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United States Patent [193 [in Patent Number: 4,514,681
Finley et al.  Date of Patent: Apr. 30, 1985
 FLUSH ELECTRICAL RESISTANCE CORROSION PROBE
 Inventors: Charles M. Finley; Clifford G. Moore,
both of Arcadia, Calif.
 Assignee: Rohrback Corporation, Santa Fe Springs, Calif.
 Appl. No.: 486,108
 Filed: Apr. 18, 1983
Related U.S. Application Data
 Continuation-in-part of Ser. No. 447,611, Dec. 7, 1982, abandoned.
 Int. CI.3 G01R 27/02
 U.S. CI 324/65 CR
 Field of Search 338/13; 73/27, 86;
 References Cited
U.S. PATENT DOCUMENTS
2,735,754 0/0000 Dravnieks 23/230
2,851,570 9/1958 SchaschI 201/63
2,994,219 8/1961 SchaschI 73/86
3,015,950 1/1962 Doctor et al 73/86
3,320,570 5/1967 Lied, Jr 338/13
3,846,795 11/1974 Jones 340/421
3,854,087 12/1974 Frenck et al 324/65
3,910,830 10/1975 Mayse 204/195
3,936,737 2/1976 Jeffries, Sr 324/65
3,948,744 4/1976 Cushing 204/195
3,980,542 9/1976 Winslow, Jr 204/195
3,996,124 12/1976 Eaton et al 204/195
4,179,653 12/1979 Davies et al 324/65
4,208,264 0/0000 Polak et al 204/195
4,226,693 0/0000 Maes 204/195
4,338,563 0/0000 Rhoades et al 324/65
Primary Examiner—Stanley T. Krawczewicz
Assistant Examiner—Jose M. Solis
Attorney, Agent, or Firm—Gausewitz, Carr, Rothenberg
An all metal-welded flush electrical resistance probe for measuring corrosion of a fluid in a pipe avoids problems of sealing dissimilar materials by using a thin, metallic test disc that is welded around its periphery to the open end of a probe body which also mounts a reference element. The very thin test element is backed up by a solid supporting medium within the probe body, and resistance of the test disc is measured between a point at the disc periphery and a point nearer to the disc center.
16 Claims, 3 Drawing Figures
FLUSH ELECTRICAL RESISTANCE CORROSION PROBE
This application is a continuation-in-part of U.S. patent application Ser. No. 447,611, filed Dec. 7, 1982, now abandoned, for FLUSH ELECTRICAL RESISTANCE CORROSION PROBE.
BACKGROUND OF THE INVENTION
The present invention relates to measurement of corrosive characteristics of a fluid and more particularly concerns a corrosion measuring probe that can be mounted flush with the interior of a fluid pipe or container.
A common method of continuous measurement of corrosive characteristics of a fluid employs resistance measurement of a metallic, corrodible test element to indicate, by change of resistance, the amount of metal that has been lost by corrosion over a period of time. A widely used instrument for this measurement is known as a Corrosometer probe manufactured by Rohrback Corporation, assignee of the present application. A probe of this type is described in U.S. Pat. No. 4,338,563 for Corrosion Measurement With Secondary Temperature Compensation, issued to Rex V. Rhoades and James L. Geer. In one form, the probe employs a long, tubular, metallic test element carrying an inner reference element made of the same material as the test element. The interior of the tubular test element is filled with a thermally conductive, electrically nonconductive compound. Alternating current is passed through the elements, and electrical resistance of each is measured while or after the probe has been immersed in an environment of which corrosive tendencies are to be monitored. Because electrical resistance of the metal changes with the amount of metal in the test element, measurement of test element resistance provides an indication of corrosion. Because electrical resistance of the metal also changes with temperature, a reference element is provided made of the same material as the test element and having the same temperature resistance characteristic. Therefore, changes in resistance of the test element that are due to long term temperature variation are eliminated by comparison of resistances of the test and reference elements.
