Method and apparatus for determining liquid level

Abstract

An ultrasonic level measuring system is disclosed for determining by a time-ratioing technique the level of a liquid within a storage tank. An instrument sensor assembly, including a waveguide formed of a tensioned, ferromagnetic wire having a low thermoelastic coefficient is vertically supported within a storage tank containing a liquid. The waveguide is positioned to span both the tank liquid and vapor spaces. Two permanent magnets are positioned in close proximity to the waveguide at a known dimensional separation which defines the calibration span of the system. The waveguide and magnets are housed in a protective shield which acts as a process barrier and which is convoluted over a substantial portion of its length allowing the instrument sensor assembly to be easily transported and installed. A float, housing a permanent magnet, is positioned concentrically about the instrument sensor assembly to rise and fall with the liquid level. An ultrasonic transducer is formed of a field coil wrapped about a section of permanently magnetized, magnetostrictive wire. The magnetostrictive wire is butt welded to the top end of the waveguide. The field coil is excited by a current pulse introducing a torsional ultrasonic pulse into the waveguide. As the ultrasonic pulse passes each of the three magnets (i.e., the top, float and bottom magnets), a low level voltage pulse is generated across the waveguide. The voltage pulses are detected and amplified by sensing circuitry and coupled to a signal processing unit. The processing unit produces an output signal indicative of the liquid level by ratioing the time interval between the float and bottom magnets to the time interval between the top and bottom magnets. The resulting ratio is scaled by being multiplied by the known distance between the top and bottom magnets, (i.e., the calibration span) to produce a measure of the liquid level in the tank.