GB2533374A

GB2533374A - A gauge for indicating a height of a liquid

Info

Publication number: GB2533374A
Application number: GB1422603.9A
Authority: GB
Inventors: Kemp Mark
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2014-12-18
Filing date: 2014-12-18
Publication date: 2016-06-22

Abstract

A liquid level gauge 10 (fig. 1) for indicating a liquid height 42 in a container 40, comprises a gauge body 11 providing an internal cavity 12, with first 14 and second 15 ends, and at least one hole 18, 19, such that liquid 41 in the container flows into the cavity and the resulting height of the liquid indicates the height of the liquid in the container, a light transmitter 20 extends at least partially along the length of the cavity, to transmit light from a light source 21 into the internal cavity, and a light receiver 23 located towards the second end of the cavity, such that light transmitted between the height of the liquid in the cavity and the second end is received at the light receiver, and light is not received in between the height of the liquid in the cavity and the first end. The gauge may further comprise a float 30 comprising a substantially opaque portion to prevent light being transmitted between a first location below the level of the liquid and a second location above the level of the liquid. The invention also discloses a gauge 110 (fig. 2) for indicating a liquid density which comprises the same optical measurement system. In use the gauge body 111 is submerged in a liquid and the liquid fills the internal cavity of the gauge. A float 130 exerts force on a compressible member 131 which is dependent on the density of the liquid. The float comprises a substantially opaque portion and the position of the float is measured using the optical measuring system to provide indication of the fluid density.

Description

A GAUGE FOR INDICATING A HEIGHT OF A LIQUID

BACKGROUND OF THE INVENTION

[0001] The present invention concerns a gauge for indicating a height of a liquid in a container. More particularly, but not exclusively, this invention concerns a gauge comprising a gauge body.

[0002] The invention also concerns a gauge for indicating a density of a liquid in a container, a fuel tank, a gauge assembly, an aircraft, a method of indicating a height of a liquid in a container and a method of indicating a density of a liquid in a container.

[0003] There are various different gauges for measuring the height of a liquid in a container, for example fuel in a fuel tank. Many of these are electrical in nature and may, for example, measure the height of fuel in a fuel tank, based on the capacitance of the fuel in the fuel tank. However, such electrically based gauges may introduce electrical energy into the flammable liquid and may act as an electrical path into the fuel tank during a lightning strike. These present safety concerns that have to be addressed. In addition, these gauges also require a separate measure of the permittivity of the fuel and their accuracy can be affected by any water in the fuel and electromagnetic interference.

100041 Other gauges are optical in nature. For example, GB 893,502 discloses a liquid level gauge for a tank. The tank is provided with a transparent window on one wall. A user can look at the level of the fuel in the tank by looking through the window. The internal wall of the window can be provided with notches to help with the reading of the level of the fuel. However, this relies on the fuel being measured being substantially opaque (or at least of a different colour/darkness than the air above the fuel). It also relies upon a user having access to the window of the tank and, of course, the measurement is entirely manual.

[0005] Another example of an optical gauge is given in US2003/1 55538 which uses a reflection rod inserted into the fuel. The level of the fuel is measured by the amount of internal reflection in the rod. This relies upon knowing the refractive index of the fuel and -2 -requires re-calibration for different liquids/fuels. It can also only work for liquids with a relatively high refractive index.

100061 A third optical gauge example is described in US2003/0019292. Here, various internal reflection rods (of different lengths) are placed in a tank, each one having a light sensing device. The amount of light received by each sensing device depends on how much light has been internally reflected in the respective rod and this depends on whether or not the fuel has reached the end of the respective rod. Based on the results of the sensing devices of the different rods, the approximate level of the fuel (i.e. that the level of the fuel is between the ends of two rods) can be worked out. These gauges again rely on knowing the refractive index (with the measurement system being calibrated for that refractive index) and only works for fuels with a relatively high refractive index.

[0007] The present invention seeks to mitigate the above-mentioned problems.

Alternatively or additionally, the present invention seeks to provide an improved gauge for indicating a height of a liquid in a container. Alternatively or additionally, the present invention seeks to provide an improved gauge for indicating a density of a liquid in a container.

SUMMARY OF THE INVENTION

100081 The present invention, according to a first aspect, provides a gauge for indicating a height of a liquid in a container, the gauge comprising a gauge body, the gauge body providing an internal cavity, with a length extending from a first end to a second end, and at least one hole to allow liquid to flow from outside the gauge body to the internal cavity, such that, in use, when the gauge body is positioned in the container with one of the first and second ends being above the other end, liquid in the container can flow into the cavity through the at least one hole, and the resulting height of the liquid in between the first and second ends of the cavity indicates the height of the liquid in the container, a light source, a light transmitter extending at least partially along the length of the cavity, such that, in use, light from the light source is transmitted by the light transmitter into the internal cavity, and a light receiver located towards the second -3 -end of the cavity, such that, in use, light transmitted from the light transmitter between the height of the liquid in the cavity and the second end is received at the light receiver, and light is not received at the light receiver from the light transmitter in between the height of the liquid in the cavity and the first end.