Long tubular probes of the type shown in U.S. Pat. No. 4,338,563 are generally used by immersion in the fluid of which corrosive tendencies are to be sensed, and the entire exterior surface of a part of the tubular probe acts as the test element. Such a probe is not nearly as suitable for measurement of fluid flowing in pipe, and, in such applications, it is preferred to use a probe having a sensing or test element that is substantially flush with the interior surface of the pipe. A flush probe will minimize disturbance to fluid flow caused by the measurement and will provide more reliable corrosion measurement. One type of such flush probe includes a probe body that extends through a pipe wall and has an end that is flush with the pipe interior. The end of the body is filled with a nonconductive insulating and sealing glass, and a metallic ribbon of a test material is mounted on the end of the glass and has electrically conductive end portions extending through the glass end seal to the interior of the probe body. Such a probe is useful only with a small number of special metals, if the probe must be employed over a wide temperature range as is commonly required. In pipelines and chemi
cal plants, corrosive fluid temperatures may be in the order of 400° F. to 500° F. At such temperatures the great difference in the coefficient of linear expansion between the metal of the test ribbon and the glass of the
5 seal frequently breaks the seal and thus prevents any extended life for such a probe. Glass seal flush probes are not sufficiently reliable nor rugged enough, and at least partly for this reason, flush probes are not as widely used as they might be. If more reliable and rug
10 ged flush probes were available, their use would be significantly increased.
Accordingly, it is an object of the present invention to provide a flush corrosion resistance probe that avoids or eliminates above-mentioned problems.
15 SUMMARY OF THE INVENTION
In carrying out principles of the present invention in accordance with a preferred embodiment thereof, a flush electrical resistance corrosion probe includes an
20 elongated tubular probe body and an electrically conductive corrosion test membrane extending across and sealing the open end of the body. The membrane has a thickness considerably less than the thickness of the probe body wall, and a reference element is mounted
25 within the probe body, which is filled with a supportive material, such as a thermally conductive and electrically nonconductive solidified cement. Acording to a feature of the invention, the corrosion test membrane comprises a thin metal disc that is continuously welded
30 around its periphery to the end of the probe body and is arranged to be positioned flush with the interior surface of a pipe. Resistance of the test element is measured between a point near its periphery and a point closer to its center. According to another feature of the inven
35 tion, the test and reference elements are integral with one another, being made from a unitary piece of metal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a corrosion measuring probe em40 bodying principles of the present invention, installed for corrosion measurement of a fluid flowing within a pipe;
FIG. 2 is an enlarged sectional view of parts of the probe of FIG. 1 showing details of its construction and relation to the wall of a pipe in which it is installed; and 45 FIG. 3 shows integral test and reference elements that may be used in the probe of FIG. 2.
DETAILED DESCRIPTION OF THE
50 As shown in FIG. 1, a probe, generally indicated at 10, is mounted to an adapter 12, which carries a plurality of electrical conductors 14 connected through a transformer 16 and current limiting safety device 18 to an instrument control and readout device 20. Probe 10
55 is mounted in any well known and suitable manner within an opening formed in the wall of a pipe 22 through which flow fluids of which corrosive tendencies are to be measured. A conventional sealing connection between the exterior of the probe 10 and the aper
60 ture in the wall of pipe 22 is provided, as is well known. Details of the probe structure are illustrated in FIG. 2 which shows a probe body 24 formed of a tubular cylinder of steel, or like metal, having a wall thickness of approximately five hundredths of an inch. The for
65 ward end of probe body 24 is mounted within and sealed to an aperture 26 formed through the wall of the pipe 22 so that the forward end of the probe body is substantially flush with the interior of the pipe. The
forward end of the probe body is formed with an inwardly facing circumferential shoulder 30 upon which is seated a metal diaphragm or thin test disc 32, also made of steel or similar metal. Test disc 32 is made with a thickness considerably less than that of the probe body, a thickness in the order of ten mils, for example, so as to provide a suitably sensitive corrosion reading. Corrosion is indicated by the electrical resistance of the disc, which, in turn, is related to the disc thickness. As the test disc is corroded by the environment to which it is exposed, its thickness decreases and, accordingly, its electrical resistance increases to provide an indication of the amount of corrosion. The test element must be extremely thin in order to provide increased sensitivity. The thinner the test disc, the greater the change in increased electrical resistance for a given amount of corrosion. Test disc 32 not only provides a large area of test material exposed to the corrosion environment, but itself seals the interior of the probe body. The disc is continuously welded about its periphery to the forward end of the probe body by means of a continuous weld, indicated at 34, to provide a rugged and reliable seal.
