Method and apparatus for testing liquid fillings in tanks

Abstract: The invention relates to a method and an apparatus for investigating a filling of liquid in a container, e.g. a fuel tank in respect of actual volume, density, temperature, opacity or other condition. From an emitter (19) at the bottom of the container there are emitted pulses of ultrasonic waves which are reflected by one or more horisontal border faces (17, 21) of the liquid and returned to the bottom of the container and detected by a receiver (20) there. The passage times of the reflected signals are compared with the passage times of direct and not-reflected signals that are detected by a plurality of receivers (1, 2, 3, 4, 5) placed above each other in fixed known positions. The result of said comparison is made the basis of a calculation of the sought qualitity. Measurement of the passage times, comparison, calculation and exposition of the result takes place by the use of electronic or data equipment (23).

Description: The invention relates to a method and an apparatus for determining the location of the level of one or more horizontal border faces of liquids in a container, e.g. a fuel tank. In order to determine the position of a liquid level it is previously known to have a radiation source emit a pulse of an ultra sonic ray which after having passed through the liquid and having been reflected against the lower side of the free surface of the liquid is sent back to a receiver placed at the container bottom and close to the radiation source. The time needed for the ultrasonic pulse to move up and down through the liquid is measured and gives an indication of the level of the reflecting surface above the container bottom. Several methods to that purpose are previously known, which all are deficient and of little accuracy by not taking into account that the transmission velocity of the ultrasonic waves in the liquid varies with certain characteristics of the liquid, which in one and the same container may differ between liquid layers having mutually different constitution, temperature or other conditions. It has been proposed in EP-A-0106677 to obtain a better accuracy by comparing the passage time of pulses reflected by the surface of the liquid with the passage times of pulses reflected by a number of reflectors placed at fixed known levels in the container. However, said method is insecure due to the risk of confusing the various echoes. Patent Abstracts of Japan, vol.4, no.86 (P-16) (586) 20.06.80 & JP-A-55 050 173 discloses the measurement of the distance of an object by the use of an emitter directing ultrasonic waves to the object, from which they are reflected to a receiver built into the emitter. The time of travel is measured. The waves are also directed to a set of receivers of different distances to calculate the speed of propagation, to obtain a compensated speed of propagation at circuit 13, which is used with the measured times of transit to calculate the distance of the object. Said risk is particularly apparent when the liquid is layered, so that there are more than one reflecting liquid face. The present invention entails an improvement of the measuring accuracy obtained in principle by comparing the passage time of the pulses having been reflected with passage times of pulses having passed through the liquid in one direction only. The features of the invention are more clearly set forth in the following claims 1 and 3, and details of preferred solutions are described herein below with reference to the accompanying drawing, in which Fig. 1 diagramatically shows an apparatus for performing the method of the invention. In the drawing, 11 designates a container or tank, holding fuel oil or crude oil at a tap station or a distribution center, where it is important to have a reliable control of the liquid contents, above all of its actual volume. In the drawing it is assumed that the tank 11 has a lower filling 13 of heavier oil and an upper filling 15 of somewhat lighter oil, and it is also assumed that the two kinds have not been mixed, but between them there is a border face 17. The reason therefor may be that they are of different origin or have been filled-in at different occations or are of different temperatures. A second border face may be formed between the oil and the dirty water collected at the bottom of the tank. The filling of the tank is examined by the use of ultrasonic sound energy emitted from a emitter 19 placed inside the tank and upon its bottom. Short pulses of ultrasonic waves are emitted with periodic interruptions from said emitter and are directed upwardly within a narrow space angle. Said pulses are reflected partly by the border face 17, partly by the free liquid surface 21 and are directed back downwardly to the bottom of the tank, where they are received by a receiver 20. Said receiver 20 may be a separat element or may be combined with the emitter 19. Said combination may comprise a crystal of some kind, by which an electric current is transformed into mechanical vibrations which are then transferred to the liquid, and vice versa. 23 designates a measuring and controlling unit, which is connected by wires in the tube 27 to the emitter 19. Current sent through said wires feed the emitter 19 and control the exact moments of emission of ultrasonic pulses therefrom. The receiver 20 receives the reflected sound pulses (echoes) and transforms them into electric signals which are sent through wires in the tube 27 to the measuring unit 23, wherein the time difference between the moment of emitting ultrasonic pulses and the moment of receiving the corresponding reflected pulses (echoes) from the faces 17 and 21 is established and registered. Knowing the transmission speed of the sound waves, said time difference gives a rough indication of the levels of the reflecting liquid faces above the bottom of the container. However, the transmission speed of the utrasonic waves varies with temperature, viscosity etc. of the liquid, and the figures thus obtained are not fully reliable. A stricter accuracy is obtainded by relating said time differences to values obtained by a separate investigation or probing of the liquid filling which may disclose possible layer formation, temperature variations or such other inhomogenities of the liquid as will affect the propagation of the sound waves. The supplementary equipment required therefore - which in many respects functions as an independent level probing device - forms a reference means comprising a plurality of mutually equal receivers (upper receivers) 1, 2, 3, 4 and 5 located in fixed positions vertically above each other and along a line coinciding with or running close to the path of the ultrasonic ray emitted from the emitter 19, so that said receivers are successively hit by parts of said ray. Said upper receivers are preferably attached to a probe stick extending vertically from a tank inspection orifice 26 to the bottom of the tank. In the drawing said probe stick forms a part of the tube 27 enclosing the wires connecting the transmitter 19 and all receivers (20, 1-5) to the control unit 23. As an alternative, the receivers 1-5 may be attached to a chain or similar suspended along the entire height of the tank. The number of upper receivers may be greater than shown and the distances between them may be equally large, but need not be so. The distances between them and to the tank bottom are accurately measured metrically and registered in the control unit 23. The volume of each stratum bordered by parallel planes extending through neighbouring receivers is calculated from the measurements of the tank, or may be ascertained when filling the tank by accurately measuring the quantities having been pumped-in at the moments when the respective receivers are successively drowned in the rising liquid. When the receivers 1-5 are hit by a wave pulse emitted by the emitter 19 they send electric signals to the unit 23, wherein their time lags relatively to each other and to the moment of emission of the pulse by the emitter 19 are measured. In all instances the measurement is taken at the steep front of the first period of the interrupted oscillations. The transmission speed of the waves passing between any two receivers is their mutual known metric distance divided by the time difference between their response to the sonic pulse. The magnitude of the speed so calculated discloses certain qualities of the liquid, such as its density; the mutual relation can be found empirically. If said speed is different in strata of different locations this may be due to different temperatures, which thus will be detected and estimated, or by inhomogenities of other kinds. The three properties: transmission speed, density and height of a liquid stratum, are dependent upon each other, so that if two of them are known, the third property may be calculated. If the described equipment is completed with means for measuring the temperature at different levels, other qualities of the liquid and variations thereof can be detected. The wave pulses detected by the receivers 1 to 5 have passed through the same strata as have been penetrated twice by the pulse reflected by the surface of the liquid and returned to the receiver 20, and therefore the transmission time of a pulse received by any of the upper receivers 1 to 5 is equal to half the transmission time of a pulse having been reflected at the same height and received by the low receiver 20. The measurements by the receivers 1-5 are not exposed to disturbances, such as by stray reflections, and will not be influenced by echoes from the reflecting faces of the liquid, which means a great advantage in comparison to known methods. In the control unit 23 there is performed by known data equipment, including i.a. time basis generators, memories, calculators etc. a registration and reworking of the signals put in from the the receivers 20, 1-5. A fictitious time scale is formed, on which there are marked the moment of sending a starting signal to the emitter 19 and the moments of receiving the signals from the receivers 1-5. Thus the distances between said marks correspond to the transmission times of sound waves directly received by the receivers 1-5 without reflections. A separate and parallel time scale is also formed, on which are marked the moment of the starting signal to the emitter 19 and the moment of receipt of the echoes by the receiver 20. Then the two time scales are laid parallel and compared. Previous to said comparison the first-mentioned scale should be shrunk in the proportion two to one, i.e. be halved, or the last-mentioned scale should be magnified in the proportion one to two, i.e. be doubled. The time position of each echo will then appear opposite the markings caused by the receivers 1-5 and can be read off from the first-mentioned scale, where the markings correspond to metrically measured true levels. The result obtained is the level of the sought face of the liquid and can be shown either graphically or numerically on the unit 23. Due to the presence of the several receivers 1-5 it is possible to ascertain the passage times of the ultrasonic waves in the liquid strata lying between each pair of receivers and thereby get knowledge of varying qualities of the liquid along the height of the tank, such as temperature, density etc. having influence upon the speed of ultrasonic waves. Of course, the descibed method and the apparatus for performing the same can be modified in several respects within the limits of the following claims. This applies especially to the functions of the measuring and controlling unit 23, where modern computer technique offers many possibilites of calculating time differences and graphically or numerically exposing the results.

