Patent GB-741827-A (GB)
Title: Improvements in impedance indicating devices
Inventor: JOHNSON IVAN BORIS DAVID
IPC: G01F23/26
Filed: 1952-11-28
Abstract: 741,827. Measuring capacitance and inductance; capacitance-type liquid contents gauges. SMITH & SONS (ENGLAND), Ltd., S. Jan. 29,1954 [Nov. 28, 1952], No. 30199/52. Class 37. Apparatus for measuring the ratio of two impedances of the same kind, i.e. both inductive or both capacitative, comprises an A.C. source and a negative feed-back high-gain amplifier with one of the impedances in the feed-back path. The invention is described in relation to capacitance-type liquid contents gauges. In the basic circuit, Fig. 1, the net amplifier input 3, 4 is zero and terminals 3, 5 are effectively strapped so that C = V/v.E, where V is amplifier output, v amplitude of source voltage 7 and E feed-back. C or E may be the main tank capacitance but desirably it is C with E as factor of proportionality and a capacitance combination including as dielectric a sample of the liquid in C and producing compensation for changes in density and permittivity of the dielectric used in C and also for other interference capacitance. E may comprise a capacitance sampling C dielectric and in parallel with a fixed capacitance where there is a functional relation between density and permittivity. The indicating meter 2 may respond to V/v or V with v stabilized. The arrangement shown in Fig. 2 has one output of amplifier 1 and one side of C earthy with 3, 5 not strapped and includes a stray compensating or meter zero control capacitance F which co-operates with a second voltage source 8 of amplitude 2v in phase with 7. A further arrangement, Fig. 6, provides a capacitance G which co-operates with an anti-phase voltage source 23 of amplitude --v to compensate for empty tank C capacitance and strays. The invention is developed and explained mathematically throughout the Specification. In a first detailed circuit, Fig. 3, translated from Fig. 2, a frequency and voltage stabilized Hartley oscillator V2a, V2b delivers v and 2v into 2-stage amplifier V3ab in connection with C9, C10 (F) and the tank capacitance C and as C varies with liquid level changes so does the anode current in T1 of V3b. The transformer T1 tuned by C16 co-operates with like poled rectifiers MR2, MR3 and a further tuned transformer T2, C17 to provide V and meter 2 bridged between centre-tap on T1 and R18/R19 responds to this voltage V. In setting up C9 (F) is adjusted to correctly zero the meter 2 and C14 in the feed-back pair C13, C14 is preset so that the scale of the meter 2 bears the appropriate relationship to the quantity of liquid in C. A modification, Fig. 4, replaces C13, C14 by (i) a tank capacitance 20 whose dielectric is a sample of the main tank liquid 15, and (ii) a trimmer capacitance 21 which provides positive feed-back (it is connected to 18 at T2). This trimmer is adjusted so that the positive feedback balances the negative feed-back through 20 when empty and counteracts any strays. Alternatively 20 itself may be replaced, Fig. 5, by a floating reference capacitance 22 as disclosed in Specification 673,988, so that V varies linearly with mass of liquid 15. In a second detailed circuit, Fig. 7, translated from Fig. 6, v is not stabilized and the Hartley oscillator V2b feeds v and V into ratiometer 2a and the compensating capacitance G is adjusted at C23 so that the - v input to amplifier V4ab from L 1 tapping 25 counteracts the v feed through the empty tank C capacitance and associated strays.