Ilkka, Koivisto
Kimmo, Hovattala
Veli-matti, Heikkinen
Mikko
Ilkka, Koivisto
Kimmo, Hovattala
Veli-matti, Heikkinen
Mikko
| 1. | Procedure for the calibration of an apparatus measuring the level of a liquid in a container (1), which measuring apparatus uses a cylindrical float (5) placed in the container (1) and having a height at least nearly equal to that of the container (1) and a transducer unit (6) mounted in conjunction with the float (5) and provided with a control unit (13), and in which measuring apparatus the buoyant force acting on the float (5), which is equal to the weight of the amount of fuel displaced, is transmitted to the transducer unit (6) and the level (2) and amount of the liquid in the container (1) can be determined in the control unit (13) by means of said buoyant force, characterized in that the float (5) is provided with at least one point of discon¬ tinuity (AD) located within the range of variation of the liquid level (2), at which point the external diameter of the float changes, that the quantity of liquid removed from or supplied into the container (1 ) is measured independently of the liquid level measurement, and that, with the aid of the liquid quantity measurement inde¬ pendent of the liquid level measurement, at least one float discontinuity (h 'h ') determined during the liquid level measurement is caused to correspond to the real value (h h ) of the point of discontinuity. |
| 2. | Apparatus for the calibration of a measuring apparatus used for the measurement of the ] ~vel of a liquid in a con¬ tainer (1 ) , which measuring apparatus has a cylindrical float (5) placed in the container (1) and having a height at least nearly equal to that of the container (1) and a transducer unit (6) mounted in conjunction with the float (5) and provided with a control unit (13), and in which measuring apparatus the buoyant force acting on the float (5), which is equal to the weight of the amount of fuel displaced, is transmitted to the transducer unit (6) and the level (2) and amount of the liquid in the container (1) can be determined in the control unit (13) by means of said buoyant force, characterized in that the float (5) has at least one point of discontinuity (AD) located within the range of variation of the liquid level (2), at which point the external diameter of the float changes, that the apparatus comprises a device (19) for measuring liquid quantity, by means of which the quantity of liquid removed from or supplied into the container (1) is measured independently of the liquid level measurement, and that the control unit (13) causes, with the aid of the liquid quantity measurement independent of the liquid level measure¬ ment, at least one float discontinuity (hA'hD') determined during the liquid level measurement to correspond to the real value (h h ) of the point of discontinuity. |
| 3. | Apparatus according to claim 2 for the calibration of a measuring apparatus used for the measurement of the level of a liquid in a container ( 1 ) , characterized in that the float (5) has one or more discontinuities (14,15) which have a length substantially smaller than that of the float, in which said discontinuity points appear and which have an external diameter differing at least in some part from the external diameter of the rest of the float within the range of varia¬ tion of the liquid level. |
| 4. | Apparatus according to claim 2 , characterized in that the transducer unit (6) contains a force transducer (10) based on a vibrating vibratory element which can be caused to vibrate in accordance with a control signal received from the control unit (13) . |
The present invention relates to a procedure and apparatus for the calibration of a liquid measuring apparatus, as defined in the preamble of claim 1.
At present, only a fraction of the containers used for the storage of fuel at service stations are equipped with level transmitters providing information about the height of the liquid surface. There are several known solutions, e.g. the magnetostrictive sensor, which involves measurement of the position of a float, or the capacitive method (most commonly used), in which the height of the liquid surface is deter- mined by measuring the capacitance of different capacitors relative to each other. Apparatuses like this involve complex electronics and are not sufficiently reliable in operation because certain fuel additives cause disturbances especially in case of the capacitive method.
US patent specification 4 914 962 presents a transducer based on the use of a vibrating stripe, which can be used for the measurement of liquid level. The level transducer presened in this specification measures the liquid level by measuring the floating of a weight unit placed at a certain height in the container and partially submersed in the liquid. The frame of the transducer serves as an upper fixing point for a thin steel st-ipe and as a fixing point for a hinge connecting the frame to a lower support which acts as the lower fixing point of the stripe and at the same time as a fixing point for the supporter of the weight unit. Placed near the stripe is a unit consisting of a permanent magnet and a winding, causing the stripe to vibrate at its resonant frequency and to pro¬ duce an electric output signal for liquid level measurement. Attached to the supporter is a supporting cable to which the weight unit is attached. The weight unit is a cylindrical body having a constant diameter over the entire range of variation of the liquid level.
Using the float technique as described in the US patent referred to, a simple structure is achieved. In addition, the apparatus has a good ability to withstand the effects of fuel additives as the float can be manufactured from a material that is immune to the additives. The apparatus can be cali¬ brated by measuring the level of the liquid in the container by means of a rod and writing down the level height. After this, the level reading given by the apparatus is compared to the rod measurement. Other calibration methods can be used as well.
The use and manufacture of a float like this are simple, but such a solution is incapable of detecting the (small) error which results from the divergence of the specific weight of liquid fuels. For diesel fuels, the divergence is about ±0.6%, and for gasolines as much as ±1.5%. The divergence in specific weight may cause an error of as much as 1 - 2 cm in the liquid level measurement performed on the basis of buoyancy.
The object of the present invention is to improve the accur¬ acy of a previously known float apparatus designed for the measurement of liquids. In the calibration procedure of the invention, the outer surface of the float is provided with points of discontinuity at which the external diameter of the float changes. With the aid of these points of discontinuity, the measurement values determined by means of the float are adapted to the real mechanical measurements of the float in accordance with the claims presented below.
