The present invention relates to a device to determine a volume of fluid present in a tank or other container. More particularly, but not exclusively, it relates to a device to determine a volume of fuel present in a vehicle fuel tank, for example so as to compare volumes present before and after use, and to a method of use of such a device.

When a car, van or other vehicle is hired out, it is common for a hire agreement to specify that the vehicle should be returned with the same volume of fuel in its fuel tank as was present when it was driven away. Should there be a deficit, the hirer is liable to pay for the vehicle to be refuelled to the initial level.

However, in practice, it is difficult to assess exactly how much fuel is present in a vehicle fuel tank, using only the vehicle's dashboard fuel gauge, which is usually connected to a float sensor within the tank. The gauge can normally be read to no better than the nearest eighth of a tank-full, which will typically represent five to ten litres of fuel. It is thus possible to return a vehicle with significantly less fuel in its tank, without this being clearly indicated by its fuel gauge. Furthermore, in borderline situations, considerations such as good customer relations will often militate against arguing with a customer over whether a fuel gauge needle is nearer to three-eighths of a tank or a quarter of a tank, for example. It is estimated that such shortfall causes loss to vehicle hirers of around £250,000 per year in the United Kingdom alone. This could be extrapolated to an annual loss for the entire vehicle hire market of approximately £80 million. There is hence a need for a precise and unequivocal means of measuring how much fuel is present in a vehicle's tank, before and after use.

Such means should, however, preferably not involve modification to the vehicles themselves, on grounds of cost and inconvenience. It should be quick and easy to use, and must be safe for use in the presence of highly inflammable fuel vapours.

Conventional liquid level sensors would not be appropriate, since they would be significantly affected by the exact attitude of the vehicle to the horizontal when the measurements are taken. In any case, it is difficult to access the fuel itself from outside the tank. However, an approach that appears to have received little attention is to measure the volume of the air above the fuel in the tank. In practice, the volume of air above the fuel will include the volume of a filler pipe leading from a filler cap or the like to the fuel tank, so the term "tank" hereinafter should be understood also to comprise such piping. Within this application, the term "air space" is used to refer to the volume within a container (which may include liquid) which is not occupied by the liquid, and is not intended to be limited to a volume occupied by atmospheric air.

Vehicle fuel tanks will be produced to a standard size for any given model. Thus, the volume of fuel in the tank plus the volume of air above it will be constant (strictly speaking, only true at a constant temperature, but such considerations can be allowed for mathematically, or by calibrating at a range of temperatures). In principle, a change of air volume in the tank will indicate a change in fuel volume, and measuring air volume has the advantage that a direct connection with this volume can be established through an existing, unmodified filler cap. While a particular problem to be addressed is the measurement of fuel volumes within vehicle fuel tanks, as discussed above, there will be many other applications in which quick and accurate measurements of liquid volumes within substantially rigid containers will be required. A device suitable for measuring fuel volumes may well also be applicable to other such measurement needs. For example, such a device would be useful in the brewing industry for determining the amount of liquid in a tank or still.

It is hence an object of some embodiments of the present invention to produce a device to determine the volume of liquid present in a container by measuring the volume of gas in the container, and to provide a method for determining such volumes and changes in such volumes, using said apparatus.

However, some embodiments of the present invention are relevant to applications where it is not necessary to determine the absolute volume of the liquid present in the container. For example, for the purposes of determining whether a customer has brought a rental car back with same amount of fuel as when they rented the vehicle, it will be sufficient only to determine whether there has been a change in the volume of the liquid. By determining the amount of the change in the volume of air space, it is possible to determine the amount by which the volume of fuel has changed and issue a bill accordingly. As changes in the volume of liquid within a container of substantially fixed volume will result in corresponding and opposite changes in the volume of the air space within the container, measuring the change in the air space within the container is sufficient to determine whether the volume of liquid within the container has changed and/or the amount of the change, if appropriate.

