WO2013162453A1

WO2013162453A1 - A liquid level measurement apparatus, arrangement and method using bubble measurement

Info

Publication number: WO2013162453A1
Application number: PCT/SE2013/050439
Authority: WO
Inventors: Carl DUMKY
Original assignee: Kockum Sonics Ab
Priority date: 2012-04-23
Filing date: 2013-04-23
Publication date: 2013-10-31

Abstract

A liquid level measuring apparatus (2a) comprising a controller (13) and a feeding conduit (8) arranged to be connected to a conduit (3) through a restrictor (11). The apparatus also comprises a pressure gauge (B1), wherein said pressure gauge (B1) is arranged to be connected to said conduit (3) and is arranged to measure a pressure (Pm) in said conduit (3). The controller (13) is configured to determine a liquid level (L) in a tank (4) having a height (H) based on said measured pressure (Pm). The feeding conduit (8) is further arranged to feed a gas flow at a feeding pressure (P1), which feeding pressure (P1) is more than or equal to twice a pressure (P2) corresponding to said tank (4) being completely filled, for providing a constant mass flow of said gas flow through said restrictor (11).

Description

A liquid level measurement apparatus, arrangement and method using bubble measurement.

TECHNICAL FIELD

This application relates to a method, an arrangement and an apparatus for improved level measurement for a liquid, and in particular to a method, an arrangement and an apparatus for improved level measurement for a liquid using bubble

measurement.

BACKGROUND

There are many circumstances where it is of importance to keep close track of the current level of a liquid in a container/tank or to measure the draught of a vessel. One example of such a circumstance is to measure the current level of a liquid in a tank, such as fuel oil in a bunker tank on an oil tanker, to avoid overfilling during a bunker operation. It will also enable to monitor the amount of received fuel oil, and to carefully monitor the fuel consumption during operation of the vessel. It is important to know the correct oil level when filling the tank so that the tank can be properly filled. If the oil or liquid level is incorrectly measured, it can cause that some of the liquid is spilt which can have dire consequences especially if the liquid in question is harmful or even poisonous. Further, levels are used to calculate volumes, weights, center of gravities, torsion, bending moments and free surface moments for carried liquids. These calculations may subsequently be used in stability and stress calculations to guarantee a safe operation of the vessel.

In many new built vessels today, manual sounding pipes are often omitted in favor for dual redundant automatic measurement systems. In this case the crew is completely dependent of a reliable automatic level gauging system.

Traditional systems for measurement of draught and levels in ballast tanks and service tanks measure the hydrostatic pressure, either by installing a pressure sensor in direct contact with the liquid inside or outside of the tank/hull, or by using an air purge system, where an air pipe is installed in the tank and compressed air is purged to the tank. The pressure is measured by a device in a cabinet located in a controlled environment often far away from the tank. The air purge systems are well proven technology and have been in use for decades in so called bubble measuring installations.

In bubble measuring a conduit, such as a pipe or a tube, is inserted into a liquid and a gas (such as air) is pumped into the conduit. When the gas pressure in the conduit becomes higher than the pressure in the liquid the gas will escape the conduit at a discharge end in a stream of bubbles, hence the name bubble measuring. It is thus possible to measure the pressure in the liquid by measuring the pressure in the conduit. Traditional bubble systems make use of constant volume flow regulators to generate the air flow to the tank. The density of the gas increase as the tank pressure increases, thus creating a larger mass flow as the tank fills up. For a 30m high tank, the mass flow

(Nl/min) will be approximately 4 times larger when the tank is full than when the tank is empty. A constant volume flow regulator is an assembled mechanical device comprises many parts, and will generally need periodic adjustment to maintain a preset constant volume flow during its lifespan. The preset volume flow generate a pressure drop along the pipe and if there is a drift in the flow, the calibrated pressure drop will no longer be valid resulting in erroneous readings.

In time-critical systems, such as control systems for ballast tanks, it is beneficial to have a constant flow so that level measurements may be provided at short time intervals or even continuously without the need for building up a gas flow pressure.

As to background art, WO97/08517 could be mentioned.

There is thus a need for an apparatus that is easy to maintain, more resistant to wear and tear, less prone to damage and can be manufactured at a reduced cost compared to prior art systems.

