Abstract:
A sensor array for detecting the distribution of fluids having different electrical characteristics, comprising a multilayer structure including a first layer which defines an array of spaced apart sensor electrodes, a second layer separated from the first layer by dielectric material and defining a conductive screen, and a third layer separated from the second layer by dielectric material and defining a series of spaced apart elongate connections, each sensor being connected to a respective connection by a respective conductive path extending through an opening in the conductive screen defined by the second layer.

Description:
This application is a 371 of PCT/GB98/02271, filed on July 29, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a flow control system, and in particular to a system for controlling the flow of a mixed phase fluid through a vessel. The term “mixed phase fluid” is used to cover fluids made up of for example suspended particulates, liquids emulsions and gas derived from different constituents for example oil and water, or liquid and gas derived from the same constituent. 
     Oil field production systems generally comprise separator plant in which raw fluid pumped from an oil bearing formation is separated into its constituent parts, that is volatile gases, liquid petroleum products, water and particulates. The nature of the input fluid to the separator plant can vary widely over relatively short periods of time. For example a large proportion of the flow may be made up of water for a first period of time and oil and gas for a second period of time. It is difficult with a separator of fixed configuration to satisfactorily process different flows when flow conditions change in an unpredictable manner. 
     In a conventional separator, an inlet flow is generally passed through a stack of inclined plates within a relatively large vessel, the inclined plate encouraging the separation of water, oil and gas into separate superimposed layers. Gas can then be extracted from an upper section of the separator vessel and the water and oil can be separated by a simple weir separator plate the height of which is arranged to be above the interface between the water and oil layers. If the input flow is such that the separator plates become largely filled with a foam or emulsion of for example oil and water the separation performance is significantly degraded. Similarly, if a large volume of water is delivered to the separator in a relatively short period of time, it can be difficult to maintain the water/oil interface below the level of the weir separator. 
     With such problems in mind, the normal approach to separator design has been to provide a relatively large capacity separator which is capable of dealing with a wide range of conditions by in effect accepting wide fluctuations in separator plate efficiency and water/oil interface levels. As a result of this design philosophy, separator plant can make up a significant proportion of the size and weight of oil field equipment. This is a particular problem in the case of offshore oil fields where the size and weight of offshore equipment determines the economic viability of some oil bearing formations. 
     Attempts have been made to monitor separator performance in particular circumstances so as to be able to match separator design to expected separator operating conditions. The equipment used has generally required the mounting of heavy gamma ray sensors on separator equipment. The use of such equipment for monitoring routine operating conditions is not appropriate. 
     Extensive work has been conducted to enable flow conditions within for example circular-section pipes to be monitored. For example, U.S. Pat. No. 5,130,661 describes a capacitance sensor system in which an array of capacitor plates is disposed around the outer periphery of a pipe through which a mixture of oil, water and gas is passing. By appropriate manipulation of output signals derived from the sensors, an image can be built up of the flow cross-section. Such sensing systems have been used for example to estimate mass flow rates of different phases but it has not been suggested that the output of such systems can be used to achieve real-time process control. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a fluid flow control system which obviates or mitigates the forementioned problems. 
     According to the present invention, there is provided a system for controlling the flow of a mixed phase fluid through a vessel, comprising at least one flow control device located in an Inlet to or outlet from the vessel, a plurality of sensors distributed about the vessel to sense characteristics of the flow adjacent the sensors, means for monitoring outputs of the sensors to detect the location of a boundary between phases within the flow, and means for controlling the flow control device to maintain the boundary location within pre-determined limits. 
     The present invention may be applied to the control of processes such as inclined plate separation and weir plate separation. The supply of fluid to or the extraction of different phases from the vessel can be controlled so as to maintain interfaces between different phases at desired levels. For example, in the case of a weir plate, used to separate water and oil, the rate at which water is removed can be controlled to maintain the water/oil interface above a water outlet and below an upper edge of the weir. Similarly, the supply of fluid to an inclined plate separator can be controlled to prevent the separator becoming ineffective for example as the result of the build-up of emulsions between the plates. 
     Preferably the sensors are embedded in vessel surfaces adjacent to which the fluid flows, for example in the surfaces of the plates of inclined plate or weir plate separators. The sensors may be capacitance sensors, pressure sensors or conductivity sensors for example. In the case of an inclined plate separator, the sensors may be supported in for example three vertical arrays, the arrays being spaced apart in the directions of fluid flow through the separator. 
     The sensors may be provided in a multilayer structure comprising a top layer of conductance sensors, a screening layer, and a layer carrying a series of connections, each layer being separated from the other by dielectric material, conducting paths between the sensors and the connections passing through the screening layer. This arrangement is advantageous because it avoids “cross-talk” between the sensors and the connections. This makes conductance measurements more accurate, and allows some sensors to be used as sources of electric fields whilst other sensors are simultaneously used as detectors of electric fields. 
