GB2549482A

GB2549482A - Charge measurement apparatus and method

Info

Publication number: GB2549482A
Application number: GB1606724.1A
Authority: GB
Inventors: Mills Richard; Ogilvy Jill; Hearn Graham; Phillips Rhys
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2016-04-18
Filing date: 2016-04-18
Publication date: 2017-10-25

Abstract

A conductor 10 is immersed in a volume of fluid (6, figure 1) at a first location and an electrometer 12 is electrically connected to the conductor 10 for a time period. An electrical property of the conductor 10, measured by the electrometer 12, is used to determine a local charge of the fluid (6, figure 1) at the location. An independent claim is included in which a control unit 3 is configured to connect the electrometer 12 and conductor 10, measure the electrical property of the conductor 10 and determine the local charge. The apparatus 1 and method may include a further conductor, connected to a further electrometer, for immersion at a second location in the volume of fluid (6, figure 1). The control unit 3 may determine a charge distribution in the fluid (6, figure 1). The electrical property may be electrostatic potential or electrical charge. The fluid (6, figure 1) may comprise aircraft fuel such as aviation gasoline or jet fuel.

Description

CHARGE MEASUREMENT APPARATUS AND METHOD

TECHNICAL FIELD

[0001] The present invention relates to an apparatus and method for measuring local charge at one or more locations in a volume of fluid, and particularly but not exclusively, to an apparatus and method for measuring local charge at one or more locations in a volume of aircraft fuel in an aircraft fuel tank.

BACKGROUND

[0002] When liquid hydrocarbons or mixtures of hydrocarbons (such as fuels) flow in a pipe, it is known that the electrical double layer formed at the solid-liquid interface can be sheared and charge can be entrained in the flowing fluid. Ordinarily, the charge relaxes by recombination of positive and negative charges. However, sometimes charge is accumulated at a rate faster than the rate of recombination, resulting in the fluid becoming electrostatically charged.

[0003] The build-up of static charges in fuels is affected by many factors, including filters, separators, the presence of water and chemical additives in the fuel, overhead splash filling of fuel tanks, and turbulent flow of fuel. Many of these conditions may occur during fuel transfer operations. A specific example is the refueling of aircraft, which involves rapid movement of aviation gasoline or jet fuel from operating storage, through a filter/separator, an underground distribution system, another filter/ separator, and then through the hose to the aircraft fuel tank at a relatively high exit velocity.

SUMMARY

[0004] A first aspect of the present invention provides an apparatus for measuring local charge at one or more locations in a volume of fluid. The apparatus comprises a conductor for immersion at a location in the volume of fluid; an electrometer, electrically connectable to the conductor; and a control unit. The control unit is configured to electrically connect the electrometer to the conductor for a time period; measure an electrical property of the conductor with the electrometer during the time period; and determine a local charge of a location in the volume of fluid based on the measured electrical property.

[0005] Embodiments of the invention therefore enable localised measurement of charge in a volume of fuel. In particular, embodiments are possible which provide a high-resolution charge probe that can be used to monitor localised charge levels in a volume of fuel. Advantageously, such embodiments can enable the electrostatic behaviour of aircraft fuel in various scenarios to be studied in detail, facilitating an improved understanding of the charge distributions likely to arise during filling of an aircraft fuel tank.

[0006] Optionally, the electrical property comprises one of: electrostatic potential and electrical charge. In some examples the capacitance of the conductor is less than 10 picofarads. In some examples the conductor is electrically isolated when not electrically connected to the electrometer.

[0007] Optionally, the apparatus comprises a switch having a first terminal connected to the conductor and a second terminal connected to the electrometer, such that closing the switch electrically connects the electrometer to the conductor. In some examples the switch is configured to be open except during the time period. In some examples the switch comprises an ultra-high impedance relay.

[0008] Optionally the duration of the time period is predetermined. In some examples the duration of the time period is determined based on the conductivity of the fluid.

[0009] Optionally, the apparatus further comprises a further conductor for immersion at a location in the volume of fluid and a further electrometer, electrically connectable to the further conductor. In such examples the control unit may be further configured to connect the further electrometer to the further conductor the further switch for a further time period; measure an electrical property of the further conductor with the further electrometer during the further time period; and determine a local charge of a further location in the volume of fluid based on the measured electrical property of the further conductor. In some examples the control unit is configured such that the further time period coincides with the time period. In some examples the control unit is further configured to determine a charge distribution of the volume of fluid based on the determined local charge of the location and the determined local charge of the further location.

[0010] A second aspect of the present invention provides a method. The method comprises (a) immersing a conductor in a volume of fluid at a first location; (b) connecting, for a time period, the conductor to an electrometer; (c) measuring an electrical property of the conductor with the electrometer during the time period; and (d) determining a local charge of the fluid at the first location based on the measured electrical property.

[0011] Optionally, the method further comprises (e) immersing a conductor in the volume of fluid at a second location; (f) connecting, for a time period, the conductor at the second location to an electrometer; (g) measuring an electrical property of the conductor at the second location with the electrometer during the time period; and (h) determining a local charge of the fluid at the second location based on the measured electrical property of the conductor at the second location. In some examples (e)-(h) are performed simultaneously with (a)-(d). In some examples the method further comprises determining a charge distribution of the volume of fluid based on the determined local charge of the fluid at the first location and the determined local charge of the fluid at the second location. In some examples the method comprises conducting a first measurement run by performing (a)-(d) under a first set of test conditions and conducting a second measurement run by performing (a)-(d) under a second set of test conditions, where the value of at least one condition comprised in the first and second sets of test conditions differs between the first set of test conditions and the second set of test conditions.