According to a first aspect of the present invention, there is provided a device for determining the volume of air space within a container, comprising means to connect the device to the container, means to alter the gas pressure within the container, gas flow restrictor means, valve means to connect the container either to the pressure altering means or to the restrictor means, and means to determine the rate of pressure change within the container while the container is connected to the restrictor means.

The volume of air space within the container can be calculated from the determined rate of pressure change. Where the volume of the container is known, it is possible to determine the volume of liquid within a container by subtracting the air space from the volume of the container. Preferably, the device includes computing means (such as a computer) to calculate the volume of liquid in a container by subtracting the air space from the volume of the container.

There is also provided a device for determining whether the volume of air space within a container has changed between two readings, comprising means to connect the device to the container, means to alter the gas pressure within the container, gas flow restrictor means, valve means to connect the container either to the pressure altering means or to the restrictor means, and means to determine the rate of pressure change within the container while the container is connected to the restrictor means.

The device is also suitable for determining whether the volume of liquid within a container has changed between two readings as, provided that the container is of substantially fixed volume, if the volume of air space has not changed between two readings, then the volume of liquid should not have changed. Again, the volume of air space within the container can be calculated from the determined rate of pressure change.

There is also provided a device for determining the change in volume of air space within a container between two readings, comprising means to connect the device to the container, means to alter the gas pressure within the container, gas flow restrictor means, valve means to connect the container either to the pressure altering means or to the restrictor means, means to measure pressure within the container, and means to determine the rate of pressure change within the container while the container is connected to the restrictor means.

The device is also suitable for determining the change in volume of liquid within a container as this will be the opposite of the change in the volume of air space between two readings provided that the volume of the container is substantially fixed. Again, the volume of air space within the container can be calculated from the determined rate of pressure change.

The means to determine the rate of pressure change within the container while the container is connected to the restrictor means preferably includes means to measure pressure within the container (such as a pressure transducer) although it would be possible in principle to use a device which measures the rate of change of pressure, or the flow rate (such as the molecular or volumetric flow rate) of gas through the restrictor means. In these cases it would remain preferable for the device to comprise a means to measure the pressure within the container as the rate of pressure change will in general depend on the pressure within the container.

The means to measure pressure may measure absolute pressure, but preferably measures the pressure difference across the restrictor means. If the restrictor means opens out into ambient air, the pressure difference across the restrictor means will be the pressure difference between the inside of the container and ambient air. It is preferable to measure the pressure difference across the restrictor means because the pressure difference is the predominant factor determining the rate at which gas flows through the restrictor means and so the rate at which pressure tends to ambient pressure within the container. Preferably, therefore, the means to measure pressure within the container comprises a pressure transducer which measures the pressure difference across the restrictor means. The pressure difference across the restrictor means is generally the pressure difference between the interior of the container and ambient air. Preferably, absolute pressure is taken into account when calculating the volume of the air space (and thus the volume of the liquid, or change in volume of the liquid). This can improve the accuracy of the resulting measurements because the rate of pressure change depends in practice not just on the pressure difference across the restrictor means, but also the absolute pressure difference. Gases of different pressures, and thus different densities, can be expected to flow differently through the restrictor means. One skilled in the art will recognise that the absolute pressure which is taken into account could be the pressure within the container or the ambient pressure.

Thus, in a preferred embodiment, the device further includes means to measure absolute pressure, such as a pressure transducer. This pressure transducer may be a different pressure transducer to that used to measure the pressure within the container, or it might be the same pressure transducer. If it is the same pressure transducer, a valve may be engageable to connect the pressure transducer to the atmosphere. This valve may also constitute or be part of the valve means to connect the container either to the pressure altering means or to the restrictor means. For example, the valve may have three positions, one which connects the pressure transducer to the container, another which connects the pressure transducer to the gas flow restrictor means, and a third which connects the pressure transducer to the atmosphere.

The temperature of the gas within the container or ambient temperature (measured by a temperature transducer such as a thermometer or other temperature gauge) may also be taken into account when calculating the volume of the air space in the container from the rate of change of pressure within the container. However, the effect of temperature is less important than the effect of absolute pressure.