SUMMARY

It is an object of the teachings of this application to overcome the problems listed above by providing an apparatus, an arrangement and a method of performing bubble measurement utilizing a simple and elegant solution. The inventor has realized that, by utilizing the phenomena of super sonic gas flow an improved apparatus, an improved arrangement and an improved method are achieved. It is an object of the teachings of this application to overcome the problems listed above by providing a liquid level measuring apparatus comprising a controller and a feeding conduit arranged to be connected to a conduit through a restrictor. The apparatus also comprises a pressure gauge, wherein said pressure gauge is arranged to be connected to said conduit and is arranged to measure a pressure in said conduit. The controller is configured to determine a liquid level in a tank having a height based on said measured pressure. The feeding conduit is further arranged to feed a gas flow at a feeding pressure, which feeding pressure is more than or equal to twice a pressure corresponding to said tank being completely filled, for providing a constant mass flow of said gas flow through said restrictor.

It is also an object of the teachings of this application to overcome the problems listed above by providing a liquid level measurement arrangement comprising a liquid level measuring apparatus according to above and at least one conduit having a discharge end for insertion or otherwise mounting in a respective tank.

It is also an object of the teachings of this application to overcome the problems listed above by providing a restrictor to be used in a liquid level measurement arrangement according to above or in a level measuring apparatus according to above.

It is also an object of the teachings of this application to overcome the problems listed above by providing a method for measuring a level of a liquid said method comprising feeding a gas flow at a feeding pressure through conduit via a restrictor to a tank, measuring a pressure in said conduit, and determining a liquid level in said tank based on said measured pressure, wherein said feeding pressure is more than or equal to twice a pressure corresponding to said tank being completely filled, for providing a constant mass flow of said gas flow through said restrictor.

These and other objects, which will appear from the following description, have now been achieved by the technique set forth in the appended independent claims; certain embodiments being defined in the dependent claims.

In one aspect of the invention, the inventor of the teachings herein has thus been able to provide a solution to the long-standing problems listed above by realizing that by using a restrictor to create a supersonic flow a very simple and elegant solution is achieved solving or at least reducing all the problems listed above. This also provides a more cost-efficient alternative to prior solutions of performing bubble measurements. The simplicity of the solution provided for herein provides a level measurement arrangement that is easy and cheap to produce, to install and to maintain.

The teachings herein find use in level measuring for liquid containers such as oil tanks and waste disposal and also for other vessels transporting fluids. The teaching herein also find use in ballast tanks and draughts readings.

Other features and advantages of the disclosed embodiments will appear from the following detailed disclosure, from the attached claims as well as from the drawing

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the [element, device, component, means, step, etc]" are to be interpreted openly as referring to at least one instance of the element, device, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in further detail through exemplifying embodiments under reference to the accompanying drawings in which:

Figure 1 shows a view of a vessel incorporating a level measuring arrangement according to one embodiment of the teachings of this application;

Figure 2 shows a schematic view of the general structure of a level measuring arrangement according to one embodiment of the teachings of this application;

Figure 3 shows a schematic view of a general restrictor for use in a level measuring arrangement according to one embodiment of the teachings of this application;

Figure 4a shows a level measuring arrangement according to one embodiment of the teachings of this application;

Figure 4b, shows a graph of how the mass flow depends on the tank pressure; Figure 5 shows a graph of how a pressure drop depends on the tank pressure; Figure 6 shows a graph of how the pressure changes as the feeding pressure is shut off;

Figure 7 shows a flow chart of a general method according to one embodiment of the teachings of this application;

Figure 8 shows a flow chart of a method for determining a pressure drop in an arrangement according to one embodiment of the teachings of this application;

Figure 9 shows a schematic view of a control apparatus according to one embodiment of the teachings of this application; and

Figure 10 shows a schematic view of a computer-readable medium according to one embodiment of the teachings of this application.

DETAILED DESCRIPTION

The disclosed embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

Figure 1 shows a vessel, in this example an oil tanker 1, which is arranged with a level measuring arrangement 2 according to the teachings herein. For clarity purposes only the aft section of the oil tanker 1 is shown as is indicated by the broken lines. The oil tanker 1 carries at least one ballast wing tank 4. The level measuring arrangement 2 comprises a level measuring apparatus housed in a level measuring unit housing 2a which is arranged on the superstructure la of the oil tanker 1 and conduits 3 extend from the level measuring unit housing 22 to the respective tanks 4, one conduit 3 for each tank 4.

