GB2043902A - Liquid Storage Tank with Measuring Device and Method for its Operation - Google Patents

Abstract

A liquid storage tank 38 is combined with a measuring device 10, 26, 28 arranged to measure either the quantity of liquid in the tank above a predetermined datum level or the change in the quantity of liquid in the combined system between two liquid levels. The measuring device comprises a body 28 suspended by a force transmission means 29 within a container 10 communicating with the storage tank 38. A gyroscopic load cell 26 measures the vertical force exerted on the body 28 and is calibrated to express the resultant vertical force as the appropriate quantity of liquid. A servo-operable inlet or outlet valve 37 to the storage tank 38 can be controlled by signals from the load cell 26 so that predetermined quantities of the liquid can be metered into or out of the storage tank 38. The apparatus may be used to calibrate dipsticks. <IMAGE>

Description

SPECIFICATION Liquid Storage Tank with Measuring Device and Methods for its Operation The present invention reiates to an improvement of the fluid measuring device disclosed in our U.K. Patent Application No.

41228/78 filed on 1 9th October 1978. The improvement enables the quantity, or change in quantity, of liquid in a large storage tank to be measured to a greater accuracy than has hitherto been practicable, and also provides fine control of predetermined volumes and weights of liquid delivered to or from such storage tanks.

Liquids are commonly stored in large storage tanks such as those used at refineries, chemical plant and gas stations. Liquids are often transported in very large volumes by bulk cargo vessels such as oil tankers and also by road tankers. Although such bulk cargo vessels and road tankers serve to transport liquids, they inherently comprise tanks of varying sizes in which the liquid is stored during transportation.

The size of such storage tanks makes it generally impracticable to measure the volume or weight of liquid present by conventional methods with sufficient accuracy. As a result the content of such storage tanks is usually assessed by means of a dip stick or by using a metering pump. The accuracy achieved by using a dip stick depends on the accuracy of its calibration and to some extent on the skill with which it is used. However it is inherently difficult to determine the exact fluid level in the dip stick due to surface tension effects, and conventionally calibrated dip sticks, even in highly skilled hands, only provide a rough measurement and are cumbersome and often inconvenient to use. Similar problems arise with manometers.

Metering pumps operate on the basis that each revolution or stroke of the pump displaces a specific volume of liquid. It is therefore possible to calculate the volume of liquid pumped into or discharged from a storage tank by multiplying the number of pump revolutions or strokes by the specific volume displaced. However the accuracy of this method is understood to be uncertain and is hardly ever better than 0.1%. The main reasons for inaccuracy being variations in the specific volume displaced, sometimes due to leakage within the pump, or to the compressibiiity of the liquid being handled. Furthermore, when small volumes are to be metered, the accuracy of a metering pump is limited to the nearest complete revolution or stroke.A metering pump essentially measures the volume of liquid handled and it is necessary to perform calculations if the weight is required. Furthermore, where a metering pump is employed, the rate of filling or emptying the storage tanks is limited by the delivery rate of the pump with the additional problem that the accuracy of a metering pump is adversely affected if its delivery rate is increased above its design speed.

In our aforesaid U.K. Patent Application we have taught an improved fluid measuring device which enables the weight of liquid in a container, or discharged into-or out of the container, to be measured exceptionally accurately by using a gyroscopic load celi. This load cell produces an electronic signal directly proportional to the weight of fluid and the signal can readily be modified electronically to convert the measured weight to the corresponding liquid volume.

Although this device could have its cylindrical body positioned directly inside a large storage tank, it would be a particularly time consuming task to calibrate the large tank to the accuracy attainable with the gyroscopic load cell, and it is also felt that, during the filling or emptying of the large storage tank, there would be substantial currents and surface waves which would necessitate delaying measurement until the liquid had been left to stand sufficiently long to achieve a substantially steady state.

The present invention is concerned with the provision of an improved device for measuring the quantity of liquid existing in or delivered to or from a large storage tank which mitigates the above disadvantages whilst enabling the achievement of exceptional accuracy.