[0009] Hence, the amount of light received by the light receiver is an indication of the amount of light transmitted between the height of the liquid in the cavity and the second end, which gives an indication of the length between the second end and the height of the liquid and hence, an indication of the height of the liquid in the cavity and therefore an indication of the level of liquid in the container at the location of the gauge body.

[0010] The present invention does not rely on reflection or refraction of light from the liquid and does not require knowledge of the permittivity of the liquid, and so the gauge can work for any liquid without any additional calibration. It also does not require any electrical inputs to the container (which could be a fuel tank).

100111 Preferably, substantially all the light transmitted from the light transmitter between the height of the liquid in the cavity and the second end is received at the light receiver. Preferably, substantially no light transmitted in between the height of the liquid in the cavity and the first end is received at the light receiver.

100121 Preferably, the gauge further comprises a float, contained in the cavity and able to move at least partially along the length of the cavity, such that, in use, the height of the float in between the first and second ends of the cavity is determined by the height of the liquid in between the first and second ends of the cavity, and wherein the float has a substantially opaque portion, such that, in use, the float prevents light being transmitted between a first location below the height of the liquid in the cavity and a second location above the height of the liquid in the cavity.

[0013] The gauge may be preferably arranged such that the second end is above the first end. In this case, the light received is transmitted above the height of the liquid in the container. In order to "block" out light transmitted below the height of the liquid in the container, either the liquid itself may be darker or more opaque than the air above it, or, if the liquid is substantially clear, the float may have a substantially opaque portion. -4 -

100141 Alternatively, the gauge may be arranged such that the first end is above the second end. In this case, the light received is transmitted below the height of the liquid in the container, and the liquid should be substantially clear. In order to "block" out light transmitted above the height of the liquid in the container, the float may have a substantially opaque portion.

[0015] Preferably, the substantially opaque portion of the float has a cross-section substantially that of the cavity such that, in use, the float prevents substantially all light being transmitted between a first location below the height of the liquid in the cavity and a second location above the height of the liquid in the cavity.

[0016] Preferably, the float is substantially spherical.

[0017] Preferably, there are at least two holes in the gauge body to allow liquid to flow into the internal cavity and simultaneously allow air displaced by the liquid to escape out of the internal cavity. More preferably, one hole is at the first end and another hole is at the second end.

100181 Preferably, the hole is sized or shaped so as to contain the float in the cavity.

This enables the hole to allow the liquid into the cavity whilst also retaining the float in the cavity. For example, if there are hole(s) on the sides of the gauge body, each hole is sized or shaped so as not to allow the float to move through the hole. Most preferably, there is a hole in the upper end of the cavity and a hole in the lower end of the cavity. Both holes may be sized smaller than the float. In this example, the float cannot float out of the upper hole or fall out of the lower hole. The upper and lower holes may be provided in end caps of the gauge body.

[0019] Preferably, the gauge body is substantially opaque such that light transmitted into the internal cavity is prevented from being transmitted through the gauge body. This is especially beneficial where there are other such gauges in other parts of the container. This prevents the light transmitted in one gauge from affecting the light received in another gauge.

100201 Preferably, the light transmitter is a substantially clear member. Alternatively, the light transmitter may be a mirror.

[0021] The light transmitter may be an elongate member. -5 -

100221 Preferably, the light transmitter is attached to an internal cavity wall of the gauge body. More preferably, the light transmitter is at least partially embedded into the internal cavity wall of the gauge body.

100231 Preferably, the light source and/or light receiver comprise a connection point for an optical fibre. This allows light to be supplied to the light source and/or received from the light receiver front/to a location remote from the container, without the use of electrical cable etc. This allows light to be supplied by electronics remote from the container and/or light received to be measured by electronics remote from the container.

[0024] Preferably, the connection point(s) comprises a substantially clear member for transmitting light to/from the optical fibre. The connection point(s) may comprise a clear lens (if the direction of the light does not need to be changed) or a clear prism or mirror (if the direction of the light does need to be changed). The direction of the light may need to be changed, due to the large radius of curvature required by optical fibre and the limited amount of space available, especially in a fuel tank.

100251 The present invention also provides a gauge, as described above, for use in a fuel tank.

100261 The present invention also provides a fuel tank comprising one or more gauges as described above. The gauges may be arranged in different locations in the container to give an overall indication of the amount of fuel in the tank. The cavity of each of the gauges may extend substantially from the bottom of the tank to the top of the tank at each respective gauge location in the tank.

[0027] The present invention also provides a gauge assembly comprising one or more gauges as described above and a first optical fibre connected at its first end to the light source and a second optical fibre connected at its first end to the light receiver.