Figure 2 shows a schematic view of a level measuring arrangement 2 according to the teachings herein. The level measuring arrangement 2 comprises a unit housing 2a from which at least one conduit 3 extends to a tank 4. The conduit 3 can be implemented using tubes or pipes or a mixture of pipes and tubes. The exact

measurement and composition of the conduit 3 depends on the liquid levels to be measured and for what kind of liquid as would be apparent to a skilled person. The tank 4 is arranged to house a liquid 6 having a surface level 7. The unit housing 2a is arranged to direct an airflow (or another gas flow) through the at least one conduit 3 to the tank 4. As the pressure of the airflow in the conduit 3 exceeds the pressure of the liquid 6 in the tank 4 the airflow will escape the conduit 3 at a discharge end 3" in the form of (a stream of) bubbles 5. At this point the pressure in the conduit 3 corresponds to the hydrostatic pressure Ph at the discharge end 3". The total pressure P which is the sum of the hydrostatic pressure Ph and Patm at the discharge end 3" is given by the equation (1) below:

P = h * relative density to water + Patm, [mmH20] (1), wherein h is the height of the liquid 6 above the discharge end 3", and Patm is the atmospheric pressure.

A pressure gauge (not shown in figure 2, but referenced B l, B2, B3 in figure 3) is housed in the unit housing 2a and arranged to measure the pressure Pm in the conduit 3. This measured pressure Pm corresponds to the hydrostatic pressure Ph, but also comprises a pressure drop Pd, that is Pm = Ph + Pd. The pressure drop Pd is the pressure drop in the conduit 3 resulting from factors such as friction to name an example. The liquid level L for the liquid 6 in the tank 4 can thus be determined using equation (2): L = (Pm - Pd)/relative density to water + O (2), wherein O is the offset, that is the height at which the discharge end 3" is arranged in the tank. In one embodiment this height is 50 mm.

Figure 3 shows a schematic view of a level measuring arrangement 2 according to the teachings herein. At least one conduit 3 extends from a unit housing 2a to a respective tank 4 holding a liquid 6 for measuring the level 7 of the liquid 6. In the example embodiment of figure 3 three tanks 4a, 4b and 4c and three conduits 3 a, 3b and 3c are shown. However, as would be clear to a skilled person, the arrangement would be easily modified to a larger number of tanks. In one embodiment 16 to 20 tanks 4 are arranged to be measured. The unit housing 2a houses one restrictor 11 for each conduit 3. In one embodiment (not shown) one restrictor may be used to restrict the airflow for more than one conduit 3. The restrictor 11 is connected to a gas feed conduit 8 configured for feeding gas, such as air, at a pressure P. The air flow passes the restrictor 11 through the conduits 3 into the tanks 4. The unit housing 2a further houses a pressure gauge B l, B2 and B3 for each conduit which measure the pressure in each conduit 3a, 3b and 3c respectively. The pressure gauges Bl, B2 and B3 are arranged to detect the pressure and transform this into an electrical signal which is fed into a controller 13, such as a Programmable Logic Controller (PLC), that is configured to receive the measurement of the pressure in each tank and to determine a corresponding liquid level 7 in the tank 4.

Figure 4a shows a schematic view of a restrictor 11 according to the teachings herein. The restrictor 11 is shown as being a part of a conduit 3 having a feeding end 3' and a discharge end 3". For illustrative purposes the conduit 3 is not shown in completeness. A gas flow is directed through the conduit 3 via the feeding end 3' at a feeding pressure PI towards a restriction. The amount and the speed of the gas mass flow Q that flows through the restriction depends on the difference between the feeding pressure PI, the diameter of the restriction and the pressure P2 after the restriction and as the feeding pressure PI increases, the speed increases until it reaches a certain speed whereby a so called super sonic flow is established. This happens when the feeding pressure PI is twice or more than the pressure P2 on the discharge side. At this point the mass flow Q of gas that passes through the restriction, assuming that the restriction has a constant diameter, is constant independent of the pressure P2 in the tank, as long as P1>=2*P2. The mass flow, for a super sonic flow, through the orifice equals (a gas unit constant, K * temperature factor, f * the feeding pressure Pl)/(the resistance in the restrictor 11). This constant flow through the restrictor 11 is shown in figure 4b where the vertical axis shows the mass flow Q and the horizontal axis shows the hydrostatic pressure from the liquid. It clearly shows that the mass flow Q does not change as the pressure in the tank increases.