The present invention or inventions reside in any feature or combination of features described herein and constituting an inventive step.

After taking into consideration the known equipment for measuring the quantity of liquid in storage tanks which has just been described, it is believed that the invention is the combination of a liquid storage tank with a measuring device, for measuring the quantity of liquid in the storage tank above a predetermined datum level, including a body positioned within a container communicating with the storage tank for receiving the liquid to a level corresponding to the level of the quantity of liquid in the storage tank, the body being immersed in the liquid to at least the predetermined datum level, a load cell capable of measuring an applied force without relative movement of the applied force, and a force transmission means retaining the body in a predetermined vertical position within the container and arranged to transmit to the load cell the resultant vertical force applied by the liquid displaced by the body above the predetermined datum level, the load cell being calibrated to express the resultant vertical force as the quantity of liquid in the storage tank. This combination enables the quantity of liquid stored at any level above the predetermined datum level to be measured accurately by the calibrated load cell.

Furthermore by computing the quantities stored before and after movements of liquid into or out of the storage tank, the quantity of liquid moved is measured.

It is also believed that the following optional features are so linked with the invention as to form a single inventive concept.

1. The load cell is calibrated by the ratio of the volume of the liquid in the storage tank above the predetermined datum level, to the volume of the liquid displaced by the body above the predetermined datum level.

2. The body is only immersed in the liquid to the predetermined datum level.

3, Liquid flow from the storage tank is controlled by a servo-operable outlet valve that is controlled by the load cell such that the servooperable outlet valve will close when the liquid in the storage tank drops to a preset level.

4. Liquid flow into the storage tank is controlled by a servo-operable inlet valve that is controlled by the load cell such that the servooperable inlet valve will close when the liquid in the storage tank rises to a preset level.

5. The combination is elaborated, to measure a change in the quantity of liquid stored when the liquid level changes from a first level to a second level, by arranging for the body to be immersed in the liquid to at least the lower of the first and second levels, by arranging for the force transmission means to retain the body in its predetermined vertical position within the container irrespective of the fluid level in the container between the first and second levels, by arranging for the force transmission means to transmit to the load cell the change in resultant vertical force applied by the liquid displaced by the body as the liquid level changes from the first level to the second level, and by calibrating the load cell to express the change in resultant force as the change in the quantity of liquid stored in the storage tank and communicating cylinder.

6. The load cell is calibrated by the ratio of the volume of the liquid in the combination between the first and second levels, to the volume of the liquid displaced by the body between the first and second levels.

7. The horizontal cross-sectional area between the container and the body is chosen to be very small in comparison with the horizontal crosssectional area of the storage tank, and the load cell is calibrated to express the change in resultant vertical force as the change in the quantity of liquid stored in the storage tank.

8. The servo-operable outlet valve is closed by the load cell when a predetermined quantity of the liquid has been discharged from the combination.

9. The servo-operable inlet valve is closed by the load cell when a predetermined quantity of the liquid has been received by the combination.

10. The load cell is a gyroscopic load cell.

11. The body is of constant cross-sectional area throughout its operational length.

12. The horizontal cross-sectional area of the storage tank varies with the liquid level and the load cell is calibrated to accomodate this variation.

13. A valve means is arranged to interrupt liquid communication between the storage tank and the container, and mixing means are provided for mixing the contents of the storage tank.

14. The combination is operated by closing the valve means to interrupt the communication between the storage tank and the container when the storage tank is substantially empty, introducing the liquid and at least one other substance into the storage tank, operating the mixing means to mix the liquid and the or each other substance within the storage tank, and subsequently restoring the communication between the storage tank and the container so that the load cell can then measure quantities of the resultant mixture.

1 5. The combination is operated to calibrate a dipstick for determining the quantity of liquid in the storage tank by introducing liquid into the storage tank in predetermined separate incremental quantities, causing the load cell to measure each separate incremental quantity as an exact depth of the liquid, marking each exact depth on the dipstick in the units of the incremental quantities, and subsequently disconnecting the measuring device permanently from the storage tank.