[0012] Optionally, the fluid comprises aircraft fuel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which: [0014] Figure lisa first schematic view of a local charge measurement apparatus according to a first embodiment; [0015] Figure 2 is a second schematic view of the local charge measurement apparatus of Figure 1; [0016] Figure 3 is a flow chart of a method of determining a local charge of a selected location in a volume of fluid, according to an embodiment; [0017] Figure 4 is a schematic view of a local charge measurement apparatus according to a second embodiment; [0018] Figure 5 is a first schematic view of a local charge measurement apparatus according to a third embodiment; [0019] Figure 6 is a second schematic view of the local charge measurement apparatus of Figure 5; and [0020] Figure 7 is an image of a test measurement run results sheet, according to an embodiment.

DETAILED DESCRIPTION

[0021] During aircraft tank filling scenarios, fuel in the aircraft fuel tank may become electrostatically charged, and it is likely that the distribution of this charge will be non-uniform over the volume of the fuel. However; there currently exists no concrete evidence for such non-uniform charge distributions. Existing techniques (e.g. which use a Faraday Pail) for measuring the electrostatic charge of volume of fluid measure the total amount of electric charge in the volume, and for aircraft applications it is often assumed that the charge is uniform throughout a fuel volume. It is desirable to gain a better understanding of the actual distribution of charge throughout an aircraft fuel tank, e g. to improve the accuracy of computational models of fuel tanks and/or to facilitate the design of improved fuel tanks. In particular, the distribution of electrostatic charge within the bulk of the fuel and across the fuel surface, are of primary importance in controlling quantities such as the peak fuel surface voltage and electric fields in the tank ullage space.

[0022] A method and apparatus have been developed, which enable localised charge measurements to be obtained. For the purposes of this specification, a “local” or “localised” measurement is a measurement of a property (e.g. charge or electrostatic potential) at either a single discrete point, or along a single axis. Performing such local measurements in multiple locations throughout a volume of fluid enables the charge distribution over the volume to be determined.

[0023] Figure lisa cross-section through an example fuel tank 5, according to a first embodiment. The fuel tank 5 is intended to represent a small-scale, simplified version of an aircraft fuel tank. It will be appreciated that real aircraft fuel tanks are generally located in the wings of an aircraft, and therefore have complex geometries dictated by the shape of the wing box. A complex tank geometry can complicate analysis of local charge measurement results, meaning that the simplified fuel tank 5 is therefore better suited for conducting initial research into the electrostatic behaviour of fuel during filling scenarios. Furthermore, unlike an aircraft fuel tank, the fuel tank 5 is open at the top, to permit a measurement apparatus and/or other research-related apparatus to be easily provided in the tank, at various locations.

[0024] The fuel tank 5 is constructed from polyamide (in this particular example the thickness of the polyamide walls of the fuel tank is 3 .4mm). The fuel tank 5 is cuboid in shape and has a total volume of 18 litres. In Figure 1, the tank is shown as containing a volume of aircraft fuel 6, which is less than the total volume of the tank 5. The fuel 6 has a known conductivity. The fuel tank 5 can be filled with fuel through the open top end. At least some electrostatic properties (e.g. electrical resistivity and charge relaxation rate) of the tank wall are known. It will be appreciated that any or all of the details of the tank design and construction may vary from those of the example fuel tank 5. For example, the method and apparatus of the invention may be used with a metallic fuel tank, a fuel tank which is fully closed, a fuel tank comprised in an aircraft, etc.

[0025] The tank 5 is provided with an apparatus 1 for measuring charge at one or more locations in a volume of fluid, according to a first embodiment of the invention. The apparatus comprises a charge probe 2, in communication with a control unit 3 by means of a communications link 4. The charge probe 2 is supported in a known, fixed position in the tank 5 by a support structure (not shown). Depending on the nature of a given application (e.g. a given test to be performed), the tank 5 may optionally also be provided with one or more of an apparatus for measuring electrical fields; a mixing device; a high voltage electrode; a grounding layer on the tank wall; a fuel inlet; a fuel outlet.

[0026] Figure 2 shows in greater detail the example apparatus 1 for measuring charge at one or more locations in a volume of fluid with which the tank 5 is provided. The charge probe 2 of the apparatus 1 comprises a conductor 10 for immersion at a location in a volume of fluid (e.g. the fuel 6), an electrometer 12, and a switch 16 which selectively electrically connects the electrometer 12 to the conductor 10. References to a “charge probe” elsewhere in this specification should be understood as referring to a combination of a conductor, a switch and an electrometer. However; such combinations (and/or individual elements thereof) need not have any or all of the particular features described below in relation to the example charge probe 2, and may have additional features not present in the example charge probe 2. For example, the switch of the example charge probe 2 is described as comprising a relay activated by an electromagnet, but it would be equally possible for a charge probe to comprise a different type of switch, indeed the switch could comprise any component suitable to perform the function of selectively connecting the conductor to the electrometer.