As has been discussed above the pressure P2 on the discharge end 3" equals the pressure P3 in the tank. The maximum possible pressure P2 on the outlet side thus depends on the height H of the tank (referenced 4 in figure 2), P2max = P3max = H * relative density to water + Patm. Thus, as long as PI is chosen to be larger than twice P2max there will be a constant mass flow through the restrictor.

The necessary feeding pressure of the gas flow to be fed to the restrictor 11 is dependent on the placement of the restrictor 11. If the restrictor is placed at a non- negligible distance from the discharge end 3", the pressure drop in the conduit 3 has to be taken into account by including it in P2max (P2max = P3max + pressure drop Pd in the conduit 3).

Returning to figure 3, an improved level measuring arrangement 2 is thereby provided by utilizing a restrictor 11 for providing a constant mass flow to be used for the bubble measuring. The simplicity of the restrictor 11 reduces the cost of the level measuring arrangement 2 and also makes it less prone to damages and wear and tear. It contains no movable parts and is not sensitive to vibrations which in turn make the arrangement more robust.

During calibration of the pressure drop, air flow to the tank 4 is stopped by closing a valve 9. A check valve 12 arranged adjacent to the restrictor 11 prevents leakage of air from the tank 4 to the supply side via the restrictor 11. The check valve 12 will thus also protect the restrictor 11 from impurities coming from the tank side.

As can be seen from equation (2), the pressure drop Pd in the conduit 3 is of importance to determine the level L of the liquid surface 7. The pressure drop is caused by the gas flow's friction against the inner walls of the conduit 3 and also the turbulence caused by the gas flow in conduit 3. Especially for an arrangement 2 having long conduits 3, such as an arrangement designed for an oil tanker (referenced 1 in figure 1) and where the pressure is measured at a distance from the tank 4 whose liquid level is to be measured (as is the case for an oil tanker where a unit housing is arranged on the superstructure), the pressure drop Pd is not negligible. The pressure drop represents the extra pressure that is needed to maintain a gas flow at a given pressure through the conduit 3. Furthermore, the pressure drop Pd is dependent on the pressure P of the gas flow as is shown in figure 5. Figure 5 shows the pressure drop Pd as a function of the pressure P in a conduit. The pressure and the pressure drop are given in the unit of meters of water (mH20). As can be seen the pressure drop Pd is inversely proportional against the pressure P. The relationship shown in figure 5 is based on an experimental model.

As the pressure drop is dependent on the friction against the conduit 3 walls, it is also dependent on the shape and condition of the conduits 3 and varies over time due to for example impurities being collected in the conduits 3. The pressure drop can be expressed as in equation (3):

Pd = K / P (3), wherein K is a constant and P is the absolute pressure in the conduit 3. K is dependent on the shape and composition of the inner walls of the conduits 3 and is thus unique for each tank installation and may change slowly over time due to for example build-up of impurities and moisture in the gas flow through the conduit 3 as mentioned above.

By utilizing a bubble measuring level measuring arrangement 2 according to the teachings herein it is possible to detect the pressure drop in a very simple manner. The detection is performed by simply shutting of the gas flow through the feeding conduit 8 whereby the feeding pressure (PI in figure 4a) and the pressure on the discharge side (P2 in figure 4a) will equalize to the hydrostatic pressure (P3 in figure 4a) in a short time as the gas flow comes to a halt, assuming that there is no or little liquid movement in a tank creating pressure changes at the discharge end 3".

