Resistivity probes

Sample tests in the field. These include coupons, stressed samples, electrical-resistance probes exposed to the plant corroding medium, or samples exposed to the atmosphere, to soils, or to fresh, brackish, or sahne waters. 

Resistance thermometers are made of a pure metal, such as platinum, nickel, or copper. The electrical resistance of such a material is almost linearly dependent on temperature. Resistance thermometers are stable, having a small drift. A widely used and the best-known resistance probe is the IW-100 probe, which is platinum, having a resistance of 100 ohms at the temperature of 0 °C. Other resistance values for PT probes are available. The resistance versus temperature values as well as tolerances for platinum probes are standardized. The shape and size of a resistance probe can vary considerably, resulting in changes in probe dynamics. 

The so-called standard instrument is used for interpolation between the fixed points and for the calibration of other thermometers lower in the metrological hierarchy. The standard instrument in the moderate temperature range is a special platinum resistance probe, as it has to fulfill set requirements. It is important in all calibration that traceability to a primary normal, here the fixed-point ITS-90 scale, exists. 

Recent developments are leading toward other materials like silica gel or polymers. Certain types of semiconductors are also used as resistive probes. The measurement range of resistive sensors varies depending on materials used. It can be as wide as 0-99% RH. The dynamics are fast enough for normal ventilation applications and the stability of good resistive sensors is high. This does not reduce the need for calibration, but the intervals of successive calibrations can be extended. 

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. 

Inhibitor control can be effected by conventional methods of chemical analysis, inspection of test specimens or by instrumentation. The application of instrumental methods is becoming of increasing importance particularly for large systems. The techniques are based on the linear (resistance) polarisation method and the use of electrical resistance probes. They have the advantage that readings from widely separated areas of the plant can be brought together at a central control point. (See Section 18.1.)... 

Fig. 19.53 Electrical resistance probe with screw-in fitting (courtesy Rohrback-Cosasco...

Choice of an appropriate reference electrode remains an enigma for non-corrosion-aware personnel-although commercially available polarisation-resistance probes can be adapted. An interesting aspect concerns localised corrosion in that, for some materials, localised corrosion only occurs within characteristic potential ranges. 

Fig. 19.55 Electrical resistance probe for assessment of cathodic protection (courtesy SSL...

Bergstrom, D. R., Case Histories-Electrical Resistance Probes Control Corrosion in Chemical Industry , Materials Performance, 20, 9, 17-20, September (1981)... 

Anderson (A2) has derived a formula relating the bubble-radius probability density function (B3) to the contact-time density function on the assumption that the bubble-rise velocity is independent of position. Bankoff (B3) has developed bubble-radius distribution functions that relate the contacttime density function to the radial and axial positions of bubbles as obtained from resistivity-probe measurements. Soo (S10) has recently considered a particle-size distribution function for solid particles in a free stream ... 

The temperature profile inside a narrow channel was determined using resistance probes with a 10 pm Pt-Ir... 

Electrical resistance probes. These probes are the next most common type of corrosion probes after coupons. This type of probe measures changes in the electrical resistance as a thin strip of met gets... 

Linear polarization resistance probes. 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 ). 

Keil, D., Luebke, R.W. and Pruett, S.B., Quantifying the relationship between multiple immunological parameters and host resistance probing the limits of reductionism, J. Immunol., 167, 4543, 2001. 

Later in the research, a variation of the resistance element technique was developed to permit the continuous measurement of a compression wave position with respect to time. With this modification the compression wave movement, during the period prior to establishment of an exothermal reaction, could be studied. The pressure-actuated resistance probe consists of a thin-walled me-... 

K. Sassa G. Larocque, Measurement of Detonation Velocity in Bore Holes by Resistance Probe , KKK 27 (4), 223—28 (1966)... 

The temperature was indicated and controlled to 0.1°C with a platinum resistance probe which extended 1 mm inside the inner surface of the cell. The pressure was adjusted using a 100 cc Ruska syringe pump and was measured to within 0.1% with a 710A Heise digital pressure gauge which is traceable to an NBS standard. The pressure varied less than 0.15 bar (2 psi) during a spectral scan. The wavelength accuracy was 0.2 nm for the Varian (Cary) 2290 spectrophotometer. 

J. Nonuniform Current and Potential Distributions to Polarization Resistance Probes... 

Fig. 5.21 Types of 2-terminal resistance probes (a) hemispheres in a conductive solid, (b) discs on a conductive surface.

T. A. Jennings, The effect if resistivity probe design on the measurement of the freezing temperature of lyophilized formulations. 7. Parent. Drug Assoc., 109-126(1980). 

The metal films polish rates (Ti, TiN, and W) were measured on Prometrix 4-point resistivity probe. Oxide loss on blanket and fully integrated wafers were measured on KLA-Tencor UV1280. In-line SEM was used for defect inspection. 

The electroresistivity probe, recently proposed by Burgess and Calder-bank (B32, B33) for the measurement of bubble properties in bubble dispersions, is a very promising apparatus. A three-dimensional resistivity probe with five channels was designed in order to sense the bubble approach angle, as well as to measure bubble size and velocity in sieve tray froths. This probe system accepts only bubbles whose location and direction coincide with the vertical probe axis, the discrimination function being achieved with the aid of an on-line computer which receives signals from five channels communicating with the probe array. Gas holdup, gas-flow specific interfacial area, and even gas and liquid-side mass-transfer efficiencies have been calculated directly from the local measured distributions of bubble size and velocity. The derived values of the disper-... 

Absolute atomic oxygen concentrations are best measured with the isothermal resistance probe. Elias et describe investigations of atomic oxygen reactions using a silver coated platinum wire coil. 

---