GB843556A - Means for following the level of the surface of a fluent material - Google Patents

Abstract

843,556. Electric tests. FIELDEN ELECTRONICS Ltd. June 10, 1958 [June 21, 1957; June 10, 1958], No. 19553/57 and 39229/57. Class 37. Means for following the level of the surface of a fluent material comprises a relay sensitive to the capacitance of an electrode suspended above the surface, a reversible motor arranged respectively to raise or lower the electrode when its displacement is less than a first predetermined value or greater than a second predetermined value and inoperative when the displacement is intermediate the first and second values. Fig. 1 shows a system for measuring the level 1 of petroleum contained in a tank 2. The measuring head comprises a hermetically sealed housing 5 containing an electrode 9, Fig. 2 and a capacity sensitive relay, 4, suspended by a metallic strip 6 via pulleys 15, 16 from a drum 17. The relay 4 is connected to a motor control relay 22 via a lead 25, the slack 27 being taken up by pulleys. The measuring head is suspended within a metal tube 13 having ports 14. A shield 10 is shaped to act as a diving bell thus preventing immersion of the electrode 9 in the event of immersion of the housing 5. In Fig. 3 (not shown) the shield 10 is omitted and the lower end of the housing 5 is closed by the electrode, a guard ring and two insulating sleeves being provided. Fig. 4 shows the electrode 9 connected to a transistor oscillator 36 so that changes in the electrode capacitance due to variation of the level 1 vary the amplitude of oscillation. Amplitude variations are detected by the transistor 41 and result in changes in the' D.C. voltage at the base of transistor 45. The difference voltage between the base of transistors 45, 44 is amplified by transistors 53, 43, and, if, sufficiently great, energizes one of relays 55, 56, the contacts 61 of which cause a reversible motor 18 to drive the drum and hence restore the position of the measuring head 5 relative to the level 1. The new level 1 is indicated at 19. Hunting is minimized by arranging that the electrode 9 takes up a position between two predetermined values relative to the level 1. The separation between these values may be adjusted by variation of the common bias on transistors 53, 54. Both values may be varied in the same direction by adjustment of the base bias transistor 44. Valve circuits may replace the transistor circuit. Specifications 645,732 and 697,253 are referred to.