Figure 6 shows an example graph of the resulting measured pressure Pm as a function of time T. At a shut-off time Tl the feeding conduit 8 is shut off and the gas flow comes to a halt. After a few seconds (at time T2) there will be a resulting drop in pressure as the gas flow comes to a halt and the pressure is no longer affected by the shape of the conduit 3 and other such factors. In figure 6 this pressure drop is indicated as Pd. In one embodiment the controller 13 (of figure 3) is configured to shut off the feeding conduit 8 through a valve 9 is controlled through a connection 10. The controller 13 is configured to determine the pressure drop of a conduit 3 by measuring the pressure in the conduit 3 at a first time Tl, shutting off the feeding gas flow and measure the pressure in the conduit 3 at a second later time T2. The time between the first and the second times T2-T1 is, in one embodiment, substantially in the range 2 to 3 seconds. The time between the first and the second times T2-T1 is in one embodiment substantially in the range 1 to 5 seconds. The time between the first and the second times T2-T1 is in one embodiment substantially in the range 1 to 10 seconds. The time between the measurements are chosen to be long enough so that the gas flow is allowed to come to a halt, but not too long so that leakage in the conduit influences the measurement. As can be seen in figure 6, the pressure continues to drop as a result of leakage in the conduit 3.

The pressure drop Pd is, as has been disclosed above, dependent on the working pressure, and by measuring the pressure drop at one or a few working pressures it is possible to extrapolate to estimate the pressure drop also at other working pressures. For example, using equation (3) the constant K can be determined through one measurement and then be used to estimate the pressure drop at other measured pressures and use the estimated pressure drop in equation (2) to provide a more accurate determination of the liquid level L. This enables a controller to calibrate the level measurements to take the pressure drop into account when determining the liquid level L. The controller 13 is further configured to routinely perform a calibration of the level measurement based on the pressure drop. The calibration can be performed on a daily basis, after specific events (such as emptying the tanks) or initiated by a user. Due to the short time span required for the calibration the calibration can be performed

automatically without disturbing the main operation.

If a further pressure measurement is taken at a still later time T3, an indication of the leakage can be determined. The leakage is also dependent on the working pressure (the higher the pressure, the more leakage). In one embodiment the controller 13 is further configured to measure the pressure at a third time T3 and to determine the air leakage of the conduit, based on the third pressure reading, and to calibrate the measurements accordingly. To be able to detect a proper reading of the pressure drop resulting from leakage the time difference between the first (or second) time and the third time T3, that is T3-T1 (or T3-T2) is in the range 20-60 seconds. In one

embodiment it is approximately 30 seconds. The leakage can be determined to be the difference between two pressure measurements, for example the pressure measurement at Tl and the pressure measurement at T3 or the pressure measurement at T2 and the pressure measurement at T3, divided by the time between the two pressure

measurements. This provides an estimation of the leakage per time unit at a certain pressure. The controller 13 is therefore configured to perform the leakage calibration at various working pressures and determine a lower pressure level where the leakage increases more slowly with increasing pressure (the remaining increase is due to leakage in the conduit 3 being in the tank 4). Should it be determined that such remaining leakage is negligible, the controller 13 is configured to calibrate the level measurements based on the leakage per time unit as determined through the two measurements. A calibration for leakage can be performed routinely or after a specific event or by manual request.

In one embodiment the controller 13 of the housing unit 2a is connected, or arranged to be connected, to an external controller, such as a computer (not shown) through an interface 14. In one embodiment the interface 14 is a universal serial field bus for distribution of relevant measurements to automatic monitoring and alarm systems. This enables for easy connection to a computer for remote control or monitoring of the level measuring arrangement 2 and the measurements made by the arrangement 2. In another embodiment the interface 14 is a wireless interface, for example according the wireless standard IEEE 802.11 or the Bluetooth™ standard. This further enables easy connection to a computer terminal as no additional cables need to be installed.

For illustrative purposes the description above does not make any distinction between the function of the controller 13 housed in the housing unit and the function of a remote controller. The controller 13 is therefore seen to include the capability of the remote controller, which capability can be called upon through the interface 14. In one embodiment the controller 13 is arranged to receive control commands, to execute them accordingly and sending pressure measurements (from the pressure gauges Bl, B2 and B3) via the interface 14. In such an embodiment the controller 13 need only be configured to operate the shut-off valve 9 and receive measurements from the pressure gauges Bl, B2 and B3. This reduces the complexity required for the controller 13. In such an embodiment the remote controller is configured to determine the level L of the liquid 6 in a tank 4 based on the pressure measurements provided by the pressure gauges Bl, B2 and B3 via the controller 13.