The invention is now further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a vertical section taken through a fluid measuring device of our aforesaid U.K.

Patent Application, the view being generally diagrammatic, and Figure 2 is a vertical section through the combination of the present invention comprising a liquid storage tank connected to a fluid measuring device similar to that shown in Figure 1.

In the drawings like characters or reference - numerals represent like or similar parts.

Referring to Figure 1 a liquid container 10 is provided with an inlet 11 controlled by a servo operable inlet valve 12 and with an outlet 13 controlled by a servo-operable outlet valve 14.

The servo-operable valves 12 and 14 are magnetically-operable in response to an electronic operating signal via respective control lines 1 5 and 1 6. The servo-operable valves 12 and 14 may alternatively be of any known type.

Inlet va!ve 12 controls the filling of the container 10 from an unshown storage tank through the inlet 11 and outlet valve 14 controls the emptying of the container 10 through the outlet 13 and a discharge conduit 1 7. The container 10 is also provided with an overflow 1 8 which controls the maximum liquid level within the container 10, any liquid passing through the overflow preferably being returned in any convenient manner to the storage tank. Conveniently the container 10 is of cylindrical form and is positioned so that its axis is vertical. The base 19 of the container 10 is consequently horizontal and is provided with a drainage channel 20 leading to a drainage passage 21 controlled by a stop-cock 22 which can be opened to enable liquid lying below the outlet 13 to be drained off. As shown by the cross-hatching, the base 1 9 is firmly supported by a rigid foundation or other rigid structure.

The top of the container 10 is open and communicates with the interior of a casing 23 which encloses a weighing linkage comprising a lever 24 pivoted from a supporting knife edge 25 carried by the casing 23. A gyroscopic load cell 26 of known construction is mounted on top of the casing 23. The construction may, for instance, be similar to that described in U.K. Patent 1468625 in which a gyroscope rotor is rotated by a motor at constant speed in an inner gyroscope gimbal that is pivoted in an outer gimbal supported for free rotation about its vertical axis.

The force to be measured is applied by a vertical rod 27 through a swivel to a lever which reacts between the inner and outer gimbals to cause a primary precessional motion directly proportional to the applied force. Inaccuracies due to secondary precession are eliminated by means of a sensing device which identifies the associated movement of the vertical rod 27 and causes an auxiliary motor to apply a compensatory torque to balance the secondary precession. This gyroscopic load cell is capable of making exceptionally fine measurements of applied downwardly acting forces with no relative movement of the vertical rod applying the force.

The rate of primary precession is measured electronically and results in a series of consecutive independent measurements of the applied force. Each of these measurements can then be compared electronically with the preceding measurement such that the accepted value of the force will only be given when consecutive measurements are of the same value.

The swivel connection between the vertical rod 27 and the lever reacting between the gyroscope gimbals is to ensure that the vertical rod 27 is not rotated by the primary precessional movement.

A cylindrical body 28 is positioned centrally within the container 10 with its axis vertical and is connected by a force transmission means, in the form of a vertical link 29, and a knife edge 30 to the lever 24. In this manner the body 28 is suspended within the container 10 by the vertical link 29 which serves to retain the body 28 in the vertical position illustrated irrespective of the liquid level in the container 10. The weight of the body 28 is chosen to be greater than the maximum weight of the liquid it displaces, that is when the liquid level coincides with the level of the overflow 18. To ensure that the body 28 remains vertical it is preferred to make a lower portion of considerably higher density than the remainder of the body 28.Instead of using the higher density lower portion, a tension spring could be connected, for instance by universal joints, between the bottom end of the body 28 and the container base 1 9 or a depression formed for the purpose in the container base 19.

The container 10 and the body 28 are carefully manufactured to ensure that their respective horizontal cross-sectional areas are constant throughout the operational range of the device, which range can extend between the levels of the outlet 13 and the overflow 1 8.