[0027] The conductor 10 comprises a thin metal wire which has a very low capacitance (when electrically isolated). The isolated capacitance of the conductor 10 may be as close as is practically possible to zero. The isolated capacitance of the conductor may be in the range 2-3 picofarads. The lower the isolated capacitance of the conductor, the less the charge distribution of the fuel 6 will be affected when a measurement is made using the apparatus. In the particular illustrated example, the conductor 10 comprises a 100mm long stainless steel wire with a diameter of 1.3mm. When the conductor 10 is immersed in charged fuel and the switch 16 is open, the charge (and therefore the potential) of the conductor 10 will respond to the local potential by virtue of its low capacitance. For as long as the switch 16 is open, electrical charge is retained on the surface of the conductor 10 and there is no measurable output to the electrometer 12.

[0028] In the particular example of Figure 2 the switch 16 comprises an ultra-high impedance high-voltage reed relay (although any other type of high insulation resistance switch or relay could equally be used). The impedance of the switch 16 may be sufficiently high that the leakage current when the switch is open is effectively zero. The impedance of the switch 16 may be sufficiently high that the leakage current which the switch is open is less than lOOpA at a 5kV potential difference. The switch may have an insulation resistance of at least IxlO^^ohms. A first terminal of the switch 16 is electrically connected to the conductor 10 and a second terminal of the switch 16 is electrically connected to the electrometer 12. In the particular example the second terminal of the switch 16 is connected to the electrometer 12 via electrical path 15. The electrical path 15 includes a 1 ΜΩ resistor 17, which acts to reduce a high voltage spike which appears at the input of the electrometer 12 when the switch 16 is initially closed. The switch 16 is arranged such that its default state is open. When the switch 16 is open the conductor 10 is electrically isolated from the electrometer 12 (i.e. there is no electrical path between the conductor 10 and the electrometer 12), and is electrically isolated from ground. When the switch 16 is closed, an electrical path between the conductor 10 and the electrometer 12 is created, and current may therefore flow between the conductor 10 and the electrometer 12.

[0029] In the particular example of Figure 2, the operation (i .e. opening and closing) of the switch 16 is effected by an electromagnet. The electromagnet is connected to the control unit 3 by a communications link 14 (e g. an electrical path) and is arranged to activate in response to a control signal (e.g. a 12V signal) sent from the control unit 3 to the electromagnet via the communications link 14. The state of the probe 2 in which the switch 16 is closed is hereinafter referred to as the “probe on” condition, and the state of the probe 2 in which the switch 16 is open is hereinafter referred to as the “probe off’ condition.

[0030] The electrometer 12 is arranged to measure an electrical property of the conductor 10. In the particular example of Figure 2, the electrometer 12 is arranged to detect a voltage (potential difference). In the particular example the electrometer comprises a capacitor and an amplifier arranged in parallel, however any suitable design of electrometer or other charge measuring circuit may alternatively be used. In some examples the electrometer may comprise a Keithley electrometer. In the particular example, the electrometer 12 has a relatively high input capacitance, which enables a measurement to be stored prior to resetting the electrometer to take a further measurement. The input capacitance may be such that the impedance of an input stage of the capacitor has an impedance of the order of lO^'^ohms. However; alternative examples are possible in which the electrometer does not have a relatively high input capacitance. The output of the electrometer is connected to the control unit 3. In the illustrated example the electrometer 12 is arranged to output a time-varying voltage (electrostatic potential) signal. However; in other examples the electrometer 12 can be arranged to output a time-varying electrical charge signal. The detected voltage can be directly converted to a charge value, e.g. to enable the electrometer to output a charge signal, because the capacitance (i.e. the capacitance of the electrometer and the probe) is known. In some examples the output of the electrometer may be selectable between a voltage signal and a charge signal. Upon closing the switch 16 (i.e. entering the “probe on” condition), the electrical charge on the immersed conductor 10 is transferred to the electrometer 12. Thus, during the “probe on” condition the conductor 10 is at virtual ground. Consequently the electric field pattern which existed during the probe off condition is significantly altered. It is therefore desirable to minimize the duration of the “probe on” condition.

[0031] The control unit 3 is configured to close the switch 16 for a time period, e g. by continuously sending a 12V signal to activate the electromagnet of the switch 16, during the time period. The time period is very short (e g. of the order of a few milliseconds). The duration of the time period may be in the range 1 millisecond to 1 second. The duration of the time period may be in the range 1-10 milliseconds. In some embodiments the duration of the time period is predetermined. The duration can be determined, for example, based on an expected time required for charge stored on the conductor 10 (e.g. charge which has built up on the conductor 10 during a “probe off’ condition) to drain from the conductor 10 following the probe entering the “probe on” condition. In some examples the duration of the time period for a given application is at least as long as the drainage time of a charge expected to become stored on the conductor 10 during that application. In some examples the duration of the time period for a given application is substantially equal to the drainage time of a charge expected to become stored on the conductor 10 during that application.

[0032] It may be advantageous to minimize the time that the switch 16 is open, for several reasons. One reason is that minimizing the “probe on” time minimizes the amount of charge removed from the body of fuel 11 by conduction. The effect of removing charge from the fuel 11 during a measurement is exacerbated with fuel of higher conductivity. A second reason is to minimize the disturbance to the electric field distribution in the body of fuel caused by the virtual grounding of the conductor 10 during the “probe on” condition. A shorter “probe on” condition therefore produces a less intrusive measurement.