The device (or a separate computing device, if appropriate) may comprise means to determine the effect of absolute pressure (and optionally absolute temperature) on the rate of pressure change while the container is connected to the restrictor means. For example, the device may comprise calibration data (such as a calibration table) or an implementation of a computing algorithm which enables the data measured while the container is connected to the restrictor means (such as the rate of pressure change within the container, or the period taken for the pressure within the container to change from a first value to a second value, or the change in pressure within the container during a period of time) to be analysed taking into account the atmospheric pressure (and optionally the ambient temperature).

In a preferred embodiment, there is provided a memory storing a calibration table for the restrictor means which allow an estimate of instantaneous molecular flow rate to be computed from the pressure difference across the restrictor means, the ambient pressure (and optionally the ambient temperature).

The means to determine the rate of pressure change within the container may time a period taken for the pressure within the container to change from a first value to a second value while the container is connected to the restrictor means. The first and second values could be predetermined. However, the first and second values could be calculated by calculating means (such as a computer); for example, the first and second values could be calculated by analysing the change in pressure with time while the means to alter the gas pressure within the container alters the gas pressure within the container.

Alternatively, the means to determine the rate of pressure change within the container may determine the change in the pressure within the container during a period of time. The time period may start when the pressure within the container reaches a predetermined value. Again, the pressure may be absolute pressure but is preferably pressure relative to ambient pressure. The period of time may be calculated by analysing the change in pressure with time while the means to alter the gas pressure within the container alters the gas pressure within the container. The means to determine the rate of pressure change within the container may measure pressure within the container at a plurality of times and analyse the successive pressure values, preferably using calculating means, such as a computer.

The volume of air space within the container can be calculated from the rate of pressure change, the time period or pressure change, as appropriate. If the volume of the container is known, then the volume of liquid within the container can be calculated by subtracting the volume of the air space from the volume of the container. If two successive reading are taken, the change in volume of air space can be determined by storing data from the first reading and comparing it with data from the second reading.

One skilled in the art will recognise that it is not necessary for the device to actually calculate the volume of air space within the container. It is sufficient to determine a parameter which can be related to the volume of air space within the container: for example, the rate of pressure change, the period taken for the pressure within the container to change from the first value to the second value, and/or the pressure difference during a period of time. Such parameters can be related to the volume of air space within the container as and when required. It is possible to determine whether the amount of liquid within the container has changed, or the amount by which it has changed, between two readings without having to actually calculate the respective volumes of air space.

The device preferably comprises one or more of a display to display the determined volume or change in volume or parameter, a memory to store the determined volume, change in volume or parameter, or an interface to transmit the volume, change in volume or parameter to an external computer or storage device. Preferably, the device includes a computer to carry out any necessary calculations. The invention also extends to a system comprising one or more computers and a plurality of devices according to the present invention. Such a system would enable readings carried out at one location (such as a car rental depot from which a car is rented) to be compared with readings carried out at another location (such as a car rental deport to which a car is returned).

Further preferred features and options correspond to those described below.

According to a second aspect of the present invention, there is provided a method of measuring the volume of a liquid in a container of known volume comprising the steps of altering the pressure of gas within the interior of a container and allowing the pressure within the container to start equilibrating with air of another pressure through gas flow restrictor means whilst carrying out at least one measurement of the rate of change of pressure within the container, thereby calculating the volume of the container which is not occupied by the liquid and thereby calculating the volume of the liquid.

There is also provided a method of determining whether the volume of a liquid in a container of substantially fixed volume has changed between first and second readings, comprising the steps of, for each reading, altering the pressure of gas within the interior of the container and allowing the pressure within the container to start equilibrating with air of another pressure through gas flow restrictor means whilst carrying out at least one measurement of the rate of change of pressure within the container, thereby calculating the volume of the air space within the container, and then comparing the calculated air space volumes. (Air space refers to the volume of the container not occupied by liquid).