[0028] Preferably, the gauge assembly further comprises a light supply at a second end of the first optical fibre and a light detector at a second end of the second optical fibre. The light supply may comprise an LED. The light detector may comprise a photodiode or a photoresistor.

100291 Preferably, the light detector is connected to a processor to convert the amount of light detected by the light detector to an indication of the height of the liquid in -6 -the container at the location of the gauge. The processor may require the input of the amount of light supplied.

100301 The present invention also provides an aircraft wing/fuselage or an aircraft comprising the gauge, fuel tank or gauge assembly, as described above.

[0031] The present invention also provides a method of indicating a height of a liquid in a container, the method comprising the steps of positioning a gauge body in the container, the gauge body having an internal cavity having a length extending from a first end to a second end, the gauge body being positioned such that one of the first and second ends is above the other end, allowing the liquid to flow into the internal cavity of the gauge body so that the resultant height of the liquid in between the first and second ends of the cavity indicates the height of the liquid in the container, transmitting light at least partially along the length of the internal cavity, such that, in use, light is emitted from the light emitter into the internal cavity, providing a light receiver located towards the second end of the cavity, allowing light transmitted between the height of the liquid in the cavity and the second end to be received at the light receiver, and preventing the light receiver from receiving light transmitted in between the height of the liquid in the cavity and the first end.

100321 Preferably, the method further comprises the step of detecting the amount of light received by the light receiver and using the detected amount to obtain an indication of the height of the liquid in the cavity.

[0033] The present invention, according to a second aspect, provides a gauge for indicating a density of a liquid in a container, the gauge comprising a gauge body, the gauge body providing an internal cavity, with a length extending from a first end to a second end, and at least one hole to allow liquid to flow from outside the gauge body to the internal cavity such that, in use, when the gauge body is submerged in the liquid in the container with one of the first and second ends being above the other end, liquid in the container can flow into the cavity through the at least one hole, and fill the cavity, a float of known volume contained in the cavity able to move at least partially along the length of the cavity, such that, in use, the height of the float in between the first and second ends of the cavity can vary, a compressible member contained towards one end in -7 -the cavity to provide a resistive force to movement of the float along the length of the cavity, a light source, a light transmitter extending at least partially along the length of the cavity, such that, in use, light from the light source is transmitted by the light transmitter into the internal cavity, and a light receiver located towards the second end of the cavity, such that, in use, light transmitted from the light transmitter between the height of the float in the cavity and the second end is received at the light receiver, and light is not received at the light receiver from the light transmitter in between the height of the float in the cavity and the first end.

[0034] Hence, the amount of light received by the light receiver is an indication of the amount of light transmitted between the height of the float in the cavity and the second end, which gives an indication of the length of the compressible member.

[0035] The compressible member provides a resistive force to movement of the float in the cavity. This could be a resistive force when the compressible member is compressed. In this case, the compressible member is located towards an upper end of the cavity and is compressed by the float (as the float will try to float to the top of the cavity). Alternatively, the compressible member provides a resistive force when it is expanded. In this case, the compressible member is located towards a lower end of the cavity and attached to the float, and is expanded by the float (as the float will try to float to the top of the cavity).

100361 The float may be of known effective density. In other words, either its density is known (if constant over its volume -for example if it is made out of a solid, non-hollow, homogenous material) or its effective density (given by its total mass / total enclosed volume) is known. The volume of the float is known.

[0037] Preferably, the length of the compressible member, and therefore the height of the float between the first and second ends of the cavity, is determined by the force exerted by the float on the compressible member, which is determined by the density of the liquid (for a known volume of float).

100381 The present invention does not rely on reflection or refraction of light from the liquid and does not require knowledge of the permittivity of the liquid, and so the -8 -gauge can work for any liquid without any additional calibration. It also does not require any electrical inputs to the container (which could be a fuel tank).

100391 Preferably, substantially all the light transmitted from the light transmitter between the height of the float in the cavity and the second end is received at the light receiver. Preferably, substantially no light transmitted in between the height of the float in the cavity and the first end is received at the light receiver.

[0040] Preferably, the compressible member is a spring, which is compressed, in use, between the float and an upper end of the cavity. The spring preferably substantially allows light to transmit though it from the light transmitter to the light receiver.

[0041] Preferably, the liquid is substantially clear.

[0042] Preferably, the float has a substantially opaque portion, such that, in use, the float prevents light being transmitted between a first location below the height of the float in the cavity and a second location above the height of the float in the cavity.

[0043] Preferably, the substantially opaque portion of the float has a cross-section substantially that of the cavity such that, in use, the float prevents substantially all light being transmitted between a first location below the height of the float in the cavity and a second location above the height of the float in the cavity.

100441 Preferably, the float is substantially spherical.

100451 Preferably, the hole is sized or shaped so as to contain the float in the cavity.

[0046] Preferably, the gauge body is substantially opaque such that light transmitted into the internal cavity is prevented from being transmitted through the gauge body. This is especially beneficial where there are other such gauges in other parts of the container. -9 -

This prevents the light transmitted in one gauge from affecting the light received in another gauge.