[0033] The control unit 3 is further configured to measure, with the electrometer 12, an electrical property of the conductor during the time period. The electrical property can be, for example, electrical charge or electrostatic potential. In some cases where the electrical property is electrical charge, measuring the electrical charge of the conductor comprises measuring the electrostatic potential of the conductor and mathematically converting the measured electrostatic potential value into a charge value (e.g. using the equation Q=VC, where Q is the charge, V is the electrostatic potential and C is the capacitance of the conductor). In the particular example of FIG. 2, the control unit 3 measures an electrical property of the conductor with the electrometer 12 by receiving an output signal (i.e. a measurement signal) from the electrometer 12. The measurement signal may comprise, e.g., a time-varying signal of the measured electrical property. The time-varying signal may comprise a time-varying electrostatic potential (voltage) signal, and/or a time-varying electrical charge signal. The control unit 3 may receive a time-series of measured values of the electrical property from the electrometer 12. The electrometer 12 outputs a measurement signal for the duration of the time period (i.e. the duration of the “probe on” condition). In some embodiments the control unit 3 is arranged to store a measurement signal received from the electrometer (e.g. on a memory comprised in or accessible by the control unit 3). In some embodiments the control unit 3 comprises a data recording function to receive and store the measurement signal from the electrometer 12.

[0034] In some examples measuring an electrical property further comprises determining a value of the electrical property in respect of the time period, based on the received measurement signal for that time period. The electrometer output signal is proportional to the charge transferred from the probe to the electrometer input during the time period. At the point when the probe is switched to the probe on condition, the initial instantaneous reading is the charge residing on the probe stem. If the probe remains on, the reading will increase with time (ramp up) at a rate dependant on the charge of the fuel and the fuel conductivity. For example, a highly charged, high conductivity fuel (not normally encountered in practice) would produce a rapidly increasing signal during the probe on time.

[0035] In some examples the determined value is determined to be equal to the first value measured after the probe is switched to the probe on condition (i.e. the first value measured during the time period). In some examples the determined value is determined based on filtering the signal. In some such examples the filtering is based on a rate of change of the signal. Determining the value is performed by a processor of the control unit 3. In embodiments in which the received measurement signal is stored in a memoiy', determining a value comprises retrieving (e g. by the processor of the control unit 3) the received measurement signal from the memory.

[0036] The control unit 3 is further configured to determine a charge of a location in the volume of fluid based on the measured electrical property. The location in the volume of fluid is the location in the fluid of the conductor 10 in respect of which the electrical property was measured. In the embodiment of Figures 1 and 2, the immersed portion of the conductor has a significant length, and the measured electrical property relates to the whole of this immersed length. In this example, therefore, the location in the volume of fluid is a line corresponding to the location of the immersed portion of the conductor 10. However; other arrangements of the conductor are possible in which the length of the conductor which is immersed is very short or negligible (e.g. the embodiment of Figure 4, discussed below). In such cases the location in the volume of fluid can be treated as a point, corresponding to the location of the immersed conductor. In some examples in which the measured electrical property of the conductor is electric charge, determining the charge of a location in the volume of fluid can comprise determining the charge of the location in the volume of fluid to be equal to the measured charge on the conductor 10. In cases where the input capacitance of the electrometer 12 is much larger than the capacitance of the conductor 10, substantially all the charge on the conductor is transferred to the electrometer 12 during the probe on time period. For example, if the capacitance of the conductor 10 is 2 picofarads and the input capacitance of the electrometer 12 is 2 nanofarads, 99.9% of the charge on the probe will be transferred. Such arrangements therefore enable all of the charge on the probe to be measured.

[0037] In some examples in which the measured electrical property of the conductor 10 is electric charge, determining the charge of a location in the volume of fluid can comprise using a defined relationship between measured charge on the conductor 10 and charge at the location of the conductor 10 in the volume of fluid (i.e. the charge of the fluid immediately surrounding the conductor 10). The defined relationship may be stored in a memory of the control unit 3. The defined relationship may be defined during a calibration process of the charge probe 2. The defined relationship may comprise, for example, a look-up table, a calibration curve; one or more rules; one or more or equations; etc. The defined relationship enables a corresponding local charge value to be determined for a given measured value of the charge on the conductor 10.

[0038] A defined relationship between measured charge on the conductor 10 and charge at the location of the conductor 10 in the volume of fluid can be determined, e.g., by applying a known charge to the conductor 10, closing the switch 16 for the time period, and then measuring an electrical property (e g. electrostatic potential or electrical charge) of the conductor 10 with the electrometer 12 during the time period. Measuring the electrical property of the conductor 10 may be performed in the manner described above in relation to the determination of an unknown local charge) In some examples this process can be repeated for a range of different known charge values, to create a data set of known charge values and corresponding measured values of the electrical property of the conductor. A relationship (e g. in the form of a calibration curve, look-up table, equation, etc.) between applied charge and the measured electrical property can then be defined based on the measured charge value(s) and the known applied charge value(s).

