US3694789A

US3694789A - Electrical resistance element

Info

Publication number: US3694789A
Application number: US14693A
Authority: US
Inventors: William L Brown
Original assignee: Rosemount Engineering Co Ltd
Current assignee: Emerson Process Management Ltd
Priority date: 1970-02-09
Filing date: 1970-02-09
Publication date: 1972-09-26
Anticipated expiration: 1989-09-26

Abstract

This disclosure teaches a new electrical resistance element construction and a method of making same. The elements are commonly used for resistance thermometer construction wherein the electrical resistance of a material, normally a fine metallic wire, changes with respect to temperature. The resistance wire is wound onto an electrically insulated or insulative mandrel and is subsequently shielded and protected with an outer shell. A cavity (which is purposely left between the mandrel and the outer shell) is sealed at one end and then filled with a liquid, normally very pure water. If necessary, the unit is centrifuged to insure that the water fills all of the air pockets in the cavity. A container is then placed around the outer shell and a plug (if the inner mandrel is hollow) inserted inside the mandrel thus extending and enlarging the cavity beyond the extremes of the unit. The extended cavity is then filled with a very finely powdered dielectric material and water slurry. The unit is again centrifuged and the heavier dielectric material (normally alumina) will replace the water originally centrifuged into the cavity between the mandrel and the outer shell and will be compacted tightly around the resistance wire. The unit is then removed from the centrifuge and dried in an oven to evaporate any of the liquid (water) which may still be present in the unit. The open end of the cavity is then sealed and the element is ready for use. Suitable lead wires are attached prior to centrifuging and means are provided for maintaining the resistance wire wrapped around the mandrel in position so that it does not electrically short out to the mandrel or the adjacent winds of the wire do not short to each other during centrifuging. As shown, one way of doing so is to affix portions of the wire to the mandrel, or the mandrel can contain grooves which will hold the wire from longitudinal displacement during centrifuging. An alternate approach, utilizing the above described centrifuging technique is also employed. That is, a resistance element of any volume is mounted within an outer shell of any volume slightly larger than the element volume. The remaining cavity is then filled with material by centrifuging as previously described thus producing a completely supported and contained resistive element.

Description

llnited States Patent Brown [54] ELECTRICAL RESISTANCE ELEMENT [72] Inventor: William L. Brown, Minneapolis,

Minn.

[73] Assignee: Rosemount Engineering Company, Minneapolis, Minn.

[22] Filed: Feb. 9, 1970 [21] Appl. No.: 14,693

Related U.S. Application Data [62] Division of Ser. No. 649,186, June 27, 1967,

Pat. No. 3,513,541.

[52] US. Cl. ..338/238, 338/28, 338/270, 338/274 [51] Int. Cl. ..H0lc l/02 [58] Field of Search ..338/238, 267, 270, 273, 274, 338/28; 73/362 AR [56] References Cited Primary Examiner-E. A. Goldberg Attorney-Dugger, Peterson, Johnson & Westman 5 7] ABSTRACT This disclosure teaches a new electrical resistance element construction and a method of making same. The elements are commonly used for resistance thermometer construction wherein the electrical resistance of a material, normally a fine metallic wire, changes with respect to temperature. The resistance wire is wound onto an electrically insulated or insulative mandrel Us] 3,694,789 45] Sept. 26 1 and is subsequently shielded and protected with an outer shell. A cavity (which is purposely left between the mandrel and the outer shell) is sealed at one end and then filled with a liquid, normally very pure water. If necessary, the unit is centrifuged to insure that the water fills all of the air pockets in the cavity. A container is then placed around the outer shell and a plug (if the inner mandrel is hollow) inserted inside the mandrel thus extending and enlarging the cavity beyond the extremes of the unit. The extended cavity is then filled with a very finely powdered dielectric material and water slurry. The unit is again centrifuged and the heavier dielectric material (normally alumina) will replace the water originally centrifuged into the cavity between the mandrel and the outer shell and will be compacted tightly around the resistance wire. The unit is then removed from the centrifuge and dried in an oven to evaporate any of the liquid (water) which may still be present in the unit. The open end of the cavity is then sealed and the element is ready for use. Suitable lead wires are attached prior to centrifuging and means are provided for maintaining the resistance wire wrapped around the mandrel in position so that it does not electrically short out to the mandrel or the adjacent winds of the wire do not short to each other during centrifuging. As shown, one way of doing so is to affix portions of the wire to the mandrel, or the mandrel can contain grooves Wl'llCl'l Wlll hold the wire from longitudinal dlS- placement during centrifuging.