     The screening layer preferably comprises a layer of conducing material. Screening conducting material is preferably located between the connections to prevent cross-talk between the connections. A most preferably way of providing the multi-layer structure is by bonding together a series of printed circuit boards. 
     The sensors may be supported in a vertical array, for example at vertically spaced positions on a weir separator, or at vertically spaced positions on an elongate support extending vertically through the fluid flow. The sensors may be arranged to have a common axis or lie in a common plane, so that measurements of electrical properties are made between adjacent sensors. 
     The elongate support may comprise a rod having two faces subtending an angle of less than 180 degrees, the faces being provided with an array of sensors. This is advantageous over known configurations of sensors because it reduces the possibility of solid material becoming trapped between opposing sensors. 
     Preferably, at least one sensor is capable of acting either as a source of an electric field or as a detector of an electric field. Providing each of the sensors with this capability allows the distribution of detectors and sensors to be optimised for any required measurement. 
     Preferably, at least one of the sensors is located on a dielectric mounting. Where the dielectric mounting forms a casing in one surface of which a sensor is located, an electric field obtained by applying a voltage to that sensor will retain its general shape in the event that the permittivity of the media surrounding the casing is increased. 
     Preferably, at least one of the sensors is separated by dielectric material from an electrical connection to another of the sensors. This allows some sensors to be used as detectors and some to be used as sources of electric fields. 
     The sensors may control for example a device for injecting flow control chemical additives, or a flow choke device. Such a flow choke device may be located upstream from or downstream from a T-junction defining one inlet and two outlets, the flow choke being controlled to control separation processes occurring at the junction Thus an inlet flow to for example an inclined plate separator can be pre-conditioned by diverting at least a proportion of unwanted components away from the inclined separator at a T-junction located upstream of that separator. 
     The invention also provides a system for monitoring the flow of a mixed phase fluid through a vessel the vessel defining surfaces adjacent to which the fluid flows, wherein sensors are embedded in the said surfaces, each sensor providing an output representative of a characteristic of the flow adjacent the surface within which it is embedded, and the sensors being distributed within the said surfaces such that the location relative to the vessel of a boundary between phases of a flow can be determined from a comparison between the sensor outputs. 
     The invention also provides a method of calibrating an array of sensors configured to monitor the location of a boundary between layers of fluid, the method comprising obtaining a series of measurements from two of the sensors while the boundary between the layers of fluid is at a series of locations between central areas of the sensors. Once the array of sensors has been calibrated for a given fluid or fluids, the position of the boundary between the layers of the fluid or fluids can be very accurately determined. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings, in which; 
     FIG. 1 is schematic representation of a sensor array capable of monitoring the separation of an oil-water mixture; 
     FIG. 2 is a schematic representation of a vertical array of capacitance and pressure sensors which can be used to monitor the position of an interface between components of an oil-water mixture: 
     FIG. 3 is a schematic illustration of a separator vessel incorporating inclined plate and weir plate separators; 
     FIGS. 4,  5  and  6  each show vertical axial sections and vertical transverse sections through three alternative flow sensing assemblies; 
     FIG. 7 schematically illustrates the distribution of capacitance sensor components in two adjacent plates of an inclined plate separator such as that incorporated in the structure illustrated in FIG. 3; 
     FIG. 8 illustrates in further detail the disposition of the sensor arrangement shown in FIG. 7; 
     FIG. 9 illustrates the structure of FIG. 8 in greater detail; 
     FIG. 10 represents the disposition of two vertical arrays of sensors in the weir plate separator of FIG. 3; 
     FIG. 11 illustrates a vertically extending structure of a type incorporated in the arrangement of FIG. 3; and 
     FIG. 12 illustrates control arrangement for a T-junction separator stage which may be incorporated upstream of the separator assembly of FIG.  3 . 
     FIG. 13 illustrates a specific configuration of a vertical array of sensors of the type which is shown schematically in FIG. 2; 
     FIG. 14 illustrates an alternative configuration of a vertical array of sensors of the type which is shown schematically in FIG. 2; 
     FIG. 15 is a cross-sectional view of an alternative configuration of a vertical array of sensors of the type which is shown schematically in FIG. 2; 
     FIG. 16 illustrates a series of layers which together comprise a vertical array of sensors of the type which is shown schematically in FIG. 2; 
     FIG. 17 illustrates schematically a cross-section of a series of layers which together comprise a vertical array of sensors of the type which is shown schematically in FIG. 2; 
     FIG. 17 a  illustrates schematically a more detailed cross-section of the layers shown in FIG. 17; 
     FIG. 18 is a schematic view of a cross-section through a source and a detector located in hemispherical casing; 
     FIG. 19 is a representation of a source and a detector showing the electric field present when a voltage is applied to the source; 
     FIG. 20 is a schematic view of a cross-section through the separator vessel of FIG. 3, showing an array of capacitance sensors, and oil and water held in the vessel in separated layers. 