The maximum liquid level in the container 10 is preferably the level of the overflow 18 as illustrated although a lower level may be selected if so desired. A very fine length adjuster 31 is perferably incorporated in the vertical link 29 so that the level of the top end face 32 of the body 28 can be adjusted precisely relative to the maximum liquid level. Although the top end face 32 can be adjusted to be exactly level with the maximum liquid level, this may cause inaccurate readings due to the possibility of the top end face 32 becoming wetted with the liquid and due to the loss of any vertically directed surface tension force when the top end face 32 is exactly level with the maximum liquid level. For this reason it is preferred to adjust the level of the top end face 32 to be slightly above the maximum liquid level.

The weight of the body 28 is counterbalanced by a tare weight 33. As the gyroscopic load cell 26 only measures downwardly acting forces, this is achieved when the liquid is at the maximum set level when it is arranged for the moments of the body 28 and of the tare weight 33 about the knife edge 25 to be in equilibrium. To facilitate fine adjustment of this equilibrium during installation, the tare weight may be mounted for fine adjustment axially along the lever 24. The moment of the surface tension acting on the body 28 can also be balanced at the same time. It will be noted that the vertical load applying rod 27 for the gyroscopic load cell 26 is connected to the lever 24 through a further knife edge 34.The equilibrium position at the maximum set fluid level can therefore be determined by adjusting the axial position of the tare weight 33 until the load cell 26 gives a zero reading.

If other types of load cell capable of measuring an applied force without relative movement of the force are used, the equilibrium position could alternatively be achieved when the body 28 lies wholly or mainly above the liquid level. In this connection it is considered best for the bottom end face 35 to be immersed in the liquid so that surface tension forces can again be counterbalanced. This alternative basis for equilibrium could also be used for a gyroscopic load cell if its load applying rod 27 were repositioned between the knife edge 25 and the tare weight 33, or some other means were used for reversing the direction of the applied force on the load applying rod 27.

In order to maximise the operational range, the bottom end face 35 is preferably at least level with the level of outlet 1 3 and the discharge conduit 1 7 is designed so that liquid can drain away to this level.

Variations in the liquid level in the container 10 cause corresponding variations in the volume of liquid displaced by the body 28, and the force applied by the vertical link 29, through the lever 24 and the vertical rod 27, to the gyroscopic load cell 26 is proportional to the weight of the liquid displaced by the body 28. It will of course be appreciated that the volume of liquid displaced by the body 28 between two liquid levels is proportional to the change in volume of the liquid in the container 10, the ratio being constant provided the cross-sectional areas of the body 28 and the container 10 are constant.The gyroscopic load cell is preferably arranged to feed its signals to a calculator or a microprocessor which has been fed with the constants necessary to express changes in the force applied to the gyroscopic load cell as the change in weight, or volume, of the liquid in the container 10.

In the case where the zero datum is the maximum liquid level, the device is calibrated by filling the container 10 through the inlet valve 12 until the liquid reaches the maximum liquid level.

The tare weight 33 is then adjusted as previously described to give a zero reading. Liquid is then removed through the outlet valve 14 down to the immersion depth H and the removed liquid is carefully collected and its exact weight is established using a second balance. Unless the load cell 26 is correctly calibrated, its display will now indicate an incorrect weight for the removed liquid. The knife edge 30 is now adjusted axially along the lever 24 to alter the leverage ratio until the display of the load cell 26 gives the correct weight. This adjustment of the knife edge 30 will slightly disturb the counterbalancing by the tare weight 33 and it will be necessary to refill the container to the maximum liquid level and readjust the tare weight to give a zero reading again. The process is again repeated until no further adjustment of the knife edge 30 is required.Liquid is then removed step-by-step at regular intervals and at each point the corresponding weight given by the display of the load cell 26 is checked by the second balance.

The full operational range of the device can be confirmed in this manner. This calibration is useful when the device is for measuring the quantity of liquid delivered through the outlet valve 14.

