Zero resistance ammeter

Separated Anode/Cathode Realizing, as noted in the preceding, that locahzed corrosion is usually active to the surrounding metal surface, a stress specimen with a limited area exposed to the test solution (the anode) is elec trically connec ted to an unstressed specimen (the cathode). A potentiostat, used as a zero-resistance ammeter, is placed between the specimens for monitoring the galvanic current. It is possible to approximately correlate the galvanic current 7g and potential to crack initiation and propagation, and, eventually, catastrophic fail-... 

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. 

The zero-resistance ammeter is seldom employed for routine testing. This instrument requires careful handling to avoid damage, in particular to the galvanometer. Usually two permanent test leads are installed at a set distance apart, and by the initial use of a zero-resistance ammeter a calibration chart of potential between the two leads and current in the structure is drawn up. Thus when routine testing is made, it is only necessary to measure the... 

Galvanic current Measurement of the galvanic current between two different metals can be easily measured using a zero resistant ammeter ". This method can have specific application, e.g. to provide a signal indicating failure of a protective coating in a process vessel. Commercial probes are available for industrial monitoring. 

Recent work [6 has been directed towards the simultaneous monitoring of potential and current noise, where the current noise signal is generated by coupling two nominally Identical electrodes with a zero resistance ammeter (ZRA), and the potential noise of the couple is monitored with respect to a reference electrode. In this manner no externally applied signal is required. 

The current noise signal was monitored by using a sensitive, low noise zero resistance ammeter (ZRA) to couple pairs of identical electrodes the ZRA acting as a current to voltage converter. This derived potential signal was then fed into a potential noise monitor. 

Figure 29 Schematic showing the important electrochemical/chemical reactions occurring inside a creviced titanium electrode and on a Ti cathode coupled to the creviced electrode through a zero resistance ammeter.

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. 

Thus, the noise in both current and potential measurement channels consists of sums of noise contributions. A similar development has been presented for zero-resistance ammeters. ... 

The fluctuations of potential - potential noise - and of current - current noise -can be made independently or together. The current measurements are made between two identical electrodes coupled by means of a zero resistance ammeter (2RA), which keeps them at the same potential. If potential and current are followed simultaneously, then the potential measurements are made between the coupled electrodes and a third electrode (Fig. 17). This third electrode can be either a reference electrode, such as the saturated calomel electrode (SCE), or even identical to those of the pair connected to the ZRA. 

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. 

Corrosion testing for galvanic corrosion may be predicted by ASTM standards in the form of potential measurements. The driving force for galvanic corrosion is the potential difference between the anode and cathode. The galvanic currents between two dissimilar metals are measured using a zero resistance ammeter (ZRA) for a chosen length of time. The ratio of anode to cathode areas is 1 1. 

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