GB672879A - Pressure measuring device - Google Patents

Abstract

672,879. Electric measuring systems. FOXBORO CO. May 12, 1948. [May. 14, 1947] No. 13054/48. Class 40 (i). [Also in Groups XIX and XXIX] Pressure measuring apparatus comprises a flexible diaphragm 46 responsive to a variable pressure and mounted in a casing 14, a tube 18 fixed to the casing at one end but readily detachable therefrom, a comparatively rigid rod 64 extending through the tube and fixed to a point thereon remote from the fixed end, and electrical signal transmission means responsive to the magnitude of the lateral deflections of the tube by the diaphragm for producing a signal substantially proportional to the value of the variable pressure, the diaphragm being separate from the rod and tube so that the latter elements can be removed from the casing without disturbing the diaphragm. The variable pressure may itself be the ultimate variable, or it may be indicative of another variable such as fluid flow or liquid level. In the latter case, the pneumatic pressure produced by variations in the liquid level in an air vessel is utilised. In Fig. 1, the fluid flow in a pipe 10 is indicated by a pneumatically-operated recording controller 32 which also regulates a pneumaticallyoperated valve 12. The flow is also measured electrically by a strain gauge 122 fixed on the tube 18. The pressure difference 'across a restricted orifice 16 is applied to the ends of the casing 14 which comprises two sections 42, 44 between which the diaphragm 46 is clamped. The adjacent end walls 52, 54 of the sections are corrugated like the diaphragm and protect it against pressure differences outside the normal operating range. The tube 18 is secured in the section 44 by a threaded bushing 62 and its upper end is secured to the rod 64 bv a nut 70 which supports a rod 22 movements of which control a pressure converter 22 (see Group XXIX). The diaphragm 46 carries a knife edge 75 bearing against the rod 64. The pressures on opposite sides of the orifice 16 act on the remote sides of two slack diaphragms 84, 106, the spaces 74, 100 between these diaphragms and the diaphragm 46 being filled with a liquid such as kerosene through passages 114. 118 closed by plugs 116, 120. The strain gauge element 122 may be as described in Specification 549,140 and comprises a resistance wire 124, Fig. 6. The wire 124 is connected in a Wheatstone bridge circuit with fixed resistances 134, 136 and a variable resistance 138. The resistance 138 is varied bv turning a knob 150 having a pointer 152. The knob 150 is adjusted until the needle 144 of the galvanometer 140 returns to its null point 148. The reading of the pointer 152 on a scale 154 is then taken. The scale 154 may be calibrated in terms of differential pressure. For more accurate measurement, apparatus as described in Specifications 646,981 and 647,025 may be used. The variations in the resistance of the strain gauge element. may be used to control the flow in the pipe 10. Ina modification, the slack diaphragms 84, 106 are dispensed with.