There is also provided a method of determining a change in the volume of a liquid in a container of substantially fixed volume between first and second readings, comprising the steps of, for each reading, altering the pressure gas within the interior of the container and allowing the pressure within the container to start equilibrating air of another pressure through gas flow restrictor means whilst carrying out at least one measurement of the rate of change of pressure within the container, thereby calculating the air space volume, and then calculating the difference in the air space volumes between readings.

Air of another pressure is preferably ambient air.

The provided methods preferably include the step of measuring either or both of absolute pressure (typically absolute ambient pressure) and ambient temperature and taking these measurements into account when calculating one or more of the volume of the air space, whether there has been a change in the volume of the air space, the amount of change in the volume of the air space, the volume of liquid in the container, whether there has been a change in the volume of liquid in the container or whether there has been a change in the volume of liquid in the container.

Values of air space volume or a parameter related thereto (e.g. the rate of pressure change, the period taken for the pressure to change from one value to another, or the pressure change in a particular period) may be stored between the first and second readings.

The step of measuring the rate of change of pressure within the container may comprise measuring the period taken for the pressure within the container to change from a first value to a second value while the container is connected to the restrictor means. The first and second values could be predetermined. However, the first and second values could be calculated; for example, the first and second values could be calculated by analysing the change in pressure with time while the means to alter the gas pressure within the container alters the gas pressure within the container.

Alternatively, the step of measuring the rate of change of pressure within the container may comprise determining the change in the pressure within the container during a period of time. The time period may start when the pressure within the container reaches a predetermined value. Again, the pressure may be absolute pressure but is preferably pressure relative to ambient pressure. The period of time may be calculated by analysing the change in pressure with time while the means to alter the gas pressure within the container alters the gas pressure within the container.

The step of measuring the rate of change of pressure within the container may comprise measuring the pressure within the container at a plurality of times and analysing the successive pressure values.

Analysis is preferably carried out using calculating means, such as a computer.

According to a third aspect of the present invention, there is provided a method of calculating a charge to be issued in relation to the rental of a vehicle having a fuel tank for containing fuel, the method comprising the steps of, on at least two occasions, determining the volume of air space in the fuel tank, or a parameter related thereto, thereby calculating the change in the amount of fuel between the occasions, and issuing a bill dependant on the calculated change in the amount of fuel.

The volume of air space, or a parameter related thereto, may be calculated using a device according to the first aspect or the fourth aspect. A different device may be used on each occasion.

The volume of air space, or a parameter related thereto, may be calculated according to a method of the second aspect or the fifth aspect.

According to a fourth aspect of the present invention, there is provided a device to measure a volume of liquid held within a container of known volume, comprising means to connect the device to the container, means to alter the gas pressure within the container, gas flow restrictor means, valve means to connect the container either to the pressure altering means or to the restrictor means, means to measure pressure within the container, and means to time a period taken for the pressure within the container to change from a first predetermined value to a second predetermined value while the container is connected to the restrictor means.

There is further provided means to measure the absolute value of pressure (either or both of ambient pressure or pressure within the container) and the device takes this measured value into account when determining the measured volume of liquid.

Preferably, the device is provided with display means to indicate said period.

Alternatively or additionally, the device may be provided with memory means adapted to record said period for future reference, or for subsequent transmission to a separate computing device.

Advantageously, said display means may comprise numeric or alphanumeric display means.

Optionally, said display means comprises a liquid crystal display.

Preferably, the device is provided with electronic control means.

Advantageously, said electronic control means is adapted to operate the valve means.

The electronic control means may be adapted to operate the valve means so as to connect the container to the restrictor means when the pressure in the container has been altered to a third preselected value, further than said first and second values from ambient pressure. The gas flow restrictor means preferably comprises calibrated orifice means through which gas may flow at a substantially constant rate.

The device may be provided with means to convert said period to a liquid volume within a particular container.

Said conversion means may comprise a set of graphs or tables of said periods against gas volume and/or liquid volume for particular preselected containers.