100471 Preferably, the light transmitter is a substantially clear member. Alternatively, the light transmitter may be a mirror.

[0048] The light transmitter may be an elongate member.

[0049] Preferably, the light transmitter is attached to an internal cavity wall of the gauge body. More preferably, the light transmitter is at least partially embedded into the internal cavity wall of the gauge body.

[0050] Preferably, the light source and/or light receiver comprise a connection point for an optical fibre. This allows light to be supplied to the light source and/or received from the light receiver from/to a location remote from the container, without the use of electrical cable etc. This allows light to be supplied by electronics remote from the container and/or light received to be measured by electronics remote from the container.

[0051] Preferably, the connection point(s) comprises a substantially clear member for transmitting light to/from the optical fibre. The connection point(s) may comprise a clear lens (if the direction of the light does not need to be changed) or a clear prism or moirror (if the direction of the light does need to be changed). The direction of the light may need to be changed, due to the large radius of curvature required by optical fibre and the limited amount of space available, especially in a fuel tank.

100521 The present invention also provides a gauge, as described above, for use in a fuel tank.

[0053] The present invention also provides a fuel tank comprising one or more gauges as described above. The gauges may be arranged in different locations in the container to give an overall indication of the amount of fuel in the tank. The cavity of one or more of the gauges may extend upwards a relatively short distance from near the bottom of the tank (compared to the length to the top of the tank) at each respective gauge location in the tank.

100541 The present invention also provides a gauge assembly comprising one or more gauges as described above and a first optical fibre connected at its first end to the light source and a second optical fibre connected at its first end to the light receiver.

100551 Preferably, the gauge assembly further comprises a light supply at a second end of the first optical fibre and a light detector at a second end of the second optical fibre. The light supply may comprise an LED. The light detector may comprise a photodiode or a photoresistor.

[0056] Preferably, the light detector is connected to a processor to convert the amount of light detected by the light detector to an indication of the height of the liquid in the container at the location of the gauge. The processor may require the input of the amount of light supplied.

[0057] The present invention also provides an aircraft wingluselage or an aircraft comprising the gauge, fuel tank or gauge assembly, as described above.

[0058] The present invention also provides a method of indicating a density of a liquid in a container, the method comprising the steps of submerging a gauge body in the liquid in the container, the gauge body having an internal cavity having a length extending from a first end to a second end, the gauge body being positioned such that one of the first and second ends is above the other end, allowing the liquid to flow into the internal cavity of the gauge body so that the resultant liquid fills the cavity, providing a float of known volume contained in the cavity and able to move at least partially along the length of the cavity, providing a compressible member contained towards one end in the cavity to provide a resistive force to movement of the float along the length of the cavity, allowing the float to at least partially compress or expand the compressible member, transmitting light at least partially along the length of the internal cavity, such that, in use, light is emitted from the light emitter into the internal cavity, providing a light receiver located towards the second end of the cavity, allowing light transmitted between the height of the float in the cavity and the second end to be received at the light receiver, and preventing the light receiver from receiving light transmitted in between the height of the float in the cavity and the first end.

[0059] Preferably, the method further comprises the step of detecting the amount of light received by the light receiver and using the detected amount to obtain an indication of the height of the float in the cavity, and therefore an indication of the (length of the

-I -

compressible member, and therefore the) force exerted on the compressible member by the submerged float and therefore an indication of the density of the liquid.

100601 It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa.

DESCRIPTION OF THE DRAWINGS

[0061] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: [0062] Figure 1 shows a side cut-away view of a fuel height gauge according to a first embodiment of the invention, in a fuel tank; [0063] Figure 2 shows a side cut-away view of a fuel density gauge according to a second embodiment of the invention, in a fuel tank; and 100641 Figure 3 shows an aircraft, including fuel tanks.

DETAILED DESCRIPTION

[0065] Figure 1 shows a side cut-away view of a fuel height gauge 10 according to a first embodiment of the invention, in a fuel tank 40. The fuel tank 40 has some fuel 41 in it and the dashed line indicates the fuel level 42 at the location of the gauge 10. The gauge 10 has a length extending substantially from the bottom of the tank to the top of the tank (at that location of the tank).

100661 The gauge 10 comprises a gauge body made of a black plastic tube 11 which is placed vertically in the tank 40. The tube 11 has an internal wall 13 and a central cylindrical cavity 12 extending the length of the tube 11. At the upper end 14 of the tube 11 there is an upper end cap 16 placed over the end of the tube to close the tube 11. The upper end cap 16 is provided with an upper hole 18 in it. This hole 18 has a diameter smaller than the diameter of the cavity 12. Similarly, at the lower end 15 of the tube 11 there is a lower end cap 17 placed over the end of the tube to close the tube 11. The lower end cap 17 is provided with a lower hole 19 in it. This hole 19 has a diameter smaller than the diameter of the cavity 12, and in this example, the diameter is the same as hole 18. The holes 18, 19 allow the fuel 41 to enter the cavity 12 and thus fill the cavity to the same level as the fuel tank 40.