However, if the device is to measure the quantity of liquid delivered into the container through the inlet valve 12, the same procedure can be followed except that the maximum dischargeabl8 quantity of liquid is set on the display of the load cell when the liquid is at the maximum level, the display being arranged to run backwards as the liquid is removed for comparison on the second balance. Conversely calibration may be effected from the minimum liquid level. Calibration may also be checked over a range of temperatures and if there is any temperature sensitivity, the operational temperature may either be controlled or also fed to the microprocessor to make the appropriate compensation.

As the cross-sectional area of the body 28 remains constant throughout the operational range of the device, it will be noted that the vertical force exerted by the surface tension of the liquid on the body will remain constant at any given temperature thereby eliminating any surface tension error. The ability of the load cell to operate without relative movement of the applied force is of course most important as any movement of the applied force would cause a corresponding vertical movement of the body 28 and would falsify the result accordingly.

Although the device described so far is arranged to measure the weight of fluid received or discharged or existing in the container 10, this quantity may alternatively be expressed as a volume by appropriately providing for the load cell display to be calibrated to take account of the liquid's density.

The electronics of the load cell display may also be provided with a floating zero facility so that the device will measure a quantity of liquid received or delivered from any starting point within the operational range of the device.

The electronics of the load cell may additionally be calibrated or programmed to give control signals for the control lines 1 5 and 1 6 to control the opening and closing of the valves 1 2 and/or 14 thereby causing the device to meter predetermined quantities of fluid by weight or by volume.

Although other forms of load cell could be utilised provided that they were capable of measuring an applied force without relative movement, particular advantages are gained by selecting a gyroscopic load cell. Apart from the exceptional accuracy of gyroscopic load cells, their ability to make a continuous series of independent force measurements and to compare consecutive measurements is useful. The independent nature of each measurement provides a fundamentally high certainty which is ideaily suited for the dispensing of fluids. When liquids are moved rapidly, there is invariably surfacedisturbance such as ripples which tend to falsify the result.The series of independent measurements taken by a gyroscopic load cell can therefore be used either to detect when such disturbances have settled to a value which does not distort consecutive measurements or can, better still, be integrated electronically to give a much earlier estimate of the mean measurement.

Other forms of load cell which could be used include pressure cells requiring minimal strokes for their operation. Although such pressure cells could be used for some applications, their accuracy would seem to be inferior in several respects to gyroscopic load cells. Also weighing cells with an unchanging equilibrium position could conceivably be used.

Instead of being heavier than the maximum volume of fluid displaced, the body could be lighter.

In the event that inflammable or explosive liquids are to be measured, the gyroscopic load cell 26 can be purged with air or an inert gas at a pressure slightly above atmospheric pressure to avoid any fire or explosion risk.

If desired the cross-sectional area of the container 10 and the body 28 may both vary with the fluid level. For instance the body may consist of a thin disk shaped so that its cross-sectional area at any level within the operational range of the device has a fixed proportion to the cross-sectional area of the container at the same level, the exact relationship being determined by their respective profiles and the careful setting of their relative vertical positions, by the operation of the very fine length adjuster.

As the ratio is constant, the effects of the two shapes cancel each other out. However, shapes of non-constant ratio could alternatively be used and the electronics of the quantity display unit driven by the load cell would then be calibrated accordingly.

It will be appreciated that the fluid measuring device just described with reference to Figure 1 is inherently suitable for measuring the quantity of fluid in the container 10, the magnitude of the quantity measured being dependant on the capacity of the container 10 after deducting the volume occupied by the body 28. If the fluid measuring device is for dispensing exact quantities of fluid, it may be necessary to refill the container 10 through the inlet valve 12 many times when a large volume of fluid is to be dispensed. Although a large volume of fluid could be measured very accurately in this way, the time for delivering the fluid would inevitably be extended by the refilling cycles.