Alternatively or additionally, the device may be provided with memory means containing data to enable conversion of said periods to gas volumes and/or liquid volumes for particular preselected containers, and the electronic control means is adapted to perform said conversion.

The device may then be provided with display means adapted to indicate a calculated volume.

The pressure-altering means preferably comprises pump means.

Advantageously, the pump means is manually- or pedally-operable.

The pump means may be adapted to raise the pressure within the container above ambient pressure.

Alternatively, the pump means may be adapted to reduce the pressure within the container below ambient pressure.

The pressure-altering means may alternatively comprise a source of gas under pressure, such as a compressed-air line or reservoir vessel containing gas under pressure. The device may be provided with pressure release means adapted to operate at a pressure differential within the container below that which might damage the container.

The connecting means may be adapted to form a gas-tight connection with a range of different container apertures, for example fuel tank filler pipe openings of different models of vehicle.

Further features or alternative features may correspond to those discussed in relation to the first and second aspects above.

According to a fifth aspect of the present invention, there is provided a method for measuring a volume of liquid held within a container of fixed volume, comprising the steps of providing a device as described in the fourth aspect above, connecting it to an aperture of the container, connecting the pressurising means to the container and raising the pressure therein to above a first predetermined value, connecting the container to the gas flow restrictor means so as to allow gas from the container to exit therethrough, timing the period taken for the pressure within the container to fall from said first predetermined value to a second predetermined value, and calculating from said period a gas volume and hence a liquid volume within the container.

Preferably, the method further comprises the steps of subsequently measuring an altered liquid volume within the container as described above and calculating a change in liquid volume between said measurements.

Further or alternative steps may correspond to those discussed in relation to the second aspect above. According to a sixth aspect of the present invention there is provided a device to measure a volume of liquid held within a container of known volume, comprising means to connect the device to the container, means to alter the gas pressure within the container, gas flow restrictor means, valve means to connect the container either to the pressure altering means or to the restrictor means, means to measure pressure within the container, and measure the change in pressure within the container to change during a period while the container is connected to the restrictor means.

Further preferred features correspond to those discussed in relation to the fourth aspect above.

According to a seventh aspect of the present invention, there is provided a method for measuring a volume of liquid held within a container of fixed volume, comprising the steps of providing a device as described in the sixth aspect above, connecting it to an aperture of the container, connecting the pressurising means to the container and raising the pressure therein to above a first predetermined value, connecting the container to the gas flow restrictor means so as to allow gas from the container to exit therethrough, determining the pressure change within the container during a period and calculating from said pressure change a gas volume and hence a liquid volume within the container.

Further preferred features correspond to those discussed in relation to the fifth aspect above.

An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawing, in which:

Figure 1 is a frontal elevation of a fuel volume meter embodying the present invention; Figure 2 is a frontal elevation of a fuel volume meter according to a second embodiment of the present invention; and

Figure 3 is a frontal elevation of a fuel volume meter according to a third embodiment of the present invention.

Referring now to Figure 1, a fuel volume meter 1 comprises a pressuring pump 2 connected to a measurement unit 3, which is in turn linked by means of a flexible pressure hose 4 to a tank connector fitting 5. The tank connector fitting 5 is shaped to form a gas-tight seal with an external opening of a fuel filler pipe leading to a fuel tank of a vehicle. (Where a wide range of different sizes of and/or shapes of openings may be encountered, it may be necessary to provide alternative, exchangeable fittings 5).

The pressuring pump 2 is here a manually-operable stirrup pump, with a reciprocally vertically-moveable pump handle 6 and a stirrup base 7 into which a user may insert a foot in order to stabilize the meter 1 during pumping. Other embodiments may use foot-operated pumps, or even (at the expense of portability) an existing or dedicated compressed-air supply, via a suitable regulator. It would even be possible to use a regulated compressed gas cylinder, for example to provide a source of inert gas for use in connection with extremely flammable fuels or other liquids.