Other calculations may also be made by the controller 13, such as volume, weight, filling rate and automatic density calculations by utilizing two measurement points in the tank separated at a known distance. Giving two pressure readings for the calculation of density = (P upper - P_lower)/(dO), where P is the measured calibrated pressure and dO is the distance between the upper and lower measurement points.

In figure 3 the level measuring arrangement 2 is shown as having one shut-off valve 9 and one feeder conduit 8, however, it should be noted that the restrictors 11 may be grouped in groups each having a feeding conduit 8 and a shut-off valve 9 per group for easier installment. This also enables for calibration to be made for a subset of the tanks 4.

Figure 7 shows a flow chart of a general method according to an embodiment of the teachings herein. A feeding gas flow is established 710 at a first pressure PI . The gas flow is fed 720 through a restrictor to establish 730 a constant mass flow having a maximum pressure P2max for bubble measuring 740. The first pressure PI is more than twice as high as the maximum pressure for the constant mass flow P2max.

Figure 8 shows a flow chart of a method of determining the pressure drop in a conduit according to an embodiment of the teachings herein. The controller 13 measures 810 a first pressure PI and then shuts off the feeding gas flow 820. After a time period (for example 2-3 seconds) the controller measures 830 a second pressure P2 and determines the pressure drop Pd to be the difference between the first and the second pressure measurement, Pd = P1-P2. The controller 13 further determines 850 a pressure drop constant, K = Pd/Pl and calibrates 860 a liquid level measurement.

Figure 9 shows a schematic view of the general structure of a remote controller

913 for a level measuring arrangement (not shown in figure 8). The controller 913 may be implemented using instructions that enable hardware functionality, for example, by using executable computer program instructions in a general-purpose or special-purpose processor that may be stored on a computer readable storage medium (disk, memory etc) 915 to be executed by such a processor. The controller 913 is configured to read instructions from the memory 915 and execute these instructions to control the operation of the level measuring arrangement. The memory may be implemented using any commonly known technology for computer-readable memories such as ROM, RAM, SRAM, DRAM, CMOS, FLASH, DDR, SDRAM or some other memory technology. The remote controller 913 further comprises one or more applications 916. The applications are set of instructions that when executed by the controller 913 control the operation of the level measuring arrangement. The applications 916 may be stored on the memory 915. The remote controller 913 further comprises an interface 914 for communicating with another controller (not shown).

Figure 10 shows a schematic view of a computer-readable medium as described in the above. The computer-readable medium 100 is in this embodiment a CD (Compact Disc) or a DVD (Digital Video Disc). The CD 100 comprises instructions 101 that when loaded into a controller such as a processor executes a method or procedure according to the embodiments disclosed above. The CD 100 is arranged to be read by a reading device 102 for loading the instructions into the controller. It should be noted that a computer-readable medium can also be other mediums such as memory sticks, flash drives, hard drives or other memory technologies commonly used.

References to 'computer-readable storage medium', 'computer program product', 'tangibly embodied computer program' etc. or a 'controller', 'computer', 'processor' etc. should be understood to encompass not only computers having different architectures such as single /multi- processor architectures and sequential (Von

Neumann)/parallel architectures but also specialized circuits such as field

programmable gate arrays (FPGA), application specific circuits (ASIC), signal processing devices and other devices. References to computer program, instructions, code etc. should be understood to encompass software for a programmable processor or firmware such as, for example, the programmable content of a hardware device whether instructions for a processor, or configuration settings for a fixed-function device, gate array or programmable logic device etc.

One benefit of the teachings herein is that a level measuring arrangement can be provided in a very simple and elegant manner by utilizing a restrictor to establish a supersonic flow of gas in turn providing a constant mass flow of gas used for bubble measuring the liquid level in a tank. Another benefit is that a continuous measuring of liquid levels can be achieved. The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

Claims

1. A liquid level measuring apparatus comprising a controller (13), a feeding conduit (8) arranged to be connected to a conduit (3; 3a, 3b, 3c) through a restrictor (11) and a pressure gauge (Bl, B2, B3), wherein said pressure gauge (B l, B2, B3) is arranged to be connected to said conduit (3; 3a, 3b, 3c) and is arranged to measure a pressure (Pm) in said conduit (3; 3a, 3b, 3c), wherein said controller (13) is configured to determine a liquid level (L) in a tank (4) having a height (H) based on said measured pressure (Pm), wherein said feeding conduit (8) is further arranged to feed a gas flow at a feeding pressure (PI), which feeding pressure (PI) is more than or equal to twice a pressure (P2) corresponding to said tank (4) being completely filled, for providing a constant mass flow of said gas flow through said restrictor (11).

