BACKGROUND OF THE INVENTION The present invention relates to the making of a pressure tight joint between components of two different, and weld incompatible, metals to provide one component for welding to a corrosion resistant pressure sensing diaphragm of a compatible material, and another component for welding to a pressure transmitter housing of a different material.

At the present time, a wide variety of pressure transmitters utilize an isolation diaphragm which is made of material that is resistant to corrosive fluids, to isolate sensitive components, and other materials that are likely to be adversely affected by the fluids being sensed for pressure. The wetted components, such as an isolator diaphragm are made of highly corrosion resistant materials, while components such as diaphragm support blocks, housings, and the like may be of less corrosion resistant and less expensive materials. The pressure transmitter components of different materials at the location of the isolator diaphragm must be welded for sealing against the high pressures involved, but the materials that are used are generally incompatible for easy welding, and require special techniques to obtain joints that are leak proof.

Weld rings for welding the edges of diaphragms of highly corrosion resistant materials, such as Monel

400, and then to also weld to parts of a nickel-iron alloy, or stainless steel have been used, but obtaining reliable welds has been difficult.

One solution for the problem of providing an adequate seal at the periphery of an isolator diaphragm relative to a support body is shown in U.S. Patent

5,184,514. Patent '514 discloses a weld ring that is first welded to the diaphragm to secure the diaphragm in place and then the weld ring is separately secured to the housing. SUMMARY OF THE INVENTION

The present invention relates to the utilization of a weld ring located in a pressure transducer for supporting an isolation diaphragm. The ring has sections of two different metals that are explosion welded together across their interface so that on one side of the ring a highly corrosion resistant material supports and is welded to an isolation diaphragm of the same material. The second, outer section of the present weld ring is made of a different metal, such as a stainless steel, for welding to the pressure sensor housing.

In the present invention, an inner annular section of material matching the isolation diaphragm is surrounded by a close fitting outer encompassing section of a material matching the weld characteristic of the supports used. The inner and outer sections are explosion welded together. Explosion welding results in a cold weld of the materials across the interfacing surfaces. Explosion welding is a cold phase weld with actual electron sharing at the interface so there is high strength bond between the two materials, even those which are not easily fused or welded together. The explosion welded interface is not subject to failure before the base materials. The welding process for attaching the isolation diaphragm to the first or inner section of the weld ring follows standard welding procedures. The stainless steel material or other easily welded material forming the second or outer section of the weld ring is

welded to supports and sensor housing parts following standard welding procedures.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a part schematic cross sectional view of the typical differential pressure transducer showing isolator diaphragms in place utilizing a weld ring made according to the present invention for retaining an isolator diaphragm;

Figure 2 is an enlarged partial sectional view showing made according to the present invention; and

Figure 3 is a further enlarged schematic view of a weld ring made in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A differential pressure sensor 10 in Figure 1 connects to a pressure transmitter housing 11, through a conduit 15 that carries electrical leads from a pressure sensing cell 12. The cell 12 has a central sensing diaphragm 14, mounted between a pair of metal housing sections 16 forming a housing 17. The housing sections 16 are welded together with the diaphragm 14 between them and forming separated cavities on opposite sides of the diaphragm 14. A pair of isolator diaphragms 18 are mounted at the opposite ends of the housing sections, as is well known. Each isolator diaphragm forms a cavity that fluidly communicates with an associated cavity formed by the sensing diaphragm 14.

Suitable non-corrosive, inert liquid, such as silicone oil fills the cavities under the isolator diaphragm and formed by the sensing diaphragm 14.

The differential pressure sensor housing 17 is held within a clamp ring 21 at one end and outer housing 20 at the other. The outer housing 20 is clamped between end caps 22, which have inlet ports 24 for

process fluids on opposite sides of the sensor housing. The process fluid generally is a very highly corrosive fluid and is generally at an elevated temperature. As can be seen, the process fluid enters chambers on the exterior of the isolating diaphragms 18, and is not permitted to come into contact with the housing sections 16, outer housing 20, or the clamp ring 21. The end caps 22 are provided with suitable seals shown at 28, to keep the process fluid sealed within the chambers open to the ports 24 and the isolator diaphragms 18.

Referring to Figure 2 in particular, it can be seen that the isolator diaphragm 18 shown is supported relative to the sensor housing section 16 by a weld ring assembly 30. The transition joint or weld ring assembly 30 is welded to the housing section 16 around the outer periphery of the weld ring assembly 30, and also is welded to the outer housing 20 with an outer peripheral weld that goes around the generally circular configuration weld ring assembly. The periphery of isolation diaphragm 18 is welded to an inner portion of the transition joint or weld ring assembly 30.

