Electrical resistance probe

Electrical resistance probes measure the change in electrical resistance of a sacrificial element exposed to the process fluid relative to a reference element sealed within the probe body. If the probe corrodes uniformly, the change in resistance of the exposed element over a fixed time period is directly proportional to the average corrosion rate for that period. 

Successive readings must be compared in order to determine fluid corrosivity over the intervening period. Electrical resistance probes may be used to measure the corrosivity of both conductive and non-conductive liquids and vapors. There are three main types  

Of the three types the tubular element is the most commonly used. Wire loop probes are less robust than their tubular element counterparts and are more susceptible to mechanical damage. Flush mounted probes can suffer preferential crevice attack at the steel element/potting compound interface, which can give rise to unrepresentative corrosivity data. 

Under high-velocity process conditions, tubular element and wire loop probes may require velocity shields for protection. However, velocity shields are prone to debris accumulation with attendant spurious results from the probe and their use should be limited accordingly. 

Wire loop or tubular element electrical resistance probes, fitted with velocity shields that extend the full length of the probe body, should not be used in conjunction with low-pressure access fittings on hydrocarbon or other hazardous duty. There is a risk that an uncontrolled fluid release could occur on retracting the probe, should the velocity shield fail by a corrosion-related or other mechanism. 

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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. 

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...

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)... 

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... 

In a pioneering investigation, Serizawa [127, 128] measured the lateral void distribution as well as the turbulent axial liquid velocity fluctuations for bubbly air/water up-flows in a vertical pipe of diameter 60 (mm) inner diameter. They used electrical resistivity probes to measure the local void fraction, the bubble impaction rate, the bubble velocity and its spectrum. Turbulence quantities, such as the liquid phase mean velocity, and the axial turbulent fluctuations were measured using a hotfilm anemometer. A supplementary... 

The techniques used for monitoring corrosion in a process plant are (i) visual inspection (ii) weight loss coupons (iii) electrical resistance probes (iv) measurement of corrosion potential (v) linear polarization measurement (vi) hydrogen probes (vii) thickness measurement and crack detection (viii) visual inspection (ix) sentinel holes. 

Electrochemical monitoring methods have also been developed for application on steel reinforcement in concrete. These methods include potential measurement on the concrete surface, linear polarization (LPR) and determination of polarization curves [9.17]. Electrical resistance probes (ER) and probes embedded in the concrete for measuring galvanic current have also been used. 

Kermabon, A., Gehin, C., and Blavier, P. 1969. A deep-sea electrical resistivity probe for measuring porosity and density of unconsolidated sediment. Geophysics, 34 554—571. 

Reference half-cell electrodes should be installed in anodic sites without disturbing the concrete around the steel to be measured. A method of installation is shown in NACE 11100 (2000). There should be at least one electrode per zone and usually more. Other probes such as macrocell probes, null probes or electrical resistance probes (Section 5.2.4) may also be installed but usually in addition to half cells. 

Measurement sensitivity divides the metal loss methods. Ultrasonics and radiography are usually considered as inspection methods. Typically, ultrasonics has a measurement resolution of around 50 pm (0.002 in.), and radiography 250 p,m (0.010 in.). Consequently, these types of measurements are typically made annually. Corrosion test specimens (coupons) assess metal loss typically over a one to three month interval. Electrical resistance probes, as an automatic coupon, assess metal loss typically over a few hours to a few weeks. 

In the same way as mass loss on corrosion test specimens, resistance metisurements on electrical resistance probes indicate metal loss by general corrosion. Pitting is geneially not noticeable until near the end of probe life, where the effect of pitting becomes "runaway" on the resistance measurement. 

Electrical Resistance Probes—Electrical resistance probes are small in size and can be easily installed in the service environment however, the walls of the test equipment must be penetrated for the probes to be installed and consequently, care must be taken to avert leakage in the system. The usefulness of the probes is limited in that they provide a measurement of uniform corrosion and can be continuously monitored, but provide no information on localized corrosion such as pitting or crevice corrosion. In addition, errors can result in the probe data if the temperature is varied during the time of the measurement [69]. 

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