GB670149A - Improvements in or relating to telemetering systems - Google Patents

Abstract

670,149. Electric indicating systems. LIQUIDOMETER CORPORATION. Dec. 7, 1949. [Dec. 14, 1948] No. 31392/49. Class 40 (i). [Also in Groups XXXVI and XL (c)] The inductance or capacitance in a series resonant circuit is varied by a condition, e.g. a liquid level, and the series resonant frequency is measured to indicate the magnitude of the condition. As shown, Fig. 1, a condenser 2 whose capacitance is varied by the liquid level is series connected with an inductor 12, the series resonant circuit so formed controlling the frequency of an oscillator 16 (see Group XL (c)). The oscillator output is amplified at 56 and applied to a frequency discriminator 60 comprising two tuned circuits 84-88, 86-92 tuned respectively to the highest and lowest possible frequencies of the oscillator. The voltages developed across the condensers 88, 92 of these tuned circuits are rectified by diodes 97, 114 and applied to the respective coils 104, 122 of a resultant field meter 106 which measures the frequency and may be graduated to read liquid level. The oscillator comprises triodes 22, 24 in which positive feedback is effected by the common bias resistor 26 and coupling condenser 32. Negative feedback effected by the condenser 48 precludes oscillation except at the series resonant frequency when the low impedance of the resonant circuit reduces the voltage across resistor 54 to a value insufficient to prevent oscillation. Modified forms of oscillator are described, the liquid level may vary the inductance 12 through a float. the capacitance 2 being fixed in this case.