[0039] Similarly, in examples in which the measured electrical property of the conductor 10 is electrostatic potential, determining the charge of a location in the volume of fluid can comprise calculating the charge transferred to the electrometer, and determining the charge of the location to be equal to the charge transferred to the electrometer, or can comprise using a defined relationship between measured electrostatic potential of the conductor 10 and charge at the location of the conductor 10 in the volume of fluid. The defined relationship between measured electrostatic potential and local charge may have any of the features described above in relation to the defined relationship between measured charge on the conductor 10 and local charge.

[0040] The control unit 3 may comprise an oscilloscope, such as a multi-channel digital oscilloscope. In some examples the control unit 3 comprises a data-logging system such as a National Instruments PXI rack. The control unit 3 may be arranged to perform one or more tests automatically, in accordance with predefined parameters (e.g. which have been selected and input to the control unit 3 by a user). Such predefined parameters can include, for example, sampling time, rate of sampling, etc. The control unit 3 may further be arranged to automatically reset between consecutive tests. The control unit 3 may further be arranged to automatically capture, store and/or process data generated by the performance of one or more tests using the apparatus.

[0041] In the particular example of Figures 1 and 2, the sensitivity of the charge probe 2 is 40mV per nanoCoulomb. A local fuel charge in the range 0.5-2.0 nanoCoulombs results in a probe voltage output in the range 20-80mV.

[0042] An example process for calibrating the charge density probe 2 will now be described. The electrometer 12 provides an output voltage (which in some examples is mathematically converted to an output charge, as described above) which is proportional to the charge transferred to its input. The constant of proportionality is dependent on the details of the electrometer circuit. The constant of proportionality can be determined or checked, for example, by transferring a known charge to the electrometer, e.g. by applying fixed current to the input of the electrometer 12 for a fixed period of time. The voltage value output by the electrometer can then be compared to the known charge to determine the constant of proportionality of the electrometer. This constant of proportionality can then be applied to the output of the electrometer 12 to yield a charge value equal to the charge transferred to the electrometer (which in some cases may be considered equal to the local charge of the fluid being measured, as described above). In some examples a range of different known charges may be applied, and the electrometer output checked for each of these known charges, to check the behaviour of the electrometer over a range of input values.

[0043] Calibrating the charge probe 2 can additionally or alternatively comprise applying a known voltage to the conductor 10 with the switch 16 in a closed configuration, and checking the voltage value output by the electrometer. This may be repeated for a range of different known voltage values, to create a data set of known applied voltage values and corresponding output voltage values, enabling the proportionality between the input voltage and the output voltage to be determined/checked.

[0044] An example method for using a charge measurement apparatus (e.g. the apparatus 1) to determine the local charge of a selected location (i.e. the location of the immersed part of a conductor of the charge measurement apparatus) in a volume of fluid (e.g. the fuel 6) will now be described, with reference to Figure 3.

[0045] In block 301 of Figure 3 a conductor (e g. the conductor 10 of the charge probe 2) is immersed in the fluid, at the selected location. The conductor is electrically isolated during the performance of block 301. As described above, electrical isolation of the conductor can be achieved by connecting it to the rest of the measurement apparatus via a normally open switch. A charge builds up on the conductor, based on the local charge of the fluid at the selected location, and on the capacitance of the conductor, creating an electrostatic potential on the conductor. Immersion of the conductor may be performed in any of the manners described above in relation to the apparatus 1 of Figures 1 and 2.

[0046] In block 302 the conductor is connected to an electrometer (e g. the electrometer 12) for a time period. Connecting the conductor to an electrometer may comprise, for example, closing a switch between the conductor and the electrometer. Connecting the conductor to an electrometer may comprise creating an electrical path between the conductor and the electrometer. The time period may have any of the features of the time period described above in relation to the operation of the apparatus 1. Connection of the conductor to an electrometer may be performed in any of the manners described above in relation to the apparatus 1. As a result of connecting the conductor to an electrometer, current flows from the conductor to the electrometer, driven by the electrostatic potential on the conductor. The current flows during the time period, and charge thereby drains from the conductor during the time period.

[0047] In block 303 an electrical property of the conductor is measured during the time period, by the electrometer. The measurement may be based on the current which flows from the conductor to the electrometer during the time period. The electrical property may be measured continuously during the time period, or alternatively may be measured at one or more discrete times comprised within the time period. The electrical property may have any of the features described above in relation to the apparatus 1. In some examples measuring an electrical property of the conductor with the electrometer comprises outputting a measurement signal from the electrometer, e g. to a control unit of a measurement apparatus. Such a measurement signal may have any of the features described above in relation to the apparatus 1. Measuring an electrical property of the conductor may be performed in any of the manners described above in relation to the apparatus 1.

[0048] In block 304 the local charge of the selected location is determined based on the measured electrical property. Block 304 may be performed, partly or entirely, by a control unit of a measurement apparatus (e.g. the control unit 3). Alternatively, block 304 may be partly or entirely performed by a processor separate from the measurement apparatus (i.e. the measurement apparatus comprising the conductor and the electrometer), or by a human operator of the measurement apparatus. Determining the local charge of the selected location may comprise processing a measurement signal received from the electrometer. Determining the local charge of the selected location may be performed in any of the manners described above in relation to the apparatus 1.