The combination illustrated in Figure 2 incorporates the fluid measuring device that has been described with reference to Figure 1 with the principal modification that the servo-operable inlet and outlet valves 12 and 14 are primarily used only when the fluid measuring device is being calibrated. For this reason the gyroscopic load cell 26 does not need to control their operation and the control lines 1 5 and 1 6 are omitted. Instead the gyroscopic load cell 26 is connected by a control line 36 to a further servooperable valve 37 controlling the flow of liquid into and out of a large storage tank 38 which communicates through a pipe 39 with the servooperable inlet valve 12 of the fluid measuring device.The servo-operable valve 37 is preferably magnetically-operable in response to an electronic operating signal via its control line 36 but may be of any known type. The storage tank 38 is of course firmly supported by a rigid foundation or other rigid structure, as indicated by the cross hatching, and is provided with a roof 40 which overlaps the side walls of the tank to provide protected ventilation. In this manner changes of the liquid level in the storage tank 38 will cause no variation in the air pressure acting on the liquid surface.

The servo-operable inlet valve 12 is usually left fully open so that the liquid will have identical levels in the container 10 and the storage tank 38. On the other hand the servo-operable outlet valve 14 and the stop cock 22 are usually kept fully closed so that the liquid can only enter and leave the cylinder 10 and storage tank 38 through the servo-operable valve 37.

The load cell 26 continually measures the resultant vertical force applied by the liquid displaced by the body 28, but is calibrated to express the resultant vertical force as the corresponding quantity of liquid available in the system. In this manner, when the quantity of liquid available increases or decreases, the load cell 26 measures the corresponding change in the resultant vertical force and is arranged to express this either as the new total quantity of liquid available in the system or as the quantity of liquid received or delivered by the system.

Where: W0=the weight of the body 28 when no liquid is available from the system.

Wx=the weight of the body 28 when the maximum quantity of liquid is available in the system.

Vx=the maximum volume of liquid available when the'system is full.

q=the constant cross-sectional area of the body 28.

Hx=the immersion depth of the body 28 when the system is full.

Wt=the total weight of liquid available when the system is full.

Then: Vx Wt=(WoWx) Equation 1 q Hx When the storage tank has a constant crosssectional area Qt as with the storage tank 38 illustrated, and the container 10 has a constant cross-sectional area Q, the maximum volume of liquid available when the system is full Vx=Qt. H+(Q-q).H Or V,=(Q,+Q--q) Hx Hx Thus: (Q+Q-q) . Hx Wt=(WOWX) .

q. Hx Or (Q+Q-q) Wt=(WoWx) q Considering a change h of overall liquid level from an immersion depth h, to an immersion depth h2.

Where: Wh,=the weight of the body 28 when the immersion depth is h, Wh2=the weight of the body 28 when the immersion depth is h2 Wh=the weight of liquid received or delivered to cause the liquid level to change from immersion depth h, to immersion depth h2.

Then: (Q+Q-q) Wh=(Wh,Wh2) ' q It will be noted that (Wh,Wh2) is the change in resultant vertical force measured exceptionally accurately by the load cell 26, and also that (Qt+Q-q) q is constant and can conveniently be used for calibrating the load cell 26 via its associated calculator or microprocessor.

Where the radial gap between the body 28 and the container 10 becomes small compared with the cross-sactional area Qt of the storage tank 38, the calibration constant tends towards Ot q The constant Q,, 0 and q are readily determined and it will be noted that the combination taught by Figure 2 has the particular advantage that the only variable for determining the weight of liquid present, delivered or received is the resultant vertical force on the body 28 and this is measured rapidly with an exceptional degree of accuracy by a gyroscopic load cell which produces a digital signal.The calibration of the load cell would therefore be generally as set out with reference to Figure 1 to ensure the appropriate degree of accuracy throughout its operating range but additionally taking into account the calibration constant either directly or indirectly via an associated calculator or microprocessor.

The combination of storage tank 38 and container 10 is preferably arranged so that the load cell 26 can measure all quantities of liquid available or present. In the event that there is any ullage which cannot conveniently be measured directly, this can readily be allowed for.