2. The liquid level measuring apparatus according to claim 1, further comprising a shut-off valve (9) being connected to the feeding conduit (8), wherein said controller (13) is further configured to:

perform a first pressure measurement in said conduit (3; 3a, 3b, 3c) at a first time (Tl);

shut off said gas flow;

perform a second pressure measurement in said conduit (3; 3a, 3b, 3c) at a second time (T2);

determine a pressure drop (Pd) in said conduit (3; 3a, 3b, 3c) based on said first pressure measurement and said second pressure measurement; and

base said determination of said liquid level (L) on said determined pressure drop (Pd) thereby providing a more accurate determination of said liquid level (L).

3. The liquid level measuring apparatus according to claim 2, wherein said controller (13) is further configured to determine a pressure drop constant (K) based on the determined pressure drop (Pd) and base said determination of said liquid level (L) on said pressure drop constant (K) by determine an estimation of a pressure drop (Pd) thereby calibrating said determination of said liquid level (L).

4. The liquid level measuring apparatus according to any one of the claims 1 to 3, wherein said second time (T2) is substantially 1 to 5 seconds after said first time (Tl).

5. The liquid level measuring apparatus according to any one of the claims 1 to 3 or 4 further comprising a shut-off valve (9) being connected to the feeding conduit (8), wherein said controller (13) is further configured to:

perform a first pressure measurement in said conduit (3; 3a, 3b, 3c) at a first time (Tl); and

shut off said gas flow, when dependent on claim 1, and wherein said controller (13) is further configured, for all claim dependencies, to:

perform a third pressure measurement in said conduit (3; 3a, 3b, 3c) at a third time (T3);

determine a leakage pressure drop in said conduit (3; 3a, 3b, 3c) based on said first pressure measurement and said third pressure measurement by dividing the difference between the third measurement and the first measurement with the difference between the third time (T3) and the first time (Tl) providing a leakage per time unit; and

base said determination of said liquid level (L) on said leakage per time unit thereby calibrating said determination of said liquid level (L) for leakage in said conduit (3).

6. The liquid level measuring apparatus according to claim 5, wherein said third time (T3) is substantially 20 to 40 seconds after said first time (Tl).

7. The liquid level measuring apparatus according to any of claims 1 to 6, wherein said controller (13) is configured to provide continuous determinations of said liquid level (L).

8. The liquid level measuring apparatus according to any of claims 1 to 7, further comprising an interface (14) for connection to a remote controller, wherein said controller (13) comprises capabilities of said remote controller.

9. A liquid level measurement arrangement comprising a liquid level measuring apparatus (2a) according to any of claims 1 to 8 and at least one conduit (3; 3a, 3b, 3c) having a discharge end (3") for insertion in a respective tank (4).

10. The liquid level measurement arrangement according to claim 9 to be used for determination of a level of oil in an oil tank (4).

11. The liquid level measurement arrangement according to claim 9 to be used for determination of a liquid level in a ballast tank (4).

12. Use of a restrictor (11) in a liquid level measurement arrangement (2) according to claims 9, 10 or 11 or in a level measuring apparatus (2a) according to any of claims 1 to 8.

13. A method for measuring a level of a liquid (6) said method comprising: feeding a gas flow at a feeding pressure (PI) through conduit (3; 3a, 3b, 3c) via a restrictor (11) to a tank (4);

measuring a pressure (Pm) in said conduit (3; 3a, 3b, 3c); and

determining a liquid level (L) in said tank (4) based on said measured pressure (Pm), wherein said feeding pressure (PI) is more than or equal to twice a pressure (P2) corresponding to said tank (4) being completely filled, for providing a constant mass flow of said gas flow through said restrictor (11).

14. The method according to claim 13 further comprising:

shut off said gas flow; perform a second pressure measurement in said conduit (3; 3a, 3b, 3c) at a second time (T2);

15. A computer readable storage medium encoded with instructions (101) that, when executed on a processor, causes the method according to claim 13 or 14 to be executed.