[0067] Along almost the entire length of the cavity 12, there is a clear plastic rod 20 embedded in a groove in one side of the internal wall 13. This clear plastic rod 20 is able to transmit light into the internal cavity 12, as will be described later. At a lower end of the rod 20, there is a clear plastic right angled prism 21, also embedded in the side of the tube 11, with one right-angled side abutting the end of the rod 20 and the other right-angled side sitting flush with an external surface of the tube 11. At the external surface, the prism 21 is connected to an optical fibre 22. This optical fibre 22 is connected to an LED (not shown). Light from the LED is transmitted through the optical fibre 22 to the prism 21. The light is then reflected by the prism 21 by 90 degrees to be transmitted along the clear plastic rod 20. The clear plastic rod 20 emits the light along its length into the internal cavity.

100681 At the upper end of the cavity 12, at an opposite side to the clear plastic rod 20, is a clear plastic lens 23 embedded in the wall of the tube 11 so that one side of the lens lies flush with the internal wall 13 and the opposite side lies flush with the external wall. At the external surface, the lens 23 is connected to an optical fibre 24. This optical fibre 24 is connected to a photodiode (not shown). Light received at the lens 23 and transmitted through the optical fibre 24 is then received by the photodiode and the photodiode can measure the amount of light received.

[0069] In the cavity 12 is an opaque spherical float 30. The float 30 is less dense than the fuel 42 and so floats at the level of the fuel in the cavity. This corresponds to the level of the fuel 42 in the tank 40. Hence, the float 30 moves up and down in the cavity 12 depending on the level of the fuel 42 in the tank 40. The float 30 has a diameter slightly less than the diameter of the internal cavity 12 so can freely move up and down within the cavity 12. However, it has a diameter larger than the holes 18, 19 and so cannot escape out of the cavity 12.

100701 In use, the LED (not shown) is turned on and its light is transmitted by the optical fibre 22 to the prism 21. It is then reflected by the prism 21 and is transmitted along the clear plastic rod 20 and emitted out of the rod 20 into the internal cavity 12.

100711 Any light that is transmitted by the rod 20 above the level of the float is then transmitted through the air above the float and is received by the clear lens 23 and the optical fibre 24. This received light is then transmitted by the optical fibre 24 to the photodiode, where the amount of light received can be measured.

[0072] However, any light that is transmitted by the rod 20 below the level of the float cannot be transmitted above the float 30 as the float 30 is opaque (despite the fact that the fuel itself may be clear, as is the case for aircraft fuel, known as kerosene).

[0073] Hence, the amount of light measured by the photodiode is a measure of the length of rod 20 above the float 30 and hence a measure of the fuel level 42.

[0074] For example, if the fuel level 42 is low, the float 30 is also low, so a large amount of light is transmitted by the rod 20 above the float 30 and so a large amount of light is received by the optical fibre 24 and measured by the photodiode.

100751 If the fuel level 42 is high, the float 30 is also high, so a small amount of light is transmitted by the rod 20 above the float 30 and so a small amount of light is received by the optical fibre 24 and measured by the photodiode.

100761 The tank 40 may have many such gauges 10 at different locations to give an overall picture of the amount of fuel in the tank. For example, this is beneficial to take account of the angle of the aircraft or the geometry of the tank 40.

[0077] Figure 2 shows a side cut-away view of a fuel density gauge 110 according to a second embodiment of the invention, in a fuel tank 140. When describing Figure 2, similar elements (to Figure 1) will be represented by the same reference numeral preceded by "1".

[0078] The fuel tank 140 has some fuel 141 in it and the solid line at the top indicates the fuel level 142 at the location of the gauge 110. It is noted that the gauge 110 is completely submerged in the fuel 141 and is entirely below the level of the fuel 142.

100791 The gauge 110 comprises a gauge body made of a black plastic tube 111 which is placed vertically in the tank 140. The tube 1 1 1 has an internal wall 113 and a central cylindrical cavity 112 extending the length of the tube 111. At the upper end 114 of the tube 111 there is an upper end cap 116 placed over the end of the tube to close the tube 111. The upper end cap 116 is provided with an upper hole 118 in it. This hole 118 has a diameter smaller than the diameter of the cavity 112. Similarly, at the lower end 115 of the tube 111 there is a lower end cap 117 placed over the end of the tube to close the tube 111. The lower end cap 117 is provided with a lower hole 119 in it. This hole 119 has a diameter smaller than the diameter of the cavity 112, and in this example, the diameter is the same as hole 118. The holes 118, 119 allow the fuel 141 to enter the cavity 112 and thus fill the cavity 112.