Although Figure 2 illustrates the use of a storage tank conveniently of constant cross sectional area Qt, the invention can readily be applied to storage tanks where cross-sectional area changes with the liquid level.

Where: Wh,=the weight of the body 28 when the immersion depth is H, Wh2=the weight of the body 28 when the immersion depth is h2 Wh,2=the weight of liquid received or delivered to cause the liquid level to change from immersion depth h, to immersion depth h2 Vh,2=the volume of liquid between the immersion depths h, and h2 Then Equation 1 becomes: Vh12 Wh,2=(Wh1 Wh2) ' q. (h1-h2) In this equation the term Vh,2 is a variable represented by the following equation:-

Where: K=a constant for the particular storage tank f(h)=a function for the cross-sectional area which changes with the variable liquid level H.

The digital signals from the gyroscopic load cell may readily be calibrated to take full account of this situation by suitably programming the associated calculator or microprocessor.

In this manner it is, by way of example, possible to cater for a spherical tank of radius r:

a cylindrical tank of length L and radius r:

a horizontal tank of length L and elliptical crosssection where a is the major elliptical axis:

In addition to measuring the instantaneous quantity of liquid available, or the quantity of liquid received by or delivered from the system, the load cell 26 can be preset to detect when a predetermined quantity of liquid has been received by or delivered from the system, and to control the operation of the servo-operable valve 37, via the control line 36, to restrict the actual receipt or delivery of liquid to the predetermined quantity.In this connection the storage tank 38 may be supplied with a separate servo-operable valve, controlled through an appropriate control line from the load cell 26, for controlling flow of liquid into the storage tank 38, the servo-operable valve 37 being reserved for use in controlling the delivery of liquid from the storage tank.

If desired a single cylinder 10 and associated load cell 26 may be connectable to a series of separate storage tanks through respective valves 12 whereby the contents of any of the tanks can be controlled by the single unit through appropriate manipulation of the valves 12.

The invention may also be applied to storage tanks which are used for mixing a liquid with other substances, such as other liquids or powders. in this event the apparatus of Figure 2 could be operated by closing the servo-operable valve 12 before the storage tank 38 was filled, opening the servo-operable valve 37 (or any other valve or inlet provided for the purpose) to introduce the liquid and the other substance or substances. The contents of the storage tank 38 would then be mixed thoroughly to form a homogenous liquid or liquid suspension. The servo-operable valve 1 2 would then be opened so that the container 10 would then receive liquid or liquid suspension of the same density as exists in the storage tank 38 thereby enabling accurate quantities to be dispensed through the servooperable valve 37.

The apparatus shown in Figure 2 may also be used to calibrate dip sticks to a hich degree of accuracy. The measuring device comprising the container 10 and the load cell 26 would be temporarily connected via their inlet 11 to the storage tank requiring a calibrated dip stick. The dip stick would have a fixed datum relative to the storage tank so that it would always occupy the same position and attitude when within its storage tank. The dip stick would be calibrated entirely whilst it was outside its storage tank, the load cell being used to measure a series of separate incremental quantities of liquid introduced into the storage tank. In this manner the load cell would be used to compute the exact depth of the liquid in the storage tank for each incremental quantity of the liquid, and these depths would be marked on the dip stick with reference to its fixed datum, the marking being of course in the same units as the incremental quantities. In addition to enabling very precise calibration of the dip stick, this method has obvious advantages over standard techniques when the dip stick is for use with noxious liquids.

Claims (18)

Claims

1. The combination of a liquid storage tank with a measuring device, for measuring the quantity of liquid in the storage tank above a predetermined datum level, including a body positioned within a container communicating with the storage tank for receiving the liquid to a level corresponding to the level of the quantity of liquid in the storage tank, the body being immersed in the liquid to at least the predetermined datum level, a load cell capable of measuring an applied force without relative movement of the applied force, and a force transmission means retaining the body in a predetermined vertical position within the container and arranged to transmit to the load cell the resultant vertical force applied by the liquid displaced by the body above the predetermined datum level, the load cell being calibrated to express the resultant vertical force as the quantity of liquid in the storage tank.

