GB858641A - Improvements in electrical measuring apparatus - Google Patents

Abstract

858,641. Pressure difference indicators. KENT Ltd., G. March 26, 1957 [March 27, 1956], No. 9571/56. Class 40 (1). An electromagnetic pump is arranged to create a pressure difference which exactly balances the pressure difference between two fluid supply lines so that the current through the pump is a measure of the pressure differences between the supply lines. Two chambers 7 are subdivided by diaphragms 8 to form chambers A, B, C and D. Supply lines 6 are connected to the chambers A and D, and chambers B and C are connected together via an electromagnetic pump P. Ferrous attachments 9 are made to the diaphragm to co-operate with a differential transformer T, the primary 11 of which is excited by a supply 12 and the secondaries of which are applied to an amplifier 12. The output of the amplifier is rectified in 13 and finally fed to the pump P. When a pressure difference occurs in the supply lines the diaphragms move and a resultant signal is fed to the amplifier which in turn causes a current to be fed to the pump so that a pressure difference opposing the initial pressure difference is set up across the pump. The rest position is obtained when the opposing pressure differences just balance. Indication of the pressure difference is given by measuring the potential drop across resistor 14. Chambers B and C are made larger than A and D so that if overloading occurs the fluid is completely expelled from chamber D (or A) and the reinforced centre of the diaphragm seals off the inlet. The other diaphragm is then supported against the excess pressure in A (or D) by the incompressible fluid in connected chambers B and C. Several pumps in series may be used to build up sufficient back pressure and the differential transformer may be replaced by means such as a high-frequency capacity probe, a photo-cell and light beam arrangement, electrical contact arrangement, a magnetic modulator, a variable inductive device or by means to measure change in resistance of the conducting fluid contained in the chambers. If the magnetic field of the pump is A.C. excited the current through the liquid may also be A.C. and in such cases rectifier 13 is not needed. In both D.C. and A.C. arrangements a linear low is obtained if a pump with independently excited magnetic field is used and a square low is obtained if the current through the liquid is used also to excite the magnetic field.