Pressure sensor designs using welded sensing capsules have been known in the prior art. Such designs have been utilized for many years. Typical designs include a silicon sensing diaphragm in a glassed header, welded to a fitting or port.
In these designs, the circumferential weld around the header can experience high hoop and tensile stresses due to the containment of the high pressure. Differential pressure measuring devices usually include one of two design variations. Both variations include at least one high pressure sensing capsule, with a stress weld. The two half bridge design includes two high pressure sensing capsules, each containing half of a Wheatstone bridge on a silicon sensing die. Each header is ported to the main or reference pressure. In this case, each weld experiences stress due to the high pressure. For a differential pressure measurement, the half bridges are electrically connected, to electrically subtract the high or main pressure from the low or reference pressure, resulting in a voltage proportional to the differential pressure. These techniques are well known. See, for example, U.S. Pat. No. 6,612,179 issued on Sep. 2, 2003 to A. D. Kurtz and assigned to the assignee herein, namely Kulite Semiconductor Products, Inc. and entitled “Method and Apparatus for the Determination of Absolute Pressure and Differential Pressure Therefrom”. This patent describes a combination of absolute and differential pressure sensing devices including a plurality of absolute pressure transducers, each transducer including a plurality of half bridge piezoresistor structures and a device for selectively coupling at least one of the half bridges to another half bridge. In a second prior art configuration, a single pressure sensing capsule is used, with the reference pressure ported to the rear side of the silicon sensing die. This design requires the use of a Wheatstone bridge on a single die. The difference of the main and reference pressure results in only the differential pressure inducing stress in the sensing diaphragm. Again, the differential pressure results in a voltage output. This design requires the reference tube to be connected to the reference pressure inlet, with high stresses in the tube. In any event, in both of the above prior art designs, the weld is typically in tension. The header is designed with a threaded element to reduce the tensile stress on the weld which increases the fatigue life of the weld. While the tensile stress is decreased considerably, the hoop stress (tangential stress) remains high due to the high pressure. In any event, in order to prevent the glass feed throughs or pins from pushing out of the header as a result of the pressure, a tapered glass/hole interface is used. In addition, the radial pressure pushing on the sides of the header further increases resistance to pin push out and failure of the header feed throughs. Thus, it is desired to produce a differential pressure sensor which eliminates the high pressure on the weld and therefore, eliminates the stress on the weld.