[0080] Along almost the entire length of the cavity 112, there is a clear plastic rod embedded in a groove in one side of the internal wall 113. This clear plastic rod 120 is able to transmit light into the internal cavity 112, as will be described later. At a lower end of the rod 120, there is a clear plastic right angled prism 121, also embedded in the side of the tube III, with one side right-angled abutting the end of the rod 120 and the other right-angled side sitting flush with an external surface of the tube 111. At the external surface, the prism 121 is connected to an optical fibre 122. This optical fibre 122 is connected to an LED (not shown). Light from the LED is transmitted through the optical fibre 122 to the prism 121. The light is then reflected by the prism 121 by 90 degrees to be transmitted along the clear plastic rod 120. The clear plastic rod 120 emits the light along its length into the internal cavity 112.

[0081] At the upper end of the cavity 112, at an opposite side to the clear plastic rod 120, is a clear plastic lens 123 embedded in the wall of the tube 111 so that one side of the lens lies flush with the internal wall 113 and the opposite side lies flush with the external wall. At the external surface, the lens 123 is connected to an optical fibre 124. This optical fibre 124 is connected to a photodiode (not shown). Light received at the lens 123 and transmitted through the optical fibre 124 is then received by the photodiode and the photodiode can measure the amount of light received.

100821 In the cavity 112 is an opaque spherical float 130. The float 130 is less dense than the fuel 142 and its volume is accurately known. The float 130 has a diameter slightly less than the diameter of the internal cavity 112 so it would freely move up and down within the cavity 112. However, it has a diameter larger than the holes 118, 119 and so cannot escape out of the cavity 112.

100831 As the float is less dense than the fuel 142 it naturally would try to float to the top of the cavity 112. However, there is a compression spring 131 in between the float 130 and the top of the cavity 112. As the float 130 pushes upwards on the spring 131, the spring 131 is compressed by the force. Hence, the position of the float 130 in the cavity 112 depends on the amount of force it exerts on the spring 131. If the spring constant (k) of the spring 131 is known, for a given position of the float 130 (i.e. for a given length of spring 131 or a given amount of compression of the spring 131), the force exerted on the spring 131 by the float 130 can be calculated.

[0084] By Archimedes' principle, the amount of force the float 130 exerts upwards on the spring 131 is equal to the buoyancy force. This equals the weight of fuel 141 that is displaced by the float 130. Hence, by knowing the position of the float 130 in the cavity 112, the force exerted by the float 130 on the spring 13I can be calculated, and from this the weight of the fuel 141 displaced can be calculated, and finally, from this the density of the fuel can be calculated (if the volume of the float 130 is known).

100851 In use, the LED (not shown) is turned on and its light is transmitted by the optical fibre 122 to the prism 121. It is then reflected by the prism 121 and is transmitted along the clear plastic rod 120 and emitted out of the rod 120 into the internal cavity 112.

100861 Any light that is transmitted by the rod 120 above the level of the float is then transmitted through the fuel (past the spring 131) above the float 130 and is received by the clear lens 123 and the optical fibre 124. This received light is then transmitted by the optical fibre 124 to the photodiode, where the amount of light received can be measured.

[0087] However, any light that is transmitted by the rod 120 below the level of the float cannot be transmitted above the float 130 as the float 130 is opaque (despite the fact that the fuel itself may be clear, as is the case for aircraft fuel, known as kerosene).

[0088] Hence, the amount of light measured by the photodiode is a measure of the length of rod 120 above the float 130 and hence a measure of the fuel density can be obtained.

100891 For example, if the fuel 142 has a relatively high density, the weight of the displaced fuel 142 is relatively high and so the buoyancy force is relatively high and so the float 130 exerts a relatively large force on the spring 131 to compress it by a relatively large amount. This results in the float 130 being relatively high in the cavity 112 and so a relatively small amount of light is transmitted by the rod 120 above the float 130 and so a relatively small amount of light is received by the optical fibre 124 and measured by the photodiode.

[0090] If the fuel 142 has a relatively low density, the weight of the displaced fuel 142 is relatively low and so the buoyancy force is relatively low and so the float 130 exerts a relatively small force on the spring 131 to compress it by a relatively small amount. This results in the float 130 being relatively low in the cavity 112 and so a relatively large amount of light is transmitted by the rod 120 above the float 130 and so a relatively large amount of light is received by the optical fibre 124 and measured by the photodiode.

100911 The tank 140 may, but not usually, have a plurality of such gauges 110 at different locations and/or at different heights.

100921 Figure 3 shows an aircraft 300, with two wings 301, 302 and a fuel tank 303, 304 in each wing. Either or both of these fuel tanks 303, 304 (or any other fuel tank on the aircraft, for example in the fuselage) could be provided with one or more fuel height gauges 10 or fuel density gauges 110, as described in Figures 1 and 2.

[0093] Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described.

[0094] The photodiode may be replaced with any suitable light measuring apparatus, such as a photoresistor.