[0049] In some examples the method of Figure 3 is repeated in respect of a different selected location in the volume of fluid. In some examples the method is performed simultaneously in respect of multiple selected locations in the volume of fluid. In such examples, a set of local charge values corresponding to different locations in the volume of fluid will be generated. In some such examples, the method includes the additional step of determining a charge distribution of the volume of fluid based on the set of local charge values. Determining a charge distribution may, for example, comprise generating a map and/or a model of the charge of the volume of fluid.

[0050] Figure 4 shows an example apparatus 41 for measuring charge at one or more locations in a volume of fluid, according to a second embodiment of the invention. The apparatus 41 is shown in use with a fuel tank 45 which contains a volume of fluid 46. The fuel tank 45 may have any or all of the features of the fuel tank 5 described above in relation to Figure 1, and the fluid 46 may have any or all of the features of the volume of fluid 6 described above in relation to Figure 1.

[0051] The apparatus comprises a charge probe 42, in communication with a control unit 43 by means of a communications link 44. The charge probe 42 is supported in a known, fixed position in the tank 45 by a support structure (not shown). The control unit 43 and communications link 44 may have any or all of the features described above in relation to the control unit 3 and the communications link 4, respectively, of Figures 1 and 2.

[0052] The charge probe 42 of the apparatus 41 comprises a conductor 50 for immersion at a location in a volume of fluid (e g. the fuel 46), an electrometer (not shown), and a first switch 47 which selectively electrically connects the electrometer to the conductor 50. The conductor 50, electrometer and first switch 47 may have any or all of the features of the conductor 10, electrometer 12 and switch 16, respectively, described above in relation to Figures 1 and 2.

[0053] Unlike with the charge probe 2 of the first embodiment, the probe stem (i.e. the part of the probe which is to be immersed in the fluid) of the charge probe 42 is coaxial. The conductor 50 is arranged within a conductive tube 51 (made from, e.g., stainless steel). The conductive tube 51 may have a low capacitance, to enable its potential to rise rapidly to match the local potential of a fluid in which it is immersed. In the particular illustrated example the tube is a 0.8m long, 3.2 mm diameter, stainless steel tube. Insulation (not shown) is arranged between the conductor 50 and the tube 51 so that the conductor 50 is electrically isolated from the tube 51. The insulation may comprise, e.g. a PTFE coating on the conductor wire. The tube 51 has a connection 49 to ground, via a second switch 48. When the second switch 48 is closed, such that the tube 51 is grounded, the tube 51 has the effect of preventing charge building up on the conductor 50 (e.g. as a result of electrical charge present in a fluid in which the probe stem is immersed). The second switch 48 may have any or all of the features of the switch 16 described above in relation to Figures 1 and 2. In some examples, the tube 51 may provide mechanical rigidity to the probe stem.

[0054] As can be seen from Figure 4, the conductor 50 extends out of the distal end of the tube 51 by a small distance, such that there exists a “probe tip” region 52 of the conductor 50 which is in electrical communication with the surrounding fluid. The probe tip region 52 is electrically insulated from the tube 51, e g. by a dielectric collar. In some examples the probe tip region 52 comprises an electrode connected to the conductor 50. In the particular illustrated example, the electrode is a cylindrical electrode of length 35mm and diameter 3.2mm.

[0055] As with the charge probe 2 of the first embodiment, the charge probe 42 may be in one of two states (“probe on” and “probe off’) at any given time. The state of the probe 42 is determined by the respective states of the first switch 47 and the second switch 48. In the “probe off’ condition both of the first switch 47 and the second switch 48 are open such that the conductor 50, the conductive tube 51 and the probe tip 52 are all electrically isolated and are permitted to float in potential.

[0056] The charge probe 42 is transitioned to the “probe on” condition (e.g. when it is desired to take a measurement) by simultaneously closing the first switch 47 and the second switch 48 (e g. by means of an applied 12V signal sent from the control unit 43). This has the effect of transferring charge from the conductive tube 51 directly to ground, and transferring charge from the probe tip 52 to the electrometer. Consequently, only an electrical property of the probe tip 52 (e g. the charge stored on the probe tip 52 or the voltage of the probe tip 52) is measured.

[0057] The electrometer and control unit function to determine a local charge of a location in the volume of fluid, based on the measured electrical property of the probe tip 52, in the same manner described above in relation to the electrometer 12 and control unit 3 of the first embodiment. However; the location for which the local charge is determined corresponds only to the probe tip 52, rather than the whole immersed length of the probe stem. In some examples the length of the probe tip is very small (e.g. of the order of a few millimetres). In such examples the location for which a local charge is determined may be treated as a point (e.g. as opposed to a line) in the volume of fluid. Other aspects of the operation of the charge measurement apparatus 41 may be the same as for the charge measurement apparatus 1 of the first embodiment.