2. The combination, according to Claim 1, in which the load cell is calibrated by the ratio of the volume of the liquid in the storage tank above the predetermined datum level, to the volume of the liquid displaced by the body above the predetermined datum level.

3. The combination, according to Claim 1 or 2, in which the body is only immersed in the liquid to the predetermined datum level.

4. The combination, according to any preceding claim, in which a servo-operable outlet valve controls liquid flow from the storage tank and is controlled by the load cell such that the servo-operable outlet valve will close when the liquid in the storage tank drops to a preset level.

5. The combination, according to any preceding claim, in which a servo-operable inlet valve controls liquid flow into the storage tank and is controlled by the load cell such that the servo-operable inlet valve will close when the liquid in the storage tank rises to a preset level.

6. The combination of a liquid storage tank having a measuring device, for measuring a change in the quantity of liquid stored when the liquid level changes from a first level to a second level, including a body positioned within a container communicating with the storage tank for receiving the liquid to the same level as the liquid in the storage tank, the body being immersed in the liquid to at least the lower of the first and second levels, a load cell capable of measuring an applied force without relative movement of the applied force, and a force transmission means retaining the body in a predetermined vertical position within the container irrespective of the fluid level in the container between the first and second levels, the force transmission means being arranged to transmit to the load cell the change in resultant vertical force applied by the liquid displaced by the body as the liquid level changes from the first level to the second level, the load cell being calibrated to express the change in resultant vertical force as the change in the quantity of liquid stored in the storage tank and communicating container.

7. The combination, according to Claim 6, in which the load cell is calibrated by the ratio of the volume of the liquid in the combination between the first and second levels, to the volume of the liquid displaced by the body between the first and second levels.

8. The combination, according to Claim 6, in which the horizontal cross-sectional area between the container and the body is chosen to be very small in comparison with the horizontal cross-sectional area of the storage tank, and the load cell is calibrated to express the change in resultant vertical force as the change in the quantity of liquid stored in the storage tank.

9. The. combination, according to any of claims 6 to 8, in which a servo-operable outlet valve controls liquid flow from the combination and is controlled by the load cell such that the servooperable outlet valve will close when a predetermined quantity of the liquid has been discharged from the combination.

10. The combination, according to any of claims 6 to 9, in which a servo-operable inlet valve controls liquid flow into the combination and is controlled by the load cell such that the servo-operable inlet valve will close when a predetermined quantity of the liquid has been received by the combination.

11. The combination, according to any preceding claim, in which the load cell is a gyroscopic load cell.

12. The combination, according to any preceding claim, in which the body is of constant cross-sectional area throughout its operational length.

13. The combination, according to any preceding claim, in which the horizontal crosssectional area of the storage tank varies with the liquid level and the load cell is calibrated to accommodate this variation.

14. The combination, according to any preceding claim, in which a valve means is arranged to interrupt liquid communication between the storage tank and the container, and mixing means are provided for mixing the contents of the storage tank.

1 5. The method of operating the combination as claimed in Claim 14 including operating the valve means to interrupt the communication between the storage tank and the container when the storage tank is substantially empty, introducing the liquid and at least one other substance into the storage tank, operating the mixing means to mix the liquid and the or each other substance within the storage tank, and subsequently restoring the communication between the storage tank and the container so that the load cell can then measure quantities of the resultant mixture.

16. The combination of a liquid storage tank and a measuring device substantially as described herein and as shown in the accompanying drawings.

17. The method of operating the combination as claimed in any of claims 1 to 13 or 16 including introducing liquid into the storage tank in predetermined separate incremental quantities, using the load cell to measure each of the separate incremental quantities as an exact depth of the liquid, marking each exact depth on a dipstick for the storage tank but expressed in the units of the incremental quantities, and subsequently disconnecting the measuring device permanently from the storage tank, the dipstick then being used for determining the quantity of liquid in the storage tank.

18. A storage tank having a dipstick calibrated by the method of claim 17.