ammeter is useful for measurement of current flow in low resistance and low current circuits. If an ordinary ammeter is used to read current in such a circuit, an appreciable margin of error would be introduced by its internal resistance, the true voltage would not be shown. The basic advantage of this instrument is its ability to read test current accurately without introducing any resistance. For instance, if the circuit has a total resistance of 0.1 ohm, and an ammeter of 0.2 ohm resistance is introduced, the current flow would be reduced by 67% by a 0.2 ohm ammeter. Thus, the margin of error would be 67%. By not introducing any resistance in the circuit the meter is able to read true current. It is designed to work on a null principle. 

The arrangement for measurement of current by zero resistance ammeter (ZRA) is shown in principle in Fig. 5.38. According to the arrangement, the open circuit potential between the pipe and the anode balanced by a potentiometer circuit. Sufficient current is allowed to flow from a battery through a variable resistor, until the IR drop balances the open circuit potential. At this point, the galvanometer registers zero current. The true current between the pipe and... 

Figure 5.38 Measurement of current flow by the zero resistance ammeter method...

Probes comprising two dissimilar metals may be used to assess the corrosivity of a conductive process fluid (see Figure 8.3). The natural current flow between the two metals is measured using a zero resistance ammeter and the magnitude of the current gives a measure of fluid corrosivity. 

If we were to place a zero-resistance meter between the two electrodes, we could monitor the amount of charge that flows. Such a meter would be called an ammeter if it measured the current, or a coulometer if it measured the charge. (In practice, most modem meters are multi-function devices and can measure both, changing from one function to another at the flick of a switch.)... 

Consider a cell made up of zinc in ZnS04 solution and copper in CUSO4 solution (the Daniell cell), the electrodes of which are connected to a variable resistance R, voltmeter V, and ammeter A, as shown in Fig. 5.1. The potential difference (emf) of zinc and copper electrodes of the cell without current flow is about IV. If a small current is allowed to flow through the external resistance, the measured potential difference falls below 1V because both electrodes polarize. The voltage continues to fall as the current increases. On complete short-circuiting (very small external resistance), maximum current flows and the potential difference of copper and zinc electrodes becomes almost zero. 

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