[0058] Figure 5 shows a second example aircraft fuel tank 35. The tank 35 contains aircraft fuel 36. The tank 35 may have any or all of the features of the tank 5 described above in relation to Figure 1. The tank 35 is of similar construction to the tank 5, except that it includes two charge probe mounting points 31 in the tank wall. The tank 35 is provided with an apparatus 30 for measuring charge at one or more locations in a volume of fluid, according to a second embodiment of the invention. The apparatus of Figure 5 comprises multiple charge probes 32 distributed throughout the volume of the tank 35, each of which is connected to a single control unit (not shown). In the particular illustrated example, the apparatus comprises three charge probes 32a, 32b and 32c, each of which is of the same general type (i.e. the probe stem is not coaxial) as the charge probe 2 described above in relation to Figures 1 and 2. However; alternative examples are possible in which one or more of the charge probes 32a, 32b and 32c is of the same general type (i.e. the probe stem is coaxial) as the charge probe 42 described above in relation to Figure 4. Each of the three charge probes 32a-c is positioned centrally with respect to three orthogonal faces of the body of fuel, and the three probes 32a-c are aligned with the x, y and z axes respectively. The charge probe 32a is supported above the open top of the tank 35 in a similar manner to the charge probe 2 of the apparatus 1 of Figure 1. The charge probes 32b and 32c are mounted to the external surface of the wall of the tank 35, at the mounting points 31. The conductor of each wall-mounted charge probe 32b, 32c passes through the wall of the fuel tank 35 by means of fuel-tight gasket. For all of the charge probes 32a-c, the same length of the conductor is immersed in the fuel. In the illustrated example, the immersed length is 70mm.

[0059] In general, the number, spacing and orientation of the charge probes will be selected depending on the particular application. As discussed above, each probe has a small effect on the local charge in the volume of fluid immediately surrounding that probe when used to take a measurement. A lower density of probes will therefore have a smaller effect on the charge distribution of the fluid being measured, enabling a more accurate determination of the charge distribution of the volume of fluid. However; a higher density of probes enables the charge distribution of the volume of fluid to be determined at a higher resolution. For each particular application, the number and arrangement of charge probes used will represent a trade-off between accuracy and resolution, and it is expected that these factors will be selected in dependence on the requirements of the particular application.

[0060] Figure 6 shows in greater detail the example apparatus 30 for measuring local charge at one or more locations in a volume of fluid with which the tank 35 is provided. The charge measurement apparatus 30 of the second embodiment comprises the three charge probes 32a-c. Each of the charge probes 32a-c comprises a conductor connected to an electrometer 33 by a switch. The conductors, switches and electrometers of the probes 32a-c may have any or all of the features of the conductor 10, the switch 16 and the electrometer 12 described above in relation to Figure 2.

[0061] The charge measurement apparatus 30 further comprises a control unit 34. The control unit 34 is connected by respective communications links 40a-c to each of the electrometers 33a-c, and is arranged to receive an output signal from each of the electrometers 33a-c when the probes 32a-c are in the “probe on” condition. In the particular example the control unit 34 has multiple channels, such that it can simultaneously receive each of the measurement signals output by the electrometers 33a-c. The control unit 34 may, e g., comprise a multi-channel data recorder. In some examples the control unit is arranged to store measurement signals (e g. in the form of time-varying voltage or charge signals) received from each of the probes 32a-c. The output signals can be stored in the form of, e.g., graphical traces and/or raw data files.

[0062] The control unit 34 is also connected by a communications link to each of the probes 32a-c, and is arranged to send control signals to the switches of each of the probes 32a-c via the communications link 39. The control unit 34 is configured to transition all of the charge probes 32a-c between the “probe off’ condition and the “probe on” condition (and vice versa) at the same time. The control unit can be arranged to achieve this, for example, by sending a control signal to each of the switches of the probes 32a-c simultaneously. The control unit 34 further comprises a reset function (which in the particular example is manual, but can alternatively be automatic), which is connected to the electrometers 33a-c by a communications link 41. The reset function “zeroes” a trace of the output signal from each electrometer. This function may be used, e.g., between each time that the probes are activated to take a measurement (otherwise the trace for the second measurement would comprise a peak on top of the peak representing the first measurement).

Table 1: Example test matrix showing combinations of conditions for which local charge values may be determined during an example test.

[0063] Operation of the charge measurement apparatus to perform a particular example test will now be described. In the example test, the fuel tank 35 is additionally provided with a high-voltage electrode for applying a charge to the fuel 36. The electrode is incorporated into a filling pipe through which fuel 36 is introduced into the tank 35 during the test. In general, the test involves determining a local charge value for each of the three locations in the fuel 36 corresponding to the locations of the immersed parts of the conductors of the probes 32a-c. The test may involve determining a local charge value for each of the three locations at multiple times during filling of the tank 35 with the fuel 36, and/or in respect of various different test conditions. Conditions which may be varied include, for example, whether or not the fuel 36 is stirred (e g with a mixing device provided in the tank 35); the conductivity of the fuel 36; the fill rate of the tank 35, etc. Table 2 shows an example combination of conditions for which local charge values may be determined.

[0064] In some examples, before starting to obtain measurements with the apparatus 30 (e.g. in accordance with the test matrix of Table 1), a preliminary test run is performed to check that the apparatus 30 is operating correctly. No charging voltage is applied by the electrode during the preliminary test run, in order to confirm zero readings from the electrometer 32 in the absence of a charging source.

[0065] Following the test run (if a test run is performed), measurement runs are performed (e g. runs 1-6 of Table 1). In each of the measurement runs, fuel flow into the tank 35 is commenced and then at least one measurement of the electrical property of the conductors is obtained. In some examples further measurements of the electrical property are taken at preselected intervals, until the full amount of fuel used for the test has entered the tank 35 . In some examples the last measurement of the electrical property of the conductors comprises the first measurement of the electrical property of the conductors to be taken after the full amount of fuel has entered the tank 35.

