Resistivity Measuring Instruments

The grounding or penetration depth of the electrical resistance in conductors is, according to Eq. (3-42), dependent on the specific resistance and the frequency. The penetration depth, t, is the distance at which the field strength has fallen by 1/e,- is the relative permeability [35]  

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Although accurate measurement of resistance is conventionally achieved by a bridge procedure, in commercial practice this is rarely used. Most resistance measuring instruments ("resistance meters ) rely either on measuring the current caused by a defined potential or the potential necessary to cause a defined current. In either case, the instrument will indicate directly the potcntial/cuirent ratio as a resistance value, or the current potential ratio as a conductance. 

Resistance measuring instrument dc and ac (potential) Resistances (soil resistance, grounding resistance) 0 - 999 kQ 1 kQ... 

As normally used in the process industries, the sensitivity and percentage of span accuracy of these thermometers are generally the equal of those of other temperature-measuring instruments. Sensitivity and absolute accuracy are not the equal of those of short-span electrical instruments used in connection with resistance-thermometer bulbs. Also, the maximum temperature is somewhat limited. 

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). 

Fig. 3-8 Circuit for determining the response time of measuring instruments with internal resistance R...

Filters have a time constant r = R x C which increases the damping of the measuring instrument. The time constant depends on the required attenuation and the interfering frequency, but not on the internal resistance of the measuring instrument. The time constants of the shielding filter are in the same range as those of the electrochemical polarization, so that errors in the off potential are increased. Since the time constants of attenuation filters connected in tandem are added, but the attenuation factors are multiplied, it is better to have several small filters connected in series rather than one large filter. 

Fig. 3-14 Circuit of a measuring instrument for determining the apparent soil resistance by the Wenner method.

Rowlands and Bentley have provided an account of the possibilities for continuously monitoring corrosion rates by polarisation resistance measurements, and they also describe the development of a commercial instrument, which uses low-frequency square-wave current to polarise the test specimens. 

The preferred potential-measuring instruments are potentiometers or electrometers, either of which permit measurements to be made without flow of sufficient current to polarise the electrodes during the determinations. It is also possible to use millivoltmeters if the internal resistance of the instrument is high enough to avoid any appreciable flow of current. 

Bridges give a mean resistance value around 7 = 0. Strictly, they should be used only for linear components. In both cases (d.c. or a.c.), the resistance measurements are made in four-wire configuration, since the resistor to be measured is at low temperature whereas the measuring instrument is at room temperature the electric connection is usually made by low thermal conductivity wires which are also poor electrical conductors (remind the Wiedeman-Franz law). 

Electrical Resistance and Percent Llgnt Transmittance. Low frequency electrical resistance measurements were made on a conductivity bridge (Model RC-18, Industrial Instrument, Cedar Grove, N.J.) at a line frequency of 1 KC. Beckman conductivity cell with cell constant 1.0 cm was used. The percent transmission was also monitored for each of the mixtures at 490 nm (Spectronlc 20, Bausch Lomb Co., Rochester, N.Y.). 

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