An alternate approach, utilizing the above described centrifuging technique is also employed. That is, a resistance element of any volume is mounted within an outer shell of any volume slightly larger than the element volume. The remaining cavity is then filled with material by centrifuging as previously described thus producing a completely supported and contained resistive element.

3 Claims, 6 Drawing Figures 1 ELECTRICAL RESISTANCE ELEMENT This application is a division of my copending application, Ser. No. 649,186, filed June 27, 1967, now US. Pat. No. 3,513,541.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention pertains to precision electrical resistance elements which can be subjected to field service conditions.

2. Description of the Prior Art In the field of electrical resistance elements and particularly in wound wire resistance elements, various methods have been advanced to hold the wire in place so that it is not adversely affected by thermal or mechanical shocks during field service conditions. One

such concept is shown in US. Pat. No. 3,114,125. In this patent, the resistance wire is formed into a small helix which is anchored to a surface by means of a holding agent. Only a small part of each turn of the helix is anchored to the surface. Thus, unwanted strain to the resistance wire, which may be caused by the difference in thermal expansion rates of the materials used, is minimized since only a portion of the wire is attached to these materials.

For some installations where there is vibration and shock loading at very high temperatures, it is desirable to support the element wire as completely as possible to prevent straining under such shock loading. This was accomplished in a standard thermometer shown in U.S. Pat. No. 3,296,572 wherein the resistance wire was laid back and forth through a plurality of small holes in an inert ceramic body in order to support the resistance wire substantially along its entire length. This is proper design where the size of the wire is relatively large (0.002 inch), where the physical size of the element is normally not too restricted, and where the resistance requirement is low. In a laboratory standard, a resistance of 25.5 ohms is a standard value. In field service applications, resistance values on the order of 50 to 1,500 ohms are common. This means that very small wire must be used because the size of the element must be maintained at a very minimum. I

The use of alumina (aluminum oxide) for insulation purposes in resistance elements in order to minimize the heat capacity is shown in U.S. Pat. No. 2,866,060. A ceramic filling powder for resistance elements is also shown in British Pat. No. 1,043,092. However, the way in which the filling powders are inserted into the cavities is not shown. It becomes a real problem in attempting to get a powdered material into the small cavities in which the resistance elements are mounted. Small cavities are desirable from the standpoint of an elements thermal response characteristics.

In any resistance element that is going to be used in field service and under high temperatures, it is desirable to support the element in a strain free manner as completely as possible, have the element as small as possible, and insure the element has good thermal response characteristics. In platinum resistance thermometers, a test of the proficiency or value of the design is a measure of alpha. The term alpha represents the percentage change in resistance for a fixed temperature span and is defined as the difierence in resistance at 100 and 0 C, divided by the resistance at 0 C. Strain free, fully annealed, reference grade (high purity) platinum wire has a typical alpha value of 0.003925. Mechanical stressing of the platinum wire will cause unwanted shifts in alpha. This stress may increase or decrease alpha, depending upon whether the stress is tension or compression. If the stress is high enough to cause plastic yielding some work hardening occurs and alpha decreases. Contamination of the platinum wire, which is accelerated at high temperatures from carelessly chosen materials near the wire, will also cause unwanted continuing decreasing values of alpha. The following rules are therefor important in the design of a good resistance element:

1. Minimize strains to the resistance wire or element caused by the insulative and sheathing materials.

2. Protect the resistance wire or element from contaminates.

3. Eliminate any air pockets around the resistance wire, thereby, providing an element with good thermal characteristics.

These are problems that exist in the known prior art.

SUMMARY OF THE INVENTION The present invention thus deals with a resistance element and a method of manufacturing such elements used primarily for resistance thermometers, in order to support the resistance wire completely and thereby minimize strain from vibrations and shock, and to support the wire in a high purity insulative powder that does not adversely affect the performance characteristics of the element. Because the cavitiescontaining resistance elements are necessarily extremely small to provide good thermal response characteristics of the element, the powder cannot be manually packed in place. These objectives are accomplished by centrifuging an assembly comprising a resistance wire mounted in a cavity to force a powdered supporting material into the cavity. The material will completely fill the cavity and will be packed firmly around the resistance wire.

This invention gives the following desirable features in resistance elements:

1. The resistance wire, or element, is in direct thermal contact with its protective insulation and containing outer shell thus providing good thermal pick-up and dissipation properties.