[0095] The clear plastic rod 20, 120 may be replaced with anything that transmits or emits light. For example, there could be an LED (or a series of LEDs) along a length of the cavity 12, 112. The rod 20, 120 may also be replaced by a different passive device, such as a mirror for reflecting light back into the cavity 12, 112.

100961 The invention applies to any suitable liquid, not just fuel.

100971 For the fuel height gauge, where the liquid is opaque (or at least sufficiently opaque), no opaque float is required for the gauge to work.

100981 For the fuel density gauge, the liquid must be sufficiently clear for light transmitted above the float 130 to reach the clear lens 123.

[0099] The fuel height gauge 10 may be used as a point level sensor. For example, the gauge may be of a short length so that over a small fuel height increase or decrease, the amount of light transmitted from the rod 20 to the clear lens 23 is substantially changed. For example, the point level sensor may establish when fuel has reached a level of A. At just below this level, substantially all the light from the rod 20 may be transmitted to the clear lens 23 and at level A substantially no light from the rod 20 may be transmitted to the clear lens 23. The point level sensor may also be used to indicate when fuel has reached a level below level B. At level B, substantially all the light from the rod 20 may be transmitted to the clear lens 23 and below level B substantially no light from the rod 20 may be transmitted to the clear lens 23.

1001001 There may be many such point level gauges at different heights in the tank, to give an overall picture of the height of the fuel in the tank. The gauges may also be located at different locations in the tank to give an overall picture of the amount of fuel in the tank. For example, this is beneficial to take account of the angle of the aircraft or the geometry of the tank 40.

[00101] The fuel density gauge 110 may instead have a compressible/biased member at the lower end 115 of the tube 111. The force exerted by the float 130 on the member may act to extend (rather than compress) the member.

[00102] Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to -1 8 -be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

