Patent GB-780849-A (GB)
Title: Methods of and apparatus for measuring liquid levels, and indicating them at a distance
IPC: G01D5/39
Filed: 1954-02-12
Abstract: 780,849. Photo-electric level sensing apparatus. RUHRCHEMIE AKT.-GES. Feb. 12, 1954 [Feb. 12, 1953; Jan. 20, 1954], No. 4237/54. Class 40 (3). [Also in Groups XXXVI and XXXVIII] The level of liquid in a gauge glass is indicated by scanning the gauge optically by a light ray or a photo-electric cell so that the surface of separation of the liquid and gaseous media produces electrical impulses the position of which in the scanning cycle is an indication of the level. Fig. 1 shows an arrangement in which a rotating light source 3 traverses a beam of light along the gauge, this beam being reflected by the surface of the liquid to a photo-cell 4. The cell and light source may be situated above the gauge as shown dotted. In an alternative arrangement (Fig. 2, not shown) the light source and cell are situated on the same 'side of the gauge and the surface of the gauge is corrugated so as to make the contrast between light reflected from the filled and unfilled part of the gauges more marked. In the arrangement of Fig. 3 (not shown) an elongated light source is situated at one side of the gauge and a rotating photo-cell at the other, while in the arrangement of Fig. 4 (not shown) the elongated light source and rotating cell are situated at the same side of the gauge. In the arrangement of Fig. 5 (not shown) the light source and photo-cell are both fixed, the scanning being effected by a rotating mirror which traverses the light beam along the gauge. A mirror situated above the gauge also reflects the light beam once per revolution of the mirror on to the photo-electric cell to provide a reference pulse from the photo-cell. The pulses produced by the liquid level are represented on the screen of a cathode-ray tube as vertical deflections of a horizontal sweep. The sweep voltage may be provided by a known time-base generator synchronized with rotation of the scanning mirror by means of a pulse generated by the rotation of the mirror. In the arrangement of Fig. 8 (not shown) a disc carried on the shaft of the mirror has a small hole near its periphery which allows a light pulse to reach a photo-electric cell once per revolution. In the arrangement of Fig. 10 the time-base is generated by a slot 52 in the disc 32 on the shaft of the mirror which allows a gradually increasing amount of light to reach a photo-electric cell which provides the sweep voltage. In the arrangement of Fig. 9 (not shown) a wiper arm on the mirror shaft sweeps over a resistance in series with a voltage source to provide the necessary sweep voltage. The time-base voltage may be produced by means of a condenser (Fig. 13) so shaped as to vary the frequency of an oscillator the output of which is fed to a frequency discriminator which provides the necessary sweep voltage. The condenser discs may be so shaped as to take into account the fact that the light beam does not move uniformly along the gauge. An arrangement is described (Fig. 14, not shown) in which a sinewave generator feeding the " X " plates of the cathode-ray tube is coupled to the rotating mirror. The phase of the signal is such that the substantially straight line portion of the sinewave only is used while the same signal 90 degrees out of phase is used to suppress the trace except during that portion of the cycle when the light beam is traversing the gauge. The output pulses of the photo-cell may be made uniform in amplitude by differentiating them and arranging them to trigger a multivibrator producing a uniform output pulse. These differentiated pulses may also be used to increase the trace brightness while tracing the deflection peaks. The time between the two pulses due to the reflection from the liquid surface and the fixed mirror may be indicated as an amplitude variation on a meter (Fig. 17, not shown). This is done by feeding the two pulses through a buffer amplifier to a scale of two counter, e.g. a multivibrator which produces a square pulse of length dependent on the interval between the two photo-cell pulses. These square pulses are averaged and indicated on a conventional meter which may also carry contacts to operate warning devices. Ambiguity due to the two possible times of measurement between pulses is removed by arranging a reference pulse to cut off the multivibrator at a particular time before the first photo-cell pulse arrives. This pulse may be generated by an auxiliary slot in the photoelectric time-base generating arrangement or a projection 56 on the condenser plate (Fig. 13). Alternatively the time-base sweep voltage may be differentiated, the resulting pulse being delayed by a suitable network to ensure that measurement of the correct period is made. As shown, Fig. 21, the time-base voltage on cathode-ray tube 47 is also fed through a differentiating circuit 48, amplifier 49 and delay network 50 to the scale of two counting circuits 42 while the pulses to be indicated are applied to the vertical deflection plates of the cathoderay tube and through a buffer amplifier 41 to the scale of two counting circuits through the averaging circuit 43 to the meter 44. This meter carries terminals 45 for operating warning devices.