Patent GB-1536894-A (GB)
Title: Monitoring the level of liquid in a vessel
IPC: G01F23/24
Filed: 1976-03-10
Abstract: 1536894 Testing liquid level SIEMENS AG 10 March 1976 [10 March 1975] 09631/76 Headings G1H and G1N Apparatus for monitoring the level of liquid in a vessel and having a self-testing facility includes a thermistor 1a connected in an arm of two arms of a bridge circuit 1 which two arms are both disposed in the vessel at a given level and which gives an electrical output signal of which the amplitude depends on whether the thermistor is  immersed in water or steam and also on which of two constant values of current is supplied to energize the bridge from a source 5. The bridge output is fed to an evaluating circut 10. The resistance la is heated by a coil 3 energized by a D.C.-A.C. converter 15 which receives an output signal from an addition stage 11 via a proportionalintegral amplifier 13 and an analogue store 14. The adding stage 11 is connected to receive the bridge output via a proportional amplifier 7 and a signal from a desired value transmitter 12. Considering the case where both resistances 1a, 1b are in the water their temperature difference is small and the bridge output voltage U Br  lies on the curve 44, Fig. 2. Hence it is below the threshold voltage, indicated by 45, of the device 8. Due to the inverter 26 a logic one signal is provided at input terminal 29 of the logic circuit 30. The testing device 31 is not actuated so a logic zero signal exists at terminal 32. Since the AND gate 35 has one inverted input 35a it has logic one signals at both inputs which produce a logic one signal via OR gate 36 to the relay 9 which closes contacts 9a, b and lights a lamp 33. Now if the resistances la, 1b are in steam, the first 1a becomes hotter due to it alone being heated by the coil 3 and the relatively poor heat transfer in steam. The bridge output rises and the threshold device 8 produces a logic zero at terminal 29, therefore relay 9 is de-energized. If the bridging plug 28 is connected as shown dashed, instead of as in full-line in Fig. 1, then the lamp 33 is caused to be lit when the measuring resistance 1a is in steam, and not in water. To test the operation of the apparatus the testing key 31 is closed to energize relay 39 which opens contacts 39a, c and changes-over contact 39b. This changes the current from source 5 from its first to its second constant value, or vice-versa, by way of the switch-over device 6. The latter includes a bridging plug 40 for setting the criterion "resistance la in water" or "resistance la in steam" as the criterion to be tested. Assuming "resistance 1a in water" is to be indicated the supply current is such that the bridge output voltage is in the indication zone I Fig. 2 and the operation is as described above for actual measurement. Thus the test checks not only the evaluation circuit 10 but also the measuring resistance. To prevent overheating of the resistance la by the coil 3 the contact 39a opens and a stored signal from amplifier 13 is supplied by store 14 to the converter 15. Open circuit in the heating coil or in a measuring resistance is indicated by lamp 22 actuated  by relay 21 and gate 20 with threshold device 19 and frequency-voltage converter 23, the latter responding to heating current from the converter 15. An external indication is provided for the signal to relay 9 via terminals 41 of the evaluation stage 10. The line 42 connecting these terminals is opened both on actuation of the testing key 31 and also on actuation of the relay 21 to avoid an erroneous external indication. The delay members 24, 37 and 38 are provided to ensure that the bridge output voltage obtained with the testing key actuated does not reach the relay 9 before the line 42 is opened by the relay 39 and contact 39c. Thus only the lamp 33 indicates the result of the testing key actuation. Equally the delay circuits ensure that no premature external indication is provided at the end of the testing period when the key 31 is again de-actuated.