CLAIMS1) A gauge for indicating a height of a liquid in a container, the gauge comprising; - a gauge body, the gauge body providing: -an internal cavity, with a length extending from a first end to a second end, and -at least one hole to allow liquid to flow from outside the gauge body to the internal cavity such that, in use, when the gauge body is positioned in the container with one of the first and second ends being above the other end, liquid in the container can flow into the cavity through the at least one hole, and the resulting height of the liquid in between the first and second ends of the cavity indicates the height of the liquid in the container, - a light source, - a light transmitter extending at least partially along the length of the cavity, such that, in use, light from the light source is transmitted by the light transmitter into the internal cavity, and - a light receiver located towards the second end of the cavity, such that, in use, light transmitted from the light transmitter between the height of the liquid in the cavity and the second end is received at the light receiver, and light is not received at the light receiver from the light transmitter in between the height of the liquid in the cavity and the first end.
2) A gauge for indicating a height of a liquid in a container, as claimed in claim 1, wherein the gauge further comprises a float, contained in the cavity and able to move at least partially along the length of the cavity, such that, in use, the height of the float in between the first and second ends of the cavity is determined by the height of the liquid in between the first and second ends of the cavity, and wherein the float has a substantially -20 -opaque portion, such that, in use, the float prevents light being transmitted between a first location below the height of the liquid in the cavity and a second location above the height of the liquid in the cavity.
3) A gauge for indicating a height of a liquid in a container, as claimed in claim 2, wherein the substantially opaque portion of the float has a cross-section substantially that of the cavity such that, in use, the float prevents substantially all light being transmitted between a first location below the height of the liquid in the cavity and a second location above the height of the liquid in the cavity.
4) A gauge for indicating a height of a liquid in a container, as claimed in claim 2 or claim 3, wherein the float is substantially spherical.
5) A gauge for indicating a height of a liquid in a container, as claimed in any of claims 2 to 4, wherein the hole is sized or shaped so as to contain the float in the cavity.
6) A gauge for indicating a height of a liquid in a container, as claimed in any preceding claim, wherein the gauge body is substantially opaque such that light transmitted into the internal cavity is prevented from being transmitted through the gauge body.
7) A gauge for indicating a height of a liquid in a container, as claimed in any preceding claim, wherein the light transmitter is a substantially clear member.
8) A gauge for indicating a height of a liquid in a container, as claimed in any preceding claim, wherein the light transmitter is attached to an internal cavity wall of the gauge body.
9) A gauge for indicating a height of a liquid in a container, as claimed in claim 8, wherein the light transmitter is at least partially embedded into the internal cavity wall of the gauge body.-21 - 10) A gauge for indicating a height of a liquid in a container, as claimed in any preceding claim, wherein the light source and/or light receiver comprises a connection point for an optical fibre.1 1) A fuel tank comprising one or more gauges according to any preceding claim.12) A gauge assembly comprising one or more gauges according to any of claims 1 to 10 and a first optical fibre connected at its first end to the light source and a second optical fibre connected at its first end to the light receiver.13) A gauge assembly as claimed in claim 12, wherein the gauge assembly further comprises a light supply at a second end of the first optical fibre and a light detector at a second end of the second optical fibre.14) An aircraft comprising the gauge, fuel tank or gauge assembly of any preceding claim.15) A method of indicating a height of a liquid in a container, the method comprising the steps of: - positioning a gauge body in the container, the gauge body having an internal cavity having a length extending from a first end to a second end, the gauge body being positioned such that one of the first and second ends is above the other end, - allowing the liquid to flow into the internal cavity of the gauge body so that the resultant height of the liquid in between the first and second ends of the cavity indicates the height of the liquid in the container, - transmitting light at least partially along the length of the internal cavity, such that, in use, light is emitted from the light emitter into the internal cavity, - providing a light receiver located towards the second end of the cavity, -22 - - allowing light transmitted between the height of the liquid in the cavity and the second end to be received at the light receiver, and - preventing the light receiver from receiving light transmitted in between the height of the liquid in the cavity and the first end.16) A method of indicating a height of a liquid in a container as claimed in claim 15, wherein the method further comprises the step of detecting the amount of light received by the light receiver and using the detected amount to obtain an indication of the height of the liquid in the cavity.17) A gauge for indicating a density of a liquid in a container, the gauge comprising; - a gauge body, the gauge body providing: -an internal cavity, with a length extending from a first end to a second end, and -at least one hole to allow liquid to flow from outside the gauge body to the internal cavity such that, in use, when the gauge body is submerged in the liquid in the container with one of the first and second ends being above the other end, liquid in the container can flow into the cavity through the at least one hole, and fill the cavity, - a float of known volume contained in the cavity able to move at least partially along the length of the cavity, such that, in use, the height of the float in between the first and second ends of the cavity can vary, - a compressible member contained towards one end in the cavity to provide a resistive force to movement of the float along the length of the cavity, - a light source, -23 - - a light transmitter extending at least partially along the length of the cavity, such that, in use, light from the light source is transmitted by the light transmitter into the internal cavity, and - a light receiver located towards the second end of the cavity, such that, in use, light transmitted from the light transmitter between the height of the float in the cavity and the second end is received at the light receiver, and light is not received at the light receiver from the light transmitter in between the height of the float in the cavity and the first end.18) A gauge for indicating a density of a liquid in a container, as claimed in claim 17, wherein the length of the compressible member, and therefore the height of the float between the first and second ends of the cavity, is determined by the force exerted by the float on the compressible member, which is determined by density of the liquid.19) A gauge for indicating a density of a liquid in a container, as claimed in claim 17 or claim 18, wherein the float has a substantially opaque portion, such that, in use, the float prevents light being transmitted between a first location below the height of the float in the cavity and a second location above the height of the float in the cavity.20) A gauge for indicating a density of a liquid in a container, as claimed in claim 19, wherein the substantially opaque portion of the float has a cross-section substantially that of the cavity such that, in use, the float prevents substantially all light being transmitted between a first location below the height of the float in the cavity and a second location above the height of the float in the cavity.21) A gauge for indicating a density of a liquid in a container, as claimed in any of claims 17 to 20, wherein the light transmitter is attached to an internal cavity wall of the gauge body.22) A fuel tank comprising one or more gauges according to any of claims 17 to 20.-24 - 23) A gauge assembly comprising one or more gauges according to any of claims 17 to 21 and a first optical fibre connected at its first end to the light source and a second optical fibre connected at its first end to the light receiver.24) An aircraft comprising the gauge, fuel tank or gauge assembly of any of claims 17 to 23.25) A method of indicating a density of a liquid in a container, the method comprising the steps of: - submerging a gauge body in the liquid in the container, the gauge body having an internal cavity having a length extending from a first end to a second end, the gauge body being positioned such that one of the first and second ends is above the other end, - allowing the liquid to flow into the internal cavity of the gauge body so that the resultant liquid fills the cavity, - providing a float of known volume contained in the cavity and able to move at least partially along the length of the cavity, - providing a compressible member contained towards one end in the cavity to provide a resistive force to movement of the float along the length of the cavity, - allowing the float to at least partially compress or expand the compressible member, - transmitting light at least partially along the length of the internal cavity, such that, in use, light is emitted from the light emitter into the internal cavity, - providing a light receiver located towards the second end of the cavity, - allowing light transmitted between the height of the float in the cavity and the second end to be received at the light receiver, and -25 - -preventing the light receiver from receiving light transmitted in between the height of the float in the cavity and the first end.26) A method of indicating a density of a liquid in a container as claimed in claim 25, wherein the method further comprises the step of detecting the amount of light received by the light receiver and using the detected amount to obtain an indication of the height of the float in the cavity, and therefore an indication of the force exerted on the compressible member by the float and therefore an indication of the density of the liquid.27) A gauge, gauge assembly, fuel tank, aircraft or method as substantially hereinbefore described with reference to the drawings.