2. The dielectric powder centrifuged in the cavity can be of the highest purity metal oxides available to reduce the possibility of contamination to the resistive wire from the dielectric. Normal methods of insulating and containing the resistance wire utilize dielectric materials containing the so-called binders to set up a fairly rigid insulative encasement. These binders and their resultant compounds are contaminating to resistance wires, especially at higher temperatures.

3. The resistance wire, or element, is fully supported in the finely compacted powdered dielectric to insure its reliability under extreme mechanical environments of shock and vibration.

4. The resistance wire can be hermetically sealed within the inner mandrel and outer shell by joining same.

5. The inner mandrel and outer shell can preferably be the same material as that of the resistance wire thus eliminating the possibility of contamination to the resistance wire from the mandrel and outer shell. The element is thus immune to contamination in addition to being fully supported in a strain free manner.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view showing a resistance element in an initial stage of being made according to the present invention;

FIG. 2 is a sectional view taken as on line 2-2 of FIG. 1;

FIG. 3 is a vertical sectional view of a completed resistance element made according to the present invention;

FIG. 4 is an end elevational view of the device of FIG. 3;

FIG. 5 is a fragmentary sectional view of a portion of the mandrel and outer covering showing a modified version of means for holding the resistance wire coil in place during manufacture; and

FIG. 6 is a flow diagram showing the detailed steps in the practice of the method of making resistance element assemblies including the optional initial forming steps.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the manufacture of resistance elements, there are a number of preliminary steps. First, the resistance element wire itself is formed. As shown in FIG. 1, a resistance element wire 10 comprises a platinum wire wound tightly into a very small coil. Before the coiled wire 10 has been wrapped onto a mandrel 11, a pair of

lead wire assemblies

12 and 13 are constructed. The lead wire assemblies each comprise a lead wire 14, an insulating (dielectric) layer 15 made for example, of quartz sleeving, and an outer metal sheath 16 encircling the dielectric or insulating material. The lead wire sheaths are usually made of a platinum alloy.

The two

lead wire assemblies

12 and 13, then are placed in provided dimples 17 and 18, respectively, which are formed in the outer surface of mandrel l 1. A pair of half-

rings

21,21 are placed between the

lead wire assemblies

12 and 13 on the outside of the mandrel and are brazed in place with a high temperature braze. At this time the lead wire assemblies and the half rings are both brazed solidly to the mandrel 11.

The mandrel is then cleaned in the element area and as disclosed is electrically insulated by plasma spraying a coating of high purity dielectric material such as alumina, on its outer surface. This insulating layer is only one or two mils thick. Then the wire or element 10 is wound into place. It should be pointed out that while a helical coil wire is shown, in other words a small coil is wrapped around the mandrel, an uncoiled wire can also be wrapped around the mandrel if the end element size is not adversely affected. When the term resistance wire is used, it will mean either a helically wound wire such as that shown at 10, or a long length of wire wound over the mandrel. The use of the helically Wound coil wire permits a longer length of resistance wire to be wound in a shorter space but the small helix is not easy to wind.

After the element or wire 10 has been wound onto the mandrel, it is affixed in minimal spots, in the first form of the invention shown, with very small amounts of a high alumina content cement that is then cured.

Elements wound with long lengths of wire are minimally held to the mandrel with small amounts of a high purity plasma sprayed alumina. In all cases, the wire has to be affixed so that it will not move in longitudinal or axial direction of the mandrel during further processing. The ends of the wire 10 are connected to the lead wires 14. The wire 10, with connected assembly, is then annealed at a required temperature. An outer sheath 22 is slipped over the resistance wire 10 and the

lead wire assemblies

12 and 13. The outer sheath is of larger diameter than the outside of the wire 10 so that it does not contact the wire.

The sheath 22 is then brazed to the lead wire assemblies and to half-

rings

21,21 with a high temperature braze so that the sheath is held solidly in place concentric with the mandrel. Together, the outer sheath and the mandrel form a cavity or annular element chamber 23 that is of sufficient size to hold the element. The brazing of the outer sheath 22 to the half-rings and lead tubes along the annular line indicated at 24 completely seals the inner end of the cavity. The inner surfaces of the half-

rings

21,21 were previously brazed to the mandrel to completely seal along that junction line. The gap of cavity is extremely small, typically 0.015 to 0.020 inch for coiled wires and 0.005 to 0.010 inch for straight lengths of wire wound on the mandrel.