The discontinuous float of the invention makes it possible to observe real height points, which ensures that possible move¬ ments and other disturbances of the container can be detect- ed. Moreover, automatic calibration as provided by the inven¬ tion is easy to handle mathematically and it can be taken into account in the normal gradual emptying of the container following each filling. The same method can also be employed
in connection with the filling of the container by recording the measurement signal at regular intervals of time during filling and analyzing the curve upon completion of filling.
In the following, the invention is described in detail by the aid of an example by referring to the attached drawing, in which
Fig. 1 presents the apparatus of the invention for the meas- urement of the level of a liquid fuel in a liquid fuel tank.
Fig. 2 presents an adjustment curve in which the discontinui¬ ties are visible.
Fig. 1 shows a cylindrical liquid fuel tank 1 containing fuel to the height of level 2. Placed in the top part of the tank is a nozzle 3 for a measuring apparatus. Mounted in the nozzle 3 is a prot^. *;ive tube 4 extending to the bottom of the tank 1 and secured at its lower end. Placed inside the tube is a cylindrical float 5 with a closed bottom and extending nearly to the lower end of the protective tube 4.
Placed on top of the float 5 is a transducer unit 6 acting as a level transmitter. It comprises a round, thin and flexible metal film placed in a cover part 7 and stretched by its ^ outer edge on a flange 9 around the cover part 7, and a transducer 10 mounted on the cover part 7. The transducer contains a metal wire vibrating at a certain frequency inside a thin vertical metal tube and in itε middle a magnet unit attached to the metal tube, with a magnet inside the magnet unit. A transducer like this is e.g. the P-200 measuring head designed for the measurement of structural stress, manufac¬ tured by Geonor A/S. The transducer unit is also provided with a protective cap 11.
The float is subject to a buoyant force which is equal to the weight of the fuel displaced. The buoyancy is transmitted via the film 8 to the force measuring transducer 10. The trans-
ducer 10 is further connected via a control cable 12 to a control unit, which in the case of Fig. 1 is placed in a filling automaton 13.
When the apparatus is turned on, the wire in the tube starts ot vibrate in accordance with a signal received from the control unit. After a few seconds, the vibrations reach a constant amplitude and the wire continues vibrating for as long as the transducer is on. The transducer 10 outputs an alternating voltage of a frequency equal to the vibration frequency, and the control unit measures the frequency of the voltage. From this frequency, the liquid level in the tank is then determined.
For calibration, the float 5 is provided with two disconti¬ nuity areas 14 and 15, which are e.g. cylinders having an ex¬ ternal diameter somewhat larger than that of the float 5, one of which is placed at the middle of the float 5 in Fig. 1 and the other at some distance above the first one so that both are within the range of variation of the liquid level 2. Both discontinuity areas 14, 15 have a constant diameter. E.g. in the case of Fig. 1, the cylinders have a length equal to about 1/10 of the height of the float 5. Points of disconti¬ nuity A - D are thus formed at the upper and lower ends of the two cylinders, at which the external diameter of the float 5 changes in a discontinuous manner and whose heights h - h in the vertical direction (arrow 16) from the bottom (horizontal axis 17) of the tank 1 are known. The disconti¬ nuities A - D in the diameter are used as calibration points, with the aid of which the calibration curve determined by the measuring apparatus can be adjusted in the manner described below.
For example, in the case of a vibrating wire, the adjustment factor for the calibration curve -used in the measurement of liquid level can be a coefficient depending on the difference between the squares of the frequencies and on the quantity of liquid removed from the tank 1.
K= f-f -fS
AV
where K is the coefficient of adjustment, f and f are two successive frequency values and ΔV is the change in the liquid volume. Fig. 2 presents an adjustment curve 21 for the tank 1 in a coordinate system in which the vertical axis represents the coefficient of adjustment K and the horizontal axis the liquid level. The starting point of the horizontal axis represents the maximum fuel level hmax and its end the minimum fuel level hmin. As can be seen from the curve, the discontinuities appear as clearly larger values than those around them. At the points of discontinuity, the derivative of the calibration curve is discontinuous.
In the system of the invention, the change ΔV in liquid vol¬ ume is obtained independently of the liquid level measuring apparatus as feedback data from the measuring device through which the liquid removed from the tank flows. Such a measur¬ ing device can be e.g. a fuel flow meter 19 which is con¬ nected via a supply hose 18 to the tank 1 and from which the data is passed to the control unit of the automaton 13 via a cable 20.
With the system of the invention, the discontinuities on the calibration curve used in the liquid level measurement can.be accurately determined, when the liquid surface 2 has moved past them, by fitting the adjustment curve 21 with its dis¬ continuities h ' - h ', which are obtained e.g. from the derivative changes or from the equation presented above, so that they correspond to the real heights h - h of the dis¬ continuity points A - D. This can be performed mathematical- ly, e.g. by the method of the least sum of squares. It is to be noted that the real heig 3 hts hA„ - lvD of the discontinuities
14, 15 from the bottom of the tank 1 are always very precise¬ ly known on mechanical grounds.
It is obvious to a person skilled in the art that different embodiments of the invention are not restricted to the ex¬ ample described above, but that they may instead be varied within the scope of the following claims.