The isolation diaphragms 18 are preferably made of a very highly corrosion resistant material such as a nickel-copper alloy sold by Inco Alloys International Inc. of Huntington, West Virginia, USA under the trademark Monel 400. The housing sections 16 typically are made of a nickel-iron alloy sold under the trademark NI-SPAN because of the desirable characteristics of this material for use with sensing diaphragm 14. The clamp ring 21 and outer housing 20 typically are made of a high performance stainless steel. Nickel-iron alloys and stainless steels are weld incompatible with Monel 400.

It is necessary to have a liquid and pressure tight seal at the periphery of the isolator diaphragms 18, so it is desirable that the periphery of each diaphragm 18 is welded to the respective weld ring assembly 30, and then the welded assembly in turn is securely welded to the housing section 16, and either clamp ring 21 or the outer housing assembly 20.

Referring to Figures 2 and 3 in particular, the weld 34 joins the outer housing assembly 20 to one end of weld ring assembly 30 as shown in Figures 2 and 3, or to the weld ring 21 as shown at the lower end of the sensor in Figure 1. Weld 32 joins the respective sensor housing section 16 and the weld ring assembly 30. The welds 32 and 34 are both made to the outer periphery section of the weld ring assemblies 30 and are spaced apart axially. In order to provide for reliable and relatively straight forward welding of the clamp ring, the outer housing assembly and the sensor housing sections to the weld rings, various solutions have been tried. It has been discovered that the nickel-copper alloy Monel 400 can be reliably clad with a layer of stainless steel using explosion welding techniques to provide a high quality metallurgical weld between the metals. The resulting weld is fluid tight, and will withstand at least as much stress as the parent materials.

Thus, in the present invention, the weld ring 30 is formed of a first or inner section 40 of a nickel- copper alloy, preferably the alloy sold under the trademark Monel 400 or of other material that is the same as the material of the isolation diaphragms 18. A second or outer weld ring section 42 is preferably made of a suitable stainless steel or other material that will weld with reliability and with suitable simplicity

to both a nickel-iron alloy, such as NI-SPAN, and to stainless steel of the same or different compositions as the outer weld ring section.

The first or inner section 40 of the weld ring assembly is an annular diaphragm support ring that is

"L" shaped in cross section as shown in Figure 3, and provides a peripheral or bounding cylindrical surface extending the full height of the weld ring assembly 30.

The second or outer weld ring section 42 is also is a peripheral ring positioned to the outside of section 40. The interface 44 of the facing surfaces of sections 40 and 42 is explosion welded.

A bar of Monel 400 material is selected to have an outside diameter corresponding to the diameter of the interface 44. The bar is then clad with a continuous outer layer of stainless steel using explosion welding techniques. The weld ring assembly 30 is then machined to the configuration shown. The outer layer is sufficiently thick to permit machining the outer weld ring section 42 to the size and shape shown. Explosion welding techniques such as those well known and performed by Regal Technology Corporation, Columbus, Ohio are used. The use of explosion welding for bonding sheets of titanium and aluminum alloys to stainless steel and low carbon steel has been known. Explosion welding generally involves having the mating surfaces forced together under the force of an explosion to cause essentially a collision between the two mating surfaces that cold bonds the surfaces uniformally and securely. This technique is also described in "The Welding Handbook", Seventh Edition, Volume III, published by the American Welding Society of Miami, Florida. As described in this reference, an explosion weld is identifiable by a wavy interface between the two

materials on a microscale. The materials are free of heat affected zones at the weld, since explosion welding involves cold bonding, that integrally secures the two metals together. The explosion welding securely holds the two different materials of weld ring sections 40 and 42, and the sections are joined along a substantial interface surface length after being machined from the stainless steel clad Monel 400 bar. The first or inner weld ring section 40 is then fusion welded, using standard heat generating welding techniques, to the periphery of its associated isolator diaphragm 18. The outer weld ring section 42, which is made of stainless steel, is fusion welded to associated housing section 16, which is generally made of a nickel-iron alloy, with the peripheral weld 32 and is fusion welded to the associated clamp ring or outer housing material, such as another stainless steel, with the peripheral weld 34. The configuration of the two different sections forming the weld ring assembly that are cold bonded together using known explosion welding techniques provides materials that can be joined to other parts with high integrity, easy to perform welds 32 and 34. The corrosion resistant properties of Monel 400 is maintained on all surfaces contacted by the process fluid. The process seal 28 rests on the leg of the "L" shaped configuration of the first weld ring section 40 and seals against a surface 50 of the end cap 22, which includes the pressure inlet ports open to the isolation diaphragms 18.

The other parts of the transmitter assembly are conventional. The two part, explosion welded weld ring assembly construction greatly simplifies the long

standing problems of easily and reliably welding the two fusion weld incompatible materials of the isolation diaphragms and sensor housings of pressure transmitters. The diaphragm and the housings have metallurgically different fusion welding characteristics, as do the two weld ring sections 40 and 42. The inner weld ring section 40 is fusion weld compatible with the diaphragm and the outer weld ring section 42 is fusion weld compatible with the housing and clamp ring. Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.