At this stage, the element assembly 25 which comprises all of the parts just described is readied for placement into a conventional centrifuge. First, a plug 26 is inserted into the interior of the mandrel and is made so that it fits tightly and seals the mandrel. The plug 26 has a conical leading surface 27. A container 28 is slipped over the outside of the outer sheath and seals against the outer periphery thereof. The container has a central opening 31. Both the container and the plug are made of a low friction material, such as material sold by E. I. du Pont de Nemours & Co. under the trademark Teflon.

The assembly 25 is then adapted to be placed in a conventional centrifuge, which is not shown. The mounting devices for mounting the assembly in the centrifuge can be made in any desired manner and the centrifuge itself can be any desired construction. Centrifuges are well known in the art.

Prior to centrifuging, the cavity or chamber 23 is filled with water, and if the chamber is very small (as shown, the gap comprises approximately 0.015 inch) the container chamber 31 is also partially filled with water. The assembly 25 is then centrifuged to make sure that the water will go into the cavity or element chamber 23 and all air is excluded. With cavities this small, water does not even run in freely.

Then, the container is filled with a slurry of very fine metal oxide powder, such as powdered alumina and the unit is run in the centrifuge again. This second centrifuging causes the heavier particles of metal oxide (alumina) from the container to be forced into the element chamber or cavity 23 under centrifugal force and completely fill it with the powder 29. Because the powder is selected to be extremely fine, it fills all of the cavity with the metal oxide and the less dense water comes back out to the container. If necessary, additional powder is added to container 28 until the cavity 23 is completely filled. Once the cavity is filled completely with the alumina powder, the water is poured off from the container 28 and the plug 26 and container 28 are removed from the assembly. The unit is then dried out or cured under known procedures. One way of doing it is to bake the unit for one hour at 200 F followed by a one-half hour bake at 700 F. This will evaporate any water which may be left inside the cavity. It does not require sintering. The powder is so tightly packed that it holds the element satisfactorily without sintering.

Because the element cavity 23 will be filled completely up to the end of the unit by the. centrifuging action, a small amount of the metal oxide powder is removed from the cavity adjacent its outer end and a sealing ring or plug 32 is pressed into place between the mandrel and the outer sheath. This plug 32 is then high temperature brazed into place. The element is thus completely assembled and is ready for use.

The resistance element can be used as a precision resistor or as a resistance thermometer element. Resistance thermometer circuitry is attached in a known manner. When the unit has been centrifuged as shown so that the element itself is supported completely by the metal oxide powder, the element will not be susceptible to damage caused by shock loads and vibration during service. The centrifuging insures that all of the air pockets are eliminated from the cavity and the use of a high purity metal oxide eliminates much of the problem that arises from contamination of the element when insulated in other manners. Since the cavity is filled, the thermal response characteristics of the element are excellent.

There are no bonding agents or cement binders for the powdered oxide that contaminate or contribute impurities to the system. At high temperatures many of the bonding agents used in other resistance elements will continue to decompose, causing a break down of the structure or loss of rigidity of theembedding mass. The highly compacted high purity metal oxide powder used in the present invention does not break down during use and is able to withstand as high temperatures as the resistance wire itself.

In some cases, if it is difficult to hold the wire to the mandrel during the centrifuging process (the coils of the wire will tend to move longitudinally on the mandrel) the structure shown in FIG. 5 can be used. In this structure, a mandrel 36 has grooves in it that are helically placed corresponding to the windings of the wire 10. The surface of the mandrel here is also insulated. The outer sheath 37 shown in FIG. 5 is also grooved to receive the wire 10. In this case, the inner surface of the sheath would have to be insulated to prevent shorting out of the coils of the wire 10. The wire 10 is wrapped around the mandrel in the provided grooves and then the outer sheath can be threaded or turned onto the coils of the wire so that the coils fit within grooves in the mandrel and the outer sheath. Then when the outer sheath is brazed to the end rings 21, the coils could not move axially because they would be held by these grooves. The construction steps may otherwise be the same.

When an uncoiled wire element is wound helically onto the mandrel, it can be held to the mandrel very satisfactorily by using a minimal amount of plasma sprayed alumina.