A tubular reference element 36 made of the same material as test disc 32 and open at both ends is welded, as indicated at 40, at its forward end to a central portion of the inner surface of the test disc 32. The reference element 36 may be made of a thicker material that is more easily handled, as it is not exposed to the corrosive environment and, therefore, is not subject to corrosion and electrical resistance changes. Electrical leads or conductors 42 and 44 are connected, respectively, to an inner end of the reference element 36 and to an inner portion of the probe body 24. These leads are connected via the conductors 14, transformer 16 and current limiting safety device 18 to a source of power that causes current to flow through both the reference element and the test disc. Measurement of resistance of the test disc 32 is made by means of a pair of leads 46, 48, of which lead 46 is connected to the junction of the forward end of the reference element and the test disc at a point near the center of the test disc, whereas lead 48 is connected to the test disc at a point adjacent its outer periphery, where it is welded to the forward end of the probe body.
Measurement of the resistance of the reference element 36 is made by means of lead 46, which is connected to both the test disc and the forward end of the reference element, and by an additional lead 50 connected to the reference element at an inner portion thereof.
The inner or rearward end of probe body 24 is fixedly connected to the adapter 12 which contains the electrical conductors and suitable connections for connecting the conductors to the instrument control.
The interior of the probe body and the interior of the reference element are filled with a thermally conductive, electrically nonconductive solid supportive material which contacts all of the interior surface of the thin test disc 32 and provides solid structural support for this very thin test element.
In the probe configuration of FIG. 2, one may encounter problems, due to the difficulty of obtaining a continuously uniform weld 40 around the junction of the reference element 36 and test element 32. This difference may be due, at least in part, to the different sizes or thickness of the materials and, in particular, to the difficulty of welding the extremely thin material of the test disc 32. Nonuniformity of the weld 40 between the
test and reference elements may give rise to undesirable nonlinearities in the measurement.
It will be noted that the resistance of the test element 32 is measured by flowing current in an essentially ra5 dial path between the outer perimeter of the test disc and the junction of the test disc with the reference element. This junction, at weld 40, accordingly carries the highest current, and precision measurement of corrosion requires the same resistance around the entire pe10 riphery of the reference element at its junction with the test element. This is so for the following reason: The test element itself may not corrode uniformly over its entire surface, due to various conditions that may exist in the corrosive environment. Thus, there may be pit15 ting of the test element, and measurement of corrosion due to a pit at one point may be different than measurement of corrosion due to an essentially similar pit at another point, if the electrical resistance of the weld 40 varies in the current paths that include the pitted areas. 20 Corrosion may also vary because of flow dynamics. For example, the leading edge of the test disc 32 (that is, the edge of the test disc that is upstream) may experience a greater corrosion than a downstream portion of the test disc, and, thus, a nonuniform resistance around the 25 perimeter of the test element, namely, at weld 40, may cause different measurements for different corroded areas of the test disc.
The arrangement of FIG. 3 substantially eliminates this cause of measurement nonlinearity. In the arrange30 ment of FIG. 3, the test and reference elements are made integral with one another. A short length of circular cross-section steel bar stock is machined to provide a solid or hollow reference element 52 having integrally formed thereon at one end a thin test disc element 54. 35 The bar stock from which the unitary structure of test and reference elements is made may be a section of three quarters of an inch in diameter and one and a half inches in length. The reference element 52 may be machined or turned down to a diameter of a quarter inch, 40 with the disc element 54 integrally attached to one end of the reference element having the original three quarter inch diameter, but having a thickness of about ten mils.
Preferably, the integral reference element is made
45 hollow, as by drilling out the center of the solid element after turning down the outside of the bar stock to form the integral test disc element. The hollow configuration reduces thermal capacity, and thus provides a faster transient response time and less thermal lag in the pres
50 ence of thermal variations. Having less thermal mass in the reference element, there is less steady state error due to thermal gradient between the environment being tested and the reference element. Moreover, the hollow reference element presents a greater electrical resis
55 tance than the solid element and is less sensitive to location. The electrical lead to the reference element is connected to this element at a point closer to the test element, and thus further decreases thermal lag. This construction surprisingly and unexpectedly pro
60 vides a number of improved results. The difficult procedure of welding a separate test disc to a reference element is entirely eliminated, and, thus, uniformity of resistance at the junction of test and reference elements is maximized to thereby minimize nonlinearities of cor
65 rosion measurement at different points of the test disc. The unitary and integral configuration of test and reference elements also eliminates thermocouples that may be generated by use of dissimilar metals for these ele