[0066] The results from a given measurement run may comprise traces of the measurement signals received from each of the electrometers 33a-c. The peaks of the traces 60 correspond to the times during which the probes 32a-c were on. A similar set of traces may be generated in respect of each measurement run. Figure 7 shows an example set of traces 60 from a measurement run in which the measured electrical property of the conductors was electrostatic potential (voltage). In some examples each measurement run is repeated a predefined number of times. In some such examples, a single set of results may be generated based on combined data from multiple repeated instances of a measurement run.

[0067] The results obtained from a given test performed using a local charge measurement apparatus (e.g. the test described above) can be used to gain a better understanding of the electrostatic environment in a fuel tank (or other type of fluid container) and thereby to validate modelling tools for modelling the electrostatic behaviour of aircraft fuel. Such modelling tools can be used, for example, to inform the design of future fuel tanks, and/or to refine refuelling restrictions.

[0068] It will be appreciated that the above described test represents a particular example, and that the apparatus 30 and/or the apparatus 1, and variations thereof, can be used to perform various different tests for the purposes of investigating the electrostatic behaviour of aircraft fuel or any other fluid. In such tests one or more of the components of the measurement apparatus, the arrangement of those components, the fluid being measured, the tank configuration, the test conditions, etc. may differ from what is described above. Results from multiple tests may be combined, e.g. in the creation of a charge distribution model.

[0069] Whilst the present invention has been described and illustrated with reference to a particular embodiment, 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.

[0070] 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.

Claims

CLAIMS:

1. An apparatus for measuring local charge at one or more locations in a volume of fluid, the apparatus comprising: a conductor for immersion at a location in the volume of fluid; an electrometer, electrically connectable to the conductor; and a control unit configured to: electrically connect the electrometer to the conductor for a time period; measure an electrical property of the conductor with the electrometer during the time period; and determine a local charge of a location in the volume of fluid based on the measured electrical property.

2. An apparatus according to claim 1, wherein the electrical property comprises one of: electrostatic potential and electrical charge.

3. An apparatus according to any preceding claim, wherein the conductor is electrically isolated when not electrically connected to the electrometer.

4. An apparatus according to any preceding claim, comprising a switch having a first terminal connected to the conductor and a second terminal connected to the electrometer, such that closing the switch electrically connects the electrometer to the conductor.

5. An apparatus according to claim 4, wherein the switch is configured to be open except during the time period.

6. An apparatus according to claim 4 or claim 5, wherein the switch comprises an ultra-high impedance relay.

7. An apparatus according to any preceding claim, wherein the duration of the time period is predetermined.

8. An apparatus according to claim 8, wherein the duration of the time period is determined based on the conductivity of the fluid.

9. An apparatus according to any preceding claim, wherein the conductor comprises a wire arranged within a conductive tube and electrically insulated from the conductive tube, wherein the length of the wire is greater than the length of the conductive tube such that a probe tip is defined at a distal end of the conductor, the probe tip comprising a portion of the wire which is not within the conductive tube.

10. An apparatus according to claim 9, further comprising a ground switch arranged to selectively connect the conductive tube to ground, wherein the ground switch is arranged to be open except during the time period.

11. An apparatus according to any preceding claim, further comprising: a further conductor for immersion at a location in the volume of fluid; a further electrometer, electrically connectable to the further conductor; wherein the control unit is further configured to: connect the further electrometer to the further conductor the further switch for a further time period; measure an electrical property of the further conductor with the further electrometer during the further time period; determine a local charge of a further location in the volume of fluid based on the measured electrical property of the further conductor.

12. An apparatus according to claim 11, wherein the control unit is configured such that the further time period coincides with the time period.

13. An apparatus according to claim 11 or claim 12, wherein the control unit is further configured to determine a charge distribution of the volume of fluid based on the determined local charge of the location and the determined local charge of the further location.

14. A method comprising: (a) immersing a conductor in a volume of fluid at a first location; (b) connecting, for a time period, the conductor to an electrometer; (c) measuring an electrical property of the conductor with the electrometer during the time period; and (d) determining a local charge of the fluid at the first location based on the measured electrical property.

15. The method of claim 14, further comprising: (e) immersing a conductor in the volume of fluid at a second location; (f) connecting, for a time period, the conductor at the second location to an electrometer; (g) measuring an electrical property of the conductor at the second location with the electrometer during the time period; and (h) determining a local charge of the fluid at the second location based on the measured electrical property of the conductor at the second location.

16. The method of claim 15, wherein (e)-(h) are performed simultaneously with (a)-(d).

17. The method of claim 15 or claim 16, further comprising determining a charge distribution of the volume of fluid based on the determined local charge of the fluid at the first location and the determined local charge of the fluid at the second location.

18. The method of any of claims 14-17, comprising conducting a first measurement run by performing (a)-(d) under a first set of test conditions and conducting a second measurement run by performing (a)-(d) under a second set of test conditions, where the value of at least one condition comprised in the first and second sets of test conditions differs between the first set of test conditions and the second set of test conditions.

19. The method of any of claims 14-18, wherein the fluid comprises aircraft fuel.