The measuring unit 3 contains a two-position valve 8. In a first position, represented by solid arrows 9, the valve 8 connects the pressurising pump 2 to the pressure hose 4, and hence to the fuel tank. In a second position, represented by broken arrows 10, the pressurising pump 2 is connected to open air, while the pressure hose 4 and fuel tank are connected to a restrictor 1 1 comprising a calibrated orifice through which air may exit the meter 1. A (first) pressure transducer 12 measures the air pressure within the pressure hose 4 (and hence within the fuel tank). The measuring unit 3 also contains electronic control apparatus (including a timing circuit), which is adapted to control the valve 8 and to receive data from the pressure transducer 12. The measuring unit 3 is also provided with a display 13, most conveniently a liquid crystal numeric or alphanumeric display, although light emitting diode or analogue displays may also be used. The electronic components are all encapsulated for safety in the presence of highly flammable fuel vapours. The portable meter 1 shown uses a low-voltage dry cell battery as power supply, which is also located within a gas-tight chamber.

To use the meter 1 , a filler cap is removed from a fuel filler pipe of a vehicle, and the fitting 5 is securely and sealingly connected to its external opening. The meter 1 is turned on, the valve 8 being it its first position 9. The user pumps the handle 6 of the pressurising pump 2, transferring air through the hose 4 to an interior of the fuel tank.

When the pressure within the fuel tank, the filler pipe and the hose 4 reaches 250 millibars above atmospheric pressure, as indicated by the pressure transducer 12, the electronic control apparatus switches the valve 8 from its first position 9 to its second position 10. The pressurising pump 2 is now connected to the atmosphere, so no more air can be pumped into the fuel tank, which is now linked, via the hose 4, to the restrictor 11. The slightly pressurised air within the fuel tank is now free to bleed out via the calibrated orifice of the restrictor 1 1, so that the pressure in the fuel tank, etc, begins to fall.

When the pressure transducer 12 registers a first pre-set pressure, for example 200 millibars, the timing circuit begins to run. Air continues to bleed out through the restrictor 11 until a second pre-set pressure is reached, for example 100 millibars, at which point the timing circuit stops. The display 13 indicates an elapsed time between reaching the first and second pre-set pressures, which the user may record. The pressure within the tank is then allowed to return to atmospheric pressure, and the fitting 5 is removed. In principle, the behaviour of a gas under pressure is governed by the ideal equation:

PV = nRT

where P is pressure, V is volume, T is temperature, R is the ideal gas constant, and n is the number of moles of gas present. In the course of the above pressurisation and depressurisation sequence, V is the free volume of the fuel tank above the fuel (which includes, as defined, the volume of the filler pipe and the hose 4), which will be substantially constant. Over the range of pressure changes envisaged, it may be assumed that the temperatures T will not vary appreciably, and R is an universal constant. Thus, the pressure is in effect directly related to n, the amount of gas present, and a change in pressure, ΔP, is directly related to a change in the amount of gas present, Δn:

ΔP = Δn.RT/V

Hence, the larger the value of V, the greater the value of Δn to give a particular value of ΔP.

The calibrated orifice in the restrictor 11 allows gas to escape at a relatively constant rate (for the overpressure ranges in question), i.e. it allows n to change at a substantially constant rate over time. That being said, the number of molecules flowing through the restrictor means and so the rate of pressure change will be a function of the pressure difference across the restrictor means, the absolute pressure, the temperature of the gas, the composition of the gas and so forth. Absolute pressure can be a significant factor. Temperature is less of a factor. Composition of the gas can also be a factor but if air is used, the variation in its composition will usually be minimal, unless perhaps the device is used with a particularly volatile liquid.

Thus, the time t, measured for a specific pressure drop to occur through the restrictor 1 1 is a measure of the amount of gas that has escaped to produce that pressure drop: Δn = k.t, where k is a constant .; ΔP = k.t.RT/V

or V = k.t.RT/ΔP Since k and R are constant, and T is effectively constant and ΔP is predetermined, the free volume V above the fuel in the tank is directly proportional to the time t.

