Electrical resistance thickness direction

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. 

The resistance of most plastics to the flow of direct current is very high. Both surface and volume electrical resistivities are important properties for applications of plastics insulating materials. The volume resistivity is the electrical resistance of the material measured in ohms as though the material was a conductor. Insulators will not sustain an indefinitely high voltage as the applied voltage is increased, a point is reached where a drastic decrease in resistance takes place accompanied by a physical breakdown of the insulator. This is known as the dielectric strength, which is the electric potential in volts, which would be necessary to cause the failure of a 1/8-in. thick insulator (Chapter 4, ELEC-TRICAL/ELECTR ONICS PRODUCT). 

Thin metal films (e.g., copper films) on insulating materials are employed in remote-controlled resistors. The electrical resistance of such a film can be regulated in both directions, by cathodic growth of the film or by anodic dissolution of the film, leading to a decrease in film thickness. 

For example, the required lower bulk electrical resistance and surface contact resistance are directly related to reducing internal power consumption in fuel cells to achieve maximum power output. The requirements of high flexural strength and flexibility (ultimate strain) are important to assure no distortion of fluid fields and no crack in a plate sustained in the large compressive loading when each unit cell is assembled together as a stack. This is particularly important when the thickness of the plate becomes thinner and thinner (can be close to or less than 1 mm [9]) and the dimension of the fluid field becomes smaller and smaller. Whether it is elastic or plastic, the large... 

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. 

Since liquid does not completely wet the packing and since film thickness varies with radial position, classical film-flow theory does not explain liquid flow behavior, nor does it predict liquid holdup (30). Electrical resistance measurements have been used for liquid holdup, assuming liquid flows as rivulets in the radial direction with little or no axial and transverse movement. These data can then be empirically fit to film-flow, pore-flow, or droplet-flow models (14,19). The real flow behavior is likely a complex combination of these different flow models, that is, a function of the packing used, the operating parameters, and fluid properties. Incorporating calculations for wetted surface area with the film-flow model allows prediction of liquid holdup within 20% of experimental values (18). 

The lateral electrical conductivity of the film is directly proportional to the electrical resistance between the electrodes which is measured by bridge 5. Film thickness can be calculated from its resistance [ 106]... 

The electrical resistivity and the Hall coefficient were measured using Van der Pauw method at 77K and 300 K. Both of the properties were meassured along the plains perpendicular to the compacting direction during hot pressing. The sample size was 3mm X 4mm and 0.5 mm thick rectangular shape. 

In the same investigation [1] Crook demonstrated another effect in a dynamically lubricated system which has an important influence on the validity of film thickness determinations by electrical resistance. Direct measurement showed that the resistivity of a commercial turbine oil decreased from 10 " ohm-cm at 273 K (32 F> to 10 ohm-cm at 549 R (530 F). It was also demonstrated that the bulk of the metal in the rotating disks acted as such a large heat sink during a run that it took considerable time for the temperature of the system to stabilize, and even then the temperature of the oil film was not exactly known. Both of these influences cast doubt on the reliability of the electrical resistance technique for evaluating fluid film thickness. The usefulness of the resistance method is restricted to special circumstances such as light loads, very slow speeds and simple linear geometry. 

The electrical resistance of the composite was measured on an l-mm (eight layers of fiber yarn)-thick cylinder ( 6.5 cm long) mounted on Micarta. The resistance was measured transverse to the winding direction by attaching two copper plates to either end of the cylinder. 

The electrical properties of an I PMC electrode can be estimated using the previous model and taking into consideration the particles penetrating into Nafion. In this study it was assumed that the inner part of the electrode on the Nafion has 1/3 of the thickness of the upper electrode (see Fig. 2.21). This is a rather typical value based on the experimental characterization. The inner electrode particles were also assumed to be cubic for the convenience of calculation. The size of the cubic particle Sp was set to 0.8 D, which makes the volume of the cubical particle same as the original spherical particle. The voids in the upper electrode were assumed to have the material properties of water and the voids in the inner electrodes the material properties of Nafion. Considering that, the electrical resistance in a direction (see Fig. 2.21) can be calculated as follows ... 

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