Monitoring electrical resistance

Electrical resistance monitors use the fact that the resistance of a conductor varies inversely as its cross-sectional area. In principle, then, a wire or strip of the metal of interest is exposed to the corrodent and its resistance is measured at regular intervals. In practice, since the resistance also varies with temperature, the resistance of the exposed element is compared in a Wheatstone bridge circuit to that of a similar element which is protected from the corrodent but which experiences the same temperature. 

The electrical resistance (ER) method to monitor physical changes in an electrically conducting material is well known(6). 10 cm x 2.5 cm coated iron foils of 10 micron thickness were electrically resistance monitored during exposure to various corrosive environments to follow metal thickness loss at the paint/metal interface. The circuit shown in Figure 1 allowed foil resistance increase AR, which is directly related to metal thickness loss Ad according to Equation (1), to be determined within 0.003 microns. 

Figure 1639 Electrical resistance monitoring with surface-mounted electrodes, (a) Surface-mounted electrodes pattern (b) schematic of delamination and matrixcracking phenomena in relation to the electrode placement [92].

Corrosion of the electrostatic precipitator that is used to remove the dust from flue gases that exit the boiler, has been a problem as design and operating techniques have evolved. An intensive research program by Thompson and Yeske [265] has overturned the widely accepted belief that acid condensation was causing the corrosion. In fact, water condensation was found to be the major cause. Flush mounted electrical resistance monitoring probes were used to measure instantaneous corrosion rates. A cooled conductivity probe was used for dewpoint measurement, and weld pad thermocouples were used for temperature measurement. 

J.H. Constable et al.. Continuous Electrical Resistance Monitoring, Pull Strength, and Eatigue Life of Isotropically Conductive Adhesive Joints, IEEE Transactions on Components and Packaging Technologies, Vol 22, 1999, p 191-199... 

Constable, J.H. Kache, T. Teichmann, H. Muhle, S. Gaynes, M.A. Continuous electrical resistance monitoring, pull strength, and fatigue life of isotropically conductive adhesive joints. IEEE Trans. Compon. Packag. Technol. June 1999, 22 (2), 191-199. 

Surface conduction is monitored in most humidity sensors through the use of porous ceramics of MgCr204—Ti02 that adsorb water molecules which then dissociate and lower the electrical resistivity. 

The initial measurement of electrical resistance must be made after considerable time. Phenomenological information has been determined based on the corrosion rate expected at what period of time to initiate readings of the electrical resistance. Since these values are based on experiential fac tors rather than on fundamental (so-called first) principles, correlation tables and lists of suggested thicknesses, compositions, and response times for usage of ER-type probes have developed over time, and these have been incorporated into the values read out of monitoring systems using the ER method. 

If changes have been made to the process (e.g. if incoming water quality cannot be maintained or other uncertainties arise concerning the corrosion behavior of the construction materials) it is possible to incorporate coupons or probes of the material into the plant and monitor their corrosion behavior. This approach may be used to assist in the materials selection process for a replacement plant. Small coupons (typically, 25 x 50 mm) of any material may be suspended in the process stream and removed at intervals for weight loss determination and visual inspection for localized corrosion. Electrical resistance probes comprise short strands for the appropriate material electrically isolated from the item of plant. An electrical connection from each end of the probe is fed out of the plant to a control box. The box senses the electrical resistance of the probe. The probe s resistance rises as its cross-sectional area is lost through corrosion. 

An electrical resistance methods which directly measures loss of metal from a probe installed in the corrosive system under study is described in Section 19.3. It is reported that corrosion equivalent to a thickness loss of as little as 2-5 X 10 cm can be detected . This technique is most useful as a means of monitoring steps taken to reduce corrosion, e.g. by inhibitors, or to detect changes in the corrosivity of process streams. Electrical methods of determining corrosion rates are considered subsequently. 

A factor which previously limited installation of automatic corrosion monitoring systems was the cost of cabling between sensors and control room instrumentation-this was particularly relevant to the electrical resistance (ER) systems. Developments to overcome this have included transmitter units at the probe location providing the standard 4-20 mA output (allowing use of standard cable) for onward transmission to data systems or the use of radio linkage which has been successfully used for other process-plant instrumentation. 

The importance of MIC has been underestimated, because most MIC occurs as a localized, pitting-type attack. In general this corrosion type results in relatively low rates of weight loss, changes in electrical resistance, and changes in total area affected. This makes MIC difficult to detect and to quantify using traditional methods of corrosion monitoring [1447]. 

The in vitro system we have been using to study the transepithelial transport is cultured Madin-Darby canine kidney (MDCK) epithelial cells (11). When cultured on microporous polycarbonate filters (Transwell, Costar, Cambridge, MA), MDCK cells will develop into monolayers mimicking the mucosal epithelium (11). When these cells reach confluence, tight junctions will be established between the cells, and free diffusion of solutes across the cell monolayer will be markedly inhibited. Tight junction formation can be monitored by measuring the transepithelial electrical resistance (TEER) across the cell monolayers. In Figure 1, MDCK cells were seeded at 2 X 104 cells per well in Transwells (0.4 p pore size) as described previously. TEER and 14C-sucrose transport were measured daily. To determine 14C-sucrose... 

While conductivities of nanocarbons dispersed in polymers fall short of those of metals, a variety of applications can be unlocked by turning an insulating matrix into a conductor, which requires only small volume fractions that can therefore keep the system viscosity at a level compatible with composite processing techniques. Of particular interest are novel functionalities of these conductive matrices that exploit the presence of a conductive network in them, such as structural health monitoring (SHM) based on changes in electrical resistance of the nanocarbon network as it is mechanically deformed [30]. 

Similarly, by directly measuring changes in electrical resistance during mechanical deformation it is possible to monitor crack propagation in hierarchical composites with non-conductive fibers and CNTs dispersed in the matrix [48]. Figure 8.7(b) shows... 

Fig. 8.7 Examples of hierarchical composites where the presence of CNTs is used for SHM. (a) Damage detection through thermal imaging of resistively-heated CNTs in an alumina composite [47] and (b) detection of crack propagation by monitoring electrical resistance (normalized by specimen length) in a CNT/glass fiber/epoxy composite [48], With kind permission from IOP (2011) and Wiley (2006).

Evaluation of the epithelial integrity can be performed by measuring the transepithelial electrical resistance (TEER). TEER values ranging from 150 ohms.cm2 up to 600 ohms.cm2 have been reported. An alternative method for assessing the monolayer integrity is to monitor the flux of hydrophilic marker molecules that pass the monolayers by the paracellular route (e.g., mannitol, Na-fluorescein, or atenolol). 

The microsystems may also serve potential applications in material science and in the growing field of nanotechnology. Microhotplates can be used for material processing, and, at the same time, for the monitoring of material properties such as the electrical resistance [10]. Moreover, the microsystems can be applied to determine thermal properties of new materials such as the melting point, especially when only small quantities of material are available [145], so that monolithic microhotplate-based devices are not only powerful sensor systems for a broad range of applications, but also new research tools for sensor science and nanotechnology. 

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