In reality, gas behaviour tends to diverge from ideal gas behaviour, but in a repeatable fashion. Thus, a graph of V versus t may not be a straight line, but it is possible to produce a reliable calibration curve for any given standard fuel tank by part-filling it with a range of known volumes of liquid, and measuring t in each case.

Thus, one may use the meter 1 on a tank of known type containing an unknown volume of fuel, to obtain an accurate assessment of the free volume within the tank, and hence the volume of fuel.

This procedure may be carried out when a vehicle is hired out, repeated when the vehicle is returned, and any difference calculated. If there is a deficit, the hire company may charge the person who has hired the vehicle for the exact shortfall.

In its basic form, the meter 1 indicates only the time taken for the pressure to drop from a first to a second preset value, leaving the user to read off a fuel volume from a calibration curve for the particular vehicle model being used. However, it is envisaged that the meter could be provided with a memory chip or the like containing the calibration curves for a range of vehicles, and a touch-pad or the like allowing the user to select a particular model. The meter display 13 would then show a calculated fuel volume. Alternatively, the meter could record the time in its memory, and then be connected to a computer, over a standard RS232 connection or the like, in order to transfer this time data. In this case, the computer could hold the calibration curves in its memory and use them to calculate the fuel volume present. It would store the "as hired" fuel volume of each vehicle, compare it with the "as returned" fuel volume of that vehicle, and calculate any refuelling charges automatically.

The meter described will be safe in use, as its electrical components are isolated from any fuel vapour mixed with the air above the fuel in the vehicle's tank. Also, the overpressures used, approximately one-quarter of atmospheric pressure at most, will be well within the range of what a vehicle fuel tank is designed to withstand. If safety regulations require, a pressure relief valve, bursting disc, or the like can be provided to release excess pressure if the valve fails to operate at 250mbar as described above.

It should also be noted that while operation of the meter 1 is described above with the fuel tank, etc, being pressurised to a slight overpressure, it is equally possible to evacuate the tank partially, creating an underpressure that sucks air into the tank through the restrictor 1 1.

A second embodiment is illustrated in Figure 2. In this embodiment, the meter includes a second pressure transducer 14 which measures ambient pressure, and a memory 15 which stores a calibration table. The calibration table includes data relating the rate at which gas flows through the restrictor 1 1 to the pressure difference across the restrictor (i.e. the difference in pressure between the inside of the container and ambient pressure), the ambient pressure (and optionally the ambient temperature measured by a temperature transducer). A computer 16 calculates the volume of the fuel tank which is not occupied by liquid taking into account the data stored in the calibration data and the pressure values measured by first pressure transducer 12 and second pressure transducer 14. The computer uses the data in the calibration table to estimate the instantaneous molecular flow rate from the pressure difference across the restrictor and ambient pressure. (In a further embodiment not shown, the meter also includes a temperature transducer and the computer uses the measured temperature in its calculations). The instantaneous molecular flow rate is evaluated periodically during the measurement process and numerically integrated over time to give an estimate of Δn and thus V.

We have found that the instantaneous molecular flow rate through the restrictor 1 1 (and hence the rate of change of pressure within the container with time) does not depend only on the pressure measured by the first pressure transducer 12, but also on the absolute value of ambient pressure measured by the second transducer 14. Accordingly, the second embodiment should provide more accurate results than the first embodiment. The second pressure transducer 14 should measure absolute pressure. The first pressure transducer 12 could measure either absolute pressure or the pressure difference between the inside of the container and the ambient air.

A third embodiment is illustrated in Figure 3. In this embodiment, the valve 10 has three positions. The first two positions correspond to the two positions of the first embodiment. The third position connects the first pressure transducer 12 to the surrounding atmosphere. Calculations are carried out as with the second embodiment. An advantage of the third embodiment is that one less pressure transducer is required. This embodiment requires that the pressure transducer measures absolute pressure as it must be able to measure the pressure of ambient air.