Claims: A method of determining the location of the level of one or more horizontal border faces of liquids in a container, e. g. a fuel tank, by the use of ultrasonic waves, wherein short ultrasonic wave pulses are intermittently passed upwardly through the liquid from an emitter (19) located at the bottom of the container(11) and echoes thereof due to reflection are passed back downwardly and received by a first receiver (20) placed close to said emitter (19) or combined therewith, and wherein the passage time of said wave pulses are measured, characterised in that said upwardly passing ultrasonic wave pulses are also, without intervening reflection, received directly by a plurality of second receivers (1, 2, 3, 4, 5) placed in fixed positions at different predetermined distances above the bottom of the container, and in that the time difference between the pulse emitting moment and the moment of reception by said first receiver (20) is compared to the time difference between the pulse emitting moment and the moments of reception by said second receivers, and in that said comparison is made the basis of a calculation of the location of the liquid level sought. Method according to claim 1, characterised in that the comparison between the time value of the passage of a reflected pulse and the time values of the simultaneous directly received wave pulses is made after first having divided the first-mentioned value by 2 or having multiplied the last-mentioned values by 2. An apparatus for determining the location of the level of one or more horizontal border faces of liquids in a container, e. g. a fuel tank, with the use of ultrasonic waves, comprising an emitter (19) and a first receiver (20) of intermittent pulses of ultrasonic waves, both placed at the bottom of the container (11), said first receiver being adapted for receiving downwardly passing wave pulses previously being reflected by a border faceof the liquid, said emitter (19) and said first receiver (20) being connected to an eletronic unit (23)for controlling the pulse emission moments and for receiving and measuring signals from said first receiver, characterised by a plurality of second receivers (1, 2, 3, 4, 5) placed at different fixed predetermined distances above the bottom of the container, said second receivers being arranged to receive upwardly passing non-reflected wave pulses, wherein said controlling and measuring electronic unit is connected also to said second receivers (1 - 5) and said unit comprises means arranged to compare the passage times of non-reflected pulses received from said second receivers to passage times of reflected pulses received by said first receiver, as well as means arranged to display the result of said comparison indicating the location of any reflecting liquid border face.

Inventors: ERIKSSON BROR ALLAN, ERIKSSON, BROR ALLAN
Applicants: ERIKSSON BROR ALLAN, ERIKSSON, BROR ALLAN
Application year: 1990
Main classification: G01F23
IPC codes: G01N29, G01K11, G01F23