Patent GB-875437-A (GB)
Title: Electronic level sensing apparatus
IPC: G01F23/24
Filed: 1960-01-29
Abstract: 875,437. Determining liquid level. GILBERT & BARKER MFG. CO. Jan. 29, 1960 [Feb. 3, 1959], No. 3232/60. Class 37. Level sensing apparatus for measuring the height of the upper and lower surfaces of a liquid p, e.g. petroleum in a tank, comprises a probe 10 suspended but insulated from a perforated metal tape 12 which passes over pulleys to a pin wheel 20 rotated by a servomotor 68 whereby the probe may be raised or lowered into a predetermined positional relationship with the upper or lower surface according to variations in the resonance frequency of the probe, and selective means whereby the frequency of an oscillator feeding the head may be varied so that the probe seeks one or the other surface in the manner described in Specification 750,858. The probe itself contains an antenna 48 shielded by a casing 50 which is filled with a dielectric material 82 such as an epoxy resin loaded with small glass beads. The antennµ 4 which is a quarter wavelength extends about # of an inch beyond the casing and carries a disc at its end for greater sensitivity, the standing wave being detected by a semi-conductor 52 and a 50-cycle wave with which the H.F. is modulated, being returned over a coaxial cable 24 which has served also to deliver the H.F. power to the electronic housing 18. The H.F. signal developed in oscillator 28 is coupled to the antennµ via loops 30, 31, 38, 46 and the return signal is confined to the desired path to amplifier 60 by capacitors 42, 44, 62 and inductive resistors 56, 58. The output of amplifier 60 is compared in comparator 64 with a reference voltage from the secondary of transformer T and the resulting signal is impressed on one winding 66 of a 2-phase induction servomotor 68, the other winding 70 being mains energized. If the probe is in air, the servomotor rotates, lowering it to contact the liquid whereby the wire 48 is detuned and the return signal is reduced until the probe reaches a predetermined position relative to the surface when the signal has an intermediate value equal to the reference value, the height of the surface being indicated by the angular position of the servomotor shaft. To cause the probe to seek the lower surface f either an interface between it and an immiscible liquid, e.g. water, or merely the bottom of the tank, the frequency of the oscillator is reduced by switching in capacitor 76, and the servomotor is caused to lower the probe to a new position e.g. “ inch above the lower surface where it avoids the scum, where the head is detuned further until the servomotor is stopped. Changes in the oscillator frequency can be made gradually by means of a variable condenser or in increments to avoid moving the probe in the wrong direction. When relay 78 is de-energized the probe seeks the upper surface. To overcome difficulty arising from the effects of humidity on the probe the probe is kept immersed and is raised to reach the height of the upper surface. The probe may be driven to the immersed position by connecting condenser 84 across the servomotor or by opening a switch 88 in the reference voltage feed. When probes of extremely high Q are used for greater accuracy, a modified circuit, Fig. 7 (not shown), is used. It includes a double triode preamplifier with degenerative feedback, in which the feedback can be disconnected for a short interval determined by a condenser discharge circuit, relay 78 which is energized from a remote source to lower the probe to the interface opening the feedback path to allow greater amplification of the signal during the initial immersion. A variable capacitor can be connected across the oscillator induction loop for calibration and tolerance compensation purposes or alternatively capacitor 76 may be used. Dimensions of parts and component values are given.