Patent GB-804855-A (GB)
Title: Improvements in or relating to liquid-level indicators and liquid-level control devices
Inventor: ROBERT EDGAR MARTIN JAMES NIGE
IPC: G01F23/26
Filed: 1956-01-20
Abstract: 804,855. Liquid level indicators. CENTRAL ELECTRICITY GENERATING BOARD, MARTIN, R. E., PREWETT, J. N., and HASLER, E. F. March 12, 1957 [Jan. 20, 1956], No. 2056/56. Class 40 (1). An indicator or other device adapted to be controlled by the level of a liquid has a coldcathode electronic tube arranged to operate as a triggered relaxation oscillator which is triggered by a signal responsive to variations in the impedance between an insulated sensing electrode and another co-operating electrode due to the presence or absence of liquid in proximity to the electrodes. A group of five electrodes 11 are shown in Fig. 1 mounted in a pipe 10 containing the liquid, the level of which may change. The electrodes are insulated from each other and co-operate with the pipe wall to form capacitances the values of which depend on the liquid level in the pipe 10. The impedance decreases when the liquid reaches an electrode if it has a dielectric constant greater than unity, or if the liquid has a sufficiently low conductivity. Otherwise a float may be arranged in the liquid to float adjacent the electrode. The electrodes are each connected to detectors 12 coupled to a waveform generator 13 and adapted to produce separate signals from each electrode indicating whether the liquid is or is not in proximity to that electrode. These signals pass to an indicator having separate glow tubes 15 for each electrode. Detectors.-The waveform generator to the left of line A, Fig. 2, consists of a cold-cathode tube 20 which conducts when the condenser 24 is charged up and discharges it through resistor 21, thereby operating as a relaxation oscillator producing an alternating voltage of sawtooth waveform. This output passes via condenser 25 and a variable potentiometer 26 to each of the detector units one of which is shown between lines A and B. The oscillator voltage is applied to parallel condensers 30, 31 connected at J to the electrode 11. The alternating voltage developed at the junction J depends upon the A.C. impedance of the electrode, i.e. upon the presence or absence of liquid at the electrode. The alternating voltage developed is applied through a condenser 32 to the triggering electrode 33 of a cold-cathode tube 34. This tube is connected through a condenser 39 so as also to operate as a relaxation oscillator. The circuit is arranged so that when the liquid level in tube 10 is below the associated electrode the voltage applied to the triggering electrode 33 is sufficient to cause the tube 34 to strike and oscillate in the normal way and the alternating plate voltage is passed via condenser 40 and rectifiers 41, 42 to the remote indicator. When the liquid rises to the associated electrode the voltage at J is insufficient to trigger the oscillating tube 34 and no A.C. voltage appears at the plate. The smoothed signal from the detector is added negatively to a positive bias voltage and the resultant voltage passed over cable 14 to the control electrode 51 of a cold-cathode tube 50 which glows when the electrode voltage is sufficient. A glow therefore indicates the presence of liquid near the associated sensing electrode. Another arrangement is described by means of which a number of remote indicator lamps can be connected to each detector. One of the detectors may be connected to a relayoperated alarm or a relay-operated control device.