Resistance measurements

The method of measuting iasulation resistance varies with each type of device or product. The iasulation resistance of iasulated wire is the resistance between the conductor and the outside of the iasulation. When the iasulation is covered by a metallic sheath or braid the measurement is made between the conductor and the sheath. Insulated wire with no sheath is usuaUy immersed ia water and the resistance measured between the conductor and the water after the wire has been immersed for a specified period of time. 

For each specific appHcation of a mbber compound as an iasulating material, there is a minimum value of resistivity below which it does not function satisfactorily. In addition, iasulating compounds are required to withstand the effect of water, moist atmosphere, or heat without their resistivity values falling below a satisfactory level. Insulation resistance measurements frequently serve as useful control tests to detect impurities and manufactuting defects ia mbber products. 

K. T. Hartwig, "An Eddy-Current Decay Technique for Low Temperature Resistivity Measurements," in G. Birnbaum and G. Eree, eds.,ASTM STP 722, American Society for Testing and Materials, Philadelphia, Pa., 1981, pp. 157—172. 

High speed machines such as impact mills begin to suffer high wear rates when processing materials above Mohs s hardness 3, unless very special wear-resisting measures can be taken. 

One measure of impact resistance is the laminate s mean break height (MBH) (102). In the standard test, there is a 50% probabiHty that a five-pound (2.27-kg) ball will not fall through a laminate if the ball is dropped at the MBH. Typical MBHs for 12 in. (- 30 cm) square laminates prepared with 30 mil (0.76 mm) thick interlayer are 10 ft ( -- 3 m) at 0°F (—18 C) and 15 ft ( -- 4.6 m) at 70°F (2I°C). Figure 2 shows a relationship between adhesion and falling ball penetration resistance measured at 2I°C. 

Fig. 2. Measuring static charge, (a) Field voltage measurement (b) Faraday cage (c) surface resistivity measurement and (d) static decay test.

Surface Resistance Measurement of Static Dissipative Planar MaterialsD SW.W, EOS/ESD Association, 1991. 

Resistivity measurements of doped, alpha-siUcon carbide single crystals from —195 to 725°C showed a negative coefficient of resistivity below room temperature, which gradually changed to positive above room temperature (45). The temperature at which the changeover occurred increased as the ionization of the donor impurity increased. This is beUeved to be caused by a change in conduction mechanism. 

The electrical-resistance measurement has nothing to do with the electrochemistry of the corrosion reaction. It merely measures a bulk property that is dependent upon the specimens cross-section area. Commercial instruments are available (Fig. 28-5). 

Linear polarization re.slstance probe.s. LPR probes are more recent in origin, and are steadily gaining in use. These probes work on a principle outlined in an ASTM guide on making polarization resistance measurements, providing instantaneous corrosion rate measurements (G59, Standard Practice for Conducting Potentiodynamic Polarization Resistance Measurements ). 

ASTM G59, Standard Prac tice for Conducting Potentiodynamic Polarization Resistance Measurements, provides instructions for the graphical plotting of data (from tests conducted using the above-noted ASTM Standard G103) as the hnear potential versus current density, from which the polarization resistance can be found. 

To obtain the corrosion current from Rp, values for the anodic and cathodic slopes must be known or estimated. ASTM G59 provides an experimental procedure for measuring Rp. A discussion or the factors which may lead to errors in the values for Rp, and cases where Rp technique cannot be used, are covered by Mansfeld in Polarization Resistance Measurements—Today s Status, Electrochemical Techniques for Corrosion Engineers (NACE International, 1992). 

This is the preferred method. The temperature of the winding is determined by observing the increase in resistance of the winding with respect to the cold resistance measured. 

The resistance must be measured with extreme care and accuracy, since a small error in measuring the resistance will cause a much larger error in determining the temperature rise. When the temperature of the winding is to be determined by the resistance, the temperature of the winding before the test, measured either by thermometer or by ETD, may be considered as the cold temperature for the resistance measured. The machine must be left cold for at least 12 to 24 hours, depending upon the size of the machine, to obtain a stable reading. 

From the resistance measured across the stator line... 

Dick and Styrus [63] report real-time resistivity measurements on shoek-loaded silver foils. The inferred vaeaney eoneentration is 1.5 x 10 per atomie site for samples shoek loaded to 10 GPa. The eombined effect of point-defect generation and reeombination to form vaeaney clusters, for example, can be influential on pulse-duration effeets such reload, release, and recovery. This topie has not yet reeeived the degree of experimental study that it deserves. 

Practical measurements providing data on corrosion risk or cathodic protection are predominantly electrical in nature. In principle they concern the determination of the three principal parameters of electrical technology voltage, current, and resistance. Also the measurement of the potential of metals in soil or in electrolytes is a high-resistance measurement of the voltage between the object and reference electrode and thus does not draw any current (see Table 3-1). 

When making measurements in the soil box, it has to be remembered that soil samples can change from their original condition and this will have an effect on the resistivity. Soil resistivity measurements in the soil box only give accurate results with cohesive soils. However, the order of magnitude of the specific resis-... 

Since the Wenner formula [Eq. (24-41)] was deduced for hemispherical electrodes, measuring errors appear for spike electrodes. To avoid errors in excess of 5%, the depth of penetration must be less than a 5. Soil resistivity increases greatly under frost conditions. While electrodes can be driven through thin layers of frost, soil resistivity measurements deeper than 20 cm in frozen ground are virtually impossible. 

Soil resistivity measurements can be affected by uncoated metal objects in the soil. Values that are too low are occasionally obtained in built-up urban areas and in streets. Measurements parallel to a well-coated pipeline or to plastic-coated cables give no noticeable differences. With measurements in towns it is recommended, if... 

The cathodic protection of reinforcing steel and stray current protection measures assume an extended electrical continuity through the reinforcing steel. This is mostly the case with rod-reinforced concrete structures however it should be verified by resistance measurements of the reinforcing network. To accomplish this, measuring cables should be connected to the reinforcing steel after removal of the concrete at different points widely separated from each other. To avoid contact resistances, the steel must be completely cleaned of rust at the contact points. 

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