Most commercial pressure gauges for general application are mass produced as relatively high production items. Processing fabrication of such gauges through the progressive production steps is normally arranged for most efficient handling in order to minimize manufacturing costs. Typically, an early production step includes attaching the inlet of a Bourdon tube sensor by means of a soft solder connection to the gauge socket. The melt temperature of the solder is on the order of about 400.degree. F. After completing assembly of the operating components, a near final step includes mounting the outer case in place about the socket to which it is usually secured by screws. Where desired or required as for liquid filled service or the like, leak-proof joints are effected about the casing-to-socket and screw openings.
For effecting the casing-to-socket joint, it is known to utilize adhesives, elastomeric seals, etc. even though the former is characterized by less than optimum reliability while the latter is usually regarded as too costly in high production lots. Welding the joint is likewise known but is limited to constructions utilizing a socket and case of weldably compatible metals. Where possible, welding is frequently a preferred technique because of its reliability and a highly limited localized heating imposed on the surrounding structure despite the high temperatures (plus 2000.degree. F.) which it incurs. Moreover, a welded joint thereat is generally of sufficient strength so that troublesome mounting screws and the associated sealing problems can be eliminated.
Brass sockets are commonly preferred because of the ease of soldering a Bourdon tube thereto. At the same time, stainless steel cases are desirable because of their corrosion resistance properties and good appearance. Consequently, a large quantity of such gauges are constructed of a stainless steel case and a brass socket. Stainless steel and brass are, however, not weldably compatible, although brazing thereof is theoretically possible. However, unlike welding, the high temperature (about 1200.degree. F.) and widespread thermal conduction associated with the brazing process tends to generate more or less uniform high temperatures throughout the surrounding structure substantially higher than the melt temperature of the soft solder connection between the Bourdon tube and the socket. For that reason, brazing after having made the solder connection tends to undo or destroy the previously made solder connection while brazing before effecting the solder connection renders the production step of forming the solder connection most difficult. Despite recognition of the problem, a solution therefor has not heretofore been known.
USPTO Backgrounds
USPTO Backgrounds