GB1256411A - Improvements in or relating to apparatus for measuring or responsive to the level of a liquid - Google Patents

Abstract

1,256,411. Measuring liquid level electrically. CENTRAL ELECTRICITY GENERATING BOARD. April 8, 1970 [Jan. 9, 1969], No.1349/69. Heading G1N. A liquid level detector comprises a temperature sensitive resistor, 12, in an upright position, and partially immersed in the liquid, the resistor carrying a current sufficient to heat it to a temperature above that of the liquid so that its resistance is indicative of liquid level. As described the arrangement is applied to a water boiler containing water and steam. Although the ambient temperatures of the water and steam are the same the temperatures of the immersed and unimmersed parts differ due to different heat transfer characteristics. Hence the linear resistivities of the two parts will be different and thus the resistance of the whole resistor 12 will be dependent on the depth of immersion X. If three equal and similar resistors 11, 12 and 13 are connected in series as shown, 11 being wholly in steam, 12 being partially immersed and 13 being wholly immersed in water then it can be shown that where R 11 , R 12 and R 13 are the respective resistances L is the length of a resistor, and X is the immersed depth. To effect the measurement a circuit as shown in Fig. 2 is provided. Four equal resistors A, B, C, D, are connected in series in a D.C. circuit, A and C being totally immersed in water, B being in steam, and D being the measuring resistor. The junction between B and C is earthed. The inter-connection of A and B (voltage +V B ) and the inter-connection of C and D (voltage -V C ) are connected to a first operational amplifier 30 whose output is proportional to sum of the voltages, i.e. V B - V c . Similarly the extreme end of A (voltage V A + V B ) and the extreme end of D (voltage - (V C +V D )) are connected to a second operational amplifier 31 whose output is proportional to the sum of these voltage i.e. V A + V B - V C - V D or (since V A = V C ) V B - V D . Hence as shown above the ratio of these two outputs V B - V D /V B - V C is equal to X/L. The outputs are applied to a potentiometric recorder to provide deflections of a marked pen in two orthogonal directions, the tangent of the angle giving the required ratio. The resistors may be of nickel sheathed in a nickel or stainless steel cover, and may be disposed within a conventional glass gauge outside the boiler. The connection of the resistors may be series or parallel.