The steps, in detail as diagrammed in FIG. 6, of making the high temperature resistance element assembly including initial and optional steps, comprise the forming of an element of suitable wire, mounting the element wire and its lead wires onto a mandrel or support, and placing an outer sheath over the outside of the element wire so that the outer sheath forms an annular chamber or cavity around the mandrel in which the element is positioned. One end of the sheath is sealed with respect to the outer surface of the mandrel and this forms the cavity. A plug is inserted into the center of the mandrel and a container which opens to the cavity is mounted on the outer sheath so that material can be placed in the container. The container and cavity are first filled with water and if necessary, centrifuged to force the water into the cavity and eliminate all air bubbles. Then a slurry of high purity metal oxide powder, such as powdered aluminum oxide (alumina), is put into the container and the unit is again centrifuged. The heavier particles of insulating metal oxide powder move into the cavity housing the resistance wire under centrifugal force and water comes back out of the cavity. The unit is then cured and/or dried and sealed with a ring brazed between the mandrel and the outer sheath at the open end of the cavity.

The making of the embedding structure (packed metal oxide) can be done with various supports and cavity shapes by centrifuging the finely powdered metal oxide into place. The element assembly itself includes the feature that the powdered alumina or metal oxide surrounding the temperature sensitive wire or element is packed into place tightly enough so that the wire is adequately supported without use of any binders or without sintering the powder. This wire support structure, free from binders, tightly packed and sealed at both ends by the rings, does not cause large temperature differential induced strain as is common where a resistance wire is encapulsed with a liquid which subsequently hardens, such as those embedded in glass or epoxies. Further the compacted metal oxide powder has a tremendous mechanical stability that is not possible to obtain when foreign binders are used with powdered alumina. The element apparatus includes the support having a cavity, the resistance wire in the cavity, the tightly packed powdered insulation substantially surrounding the wire, and the seals to enclose the cavity in which the resistance wire is placed.

These principles of centrifuging to support a resistance element with respect to an outer sheath or shell can be employed even when the element is initially mounted to a mandrel in known ways. For example, if an outer sheath of only slightly larger volume than a resistance element is to be placed over the resistance element, the sheath can be placed over the element and the two components centrifuged to force the powdered supporting material into the spaces between the sheath and the element.

This technique insures that the sheath will be securely positioned with respect to the element and also that there will be good thermal response for the assembly.

The sheath thus becomes a support having a cavity. The resistance element is supported with respect to this sheath by the centrifuging. The resistance element may or may not be supported on a mandrel and may or may not be separately encapsulated before insertion into the sheath.

What is claimed is: comprising an inner mandrel, a temperature sensitive 1. A' temperature sensor comprising a sensing eleresistance wire wound on the exterior of said mandrel, having a resistance Wife wound on a g n y "a sheath surrounding said wire and spaced therefrom, cylindrical center mandrel, a tube substantially coaxial with said mandrel surrounding said mandrel and having an inner surface spaced from the outer surface of said mandrel no greater than 0.020 inches to form a narrow :"annular chamber, said chamber being filled with a said sheath and said mandrel together forming a cavity, means to seal said cavity at a first end thereof, and a filling of compacted powdered alumina centrifuged into the cavity and surrounding said wire and complete- 1y filling said cavity, and a sea] at the opposite end of said cavity from said first end, said seals at both ends of said cavity being bonded to the mandrel and to the outer sheath.

sheath and supporting said outer sheath withrespect to T combmanon sPeclfied l h mandreL said tightly packed alumina 18 free of binder material.

2. A high temperature resistance element assembly

Claims

1. A temperature sensor comprising a sensing element having a resistance wire wound on a generally cylindrical center mandrel, a tube substantially coaxial with said mandrel surrounding said mandrel and having an inner surface spaced from the outer surface of said mandrel no greater than 0.020 inches to form a narrow annular chamber, said chamber being filled with a metal oxide powder centrifuged into place in said annular chamber to form a tightly packed support without subjecting the sensing element to substantial strain, and seal means bonded to said mandrel and said outer sheath and supporting said outer sheath with respect to said mandrel.

2. A high temperature resistance element assembly comprising an inner mandrel, a temperature sensitive resistance wire wound on the exterior of said mandrel, a sheath surrounding said wire and spaced therefrom, said sheath and said mandrel together forming a cavity, means to seal said cavity at a first end thereof, and a filling of compacted powdered alumina centrifuged into the cavity and surrounding said wire and completely filling said cavity, and a seal at the opposite end of said cavity from said first end, said seals at both ends of said cavity being bonded to the mandrel and to the outer sheath.

3. The combination as specified in claim 1 wherein said tightly packed alumina is free of binder material.