     FIG. 21 illustrates a section of the array of capacitance sensors shown in FIG.  20 . 
     FIG. 22 is a graph representing a number of capacitance profiles obtained using array comprising a series of electrodes whilst an interface between two media was traversed across the separator vessel of FIG.  20 . 
     FIG. 23 is a graph representing data from FIG. 21 which data has been mathematically transformed. 
     FIG. 24 is a graph representing a computer simulation corresponding to the data shown in FIG.  23 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, the illustrated system comprises a rectangular section pipe  1  within a upper section of which is embedded a source electrode  2 . Four detection electrodes  3  are embedded in axially spaced lower portions of the pipe  1 . Signals detected at the electrodes  3  are delivered to a capacitance meter  4  the output of which is applied to a data acquisition system  5 . The output of the data acquisition system  5  is applied to a computer  6  which produces an estimate of the position of an interface  7  between oil and water components of the flow (represented by arrow  8 ) within the pipe  1 . 
     It will be appreciated that the position of the interfaces  7  within the pipe will vary over time and the resulting changes in the dielectric coefficient of the region between each electrode  3  and the source electrode  2  will vary. Such variations enable an estimate to be made of the relative volumes of the different phases passing through the pipe  1 . 
     FIG. 2 is schematic illustration of a sensor array which can be used to monitor the location of an interface between different phases in a relatively large vessel across which the illustrated array extends. The illustrated arrangement comprises a support  9  on which thermocouples  10  are positioned to enable temperature compensation of sensor outputs to be achieved. Mounted on the support  9  is a sub-assembly  11  supporting a vertically spaced array of piezoelectric pressure transducers  12  and a vertically spaced array of embedded capacitance sensors  13 . If the array is immersed in a vessel containing two vertically separated layers of for example oil and water, outputs from the sensors enable an estimate to be made of the vertical position of the interface between the different layers. 
     Referring now to FIG. 3, this schematically represents a separator for use in the separation of water, oil and gas components from an oil well production flow The separator comprises a containment vessel  14  having an inlet  15 , a gas outlet  16 , a produced water outlet  17 , and an oil outlet  18 . Pipe sensors for example of the type illustrated in FIG. 1 are represented as component  19  on the inlet  15  and components  20 ,  21  and  22  on the outlets  16 ,  17  and  18 . A further outlet  23  is provided for the discharge of accumulated solids, and inlets  24  are provided to enable the agitation of deposited solids. Solids will generally be discharged periodically and therefore the dynamic control of the output  23  and input  24  is not required. This feature of the operation of the separator will not therefore be further described herein. 
     The separator vessel  14  houses an inclined plate separator  25 , a vertically extending weir plate separator  26 , an oil/gas separator  27  and three sensor arrays  28 ,  29  and  30 . With the exception of the instrumentation supported on the units, the separators  25 ,  26  and  27  are conventional. The sensor arrays  28 ,  29  and  30  are of the general type described with reference to FIG.  2 . The separator plates  25  support three vertical arrays  31  of capacitance sensors. The weir plate separator  26  supports two vertical arrays of capacitance sensors  32 , and angled plates in the separator  27  each support a capacitance sensor. The various sensing assemblies are connected by signal processing units  33 ,  34 ,  35  and  36  to a controller  38  which in rum is arranged to control valves  39 ,  40 ,  41  and  42  provided on the inlet and gas, oil and water outlets respectively. The valve  42  can also be controlled directly by the signal processing unit  35  such that if the oil/water interface on the upstream side of the weir plate  26  rises above a predetermined level the valve  42  is open to discharge water. Similarly, the valve  41  can be directly controlled by a level gauge  43  which discharges oil through valve  41  automatically if the oil/gas interface exceeds an upper sensing level  44  and closes the valve  41  automatically if the oil/gas interface falls below a sensing level  45 . 
     The sensor arrays  31  within the plate separator  25  enable the interface between any flow of emulsion separating water and oil to be accurately located. The sensor arrays  28 ,  29 ,  30  and  32  make it possible to monitor the efficiency of the separation process in the direction of flow through the separator. This information is delivered to the controller  38  which then ensures the appropriate control of the inlet and outlet valves to maintain appropriate flow and pressure conditions to prevent undesirable circumstances developing, for example a circumstance such that significant volumes of emulsion are present in the separator downstream of the inclined plate separator  25 . 
     FIG. 4 shows the detailed structure of one possible pipe sensor which differs slightly from that of FIG.  1 . In the arrangement of FIG. 4, five rings each of eight plates  46  are disposed around the axis of a short flanged pipe insert. Appropriate signals may be applied to the plates  46  so as to derive the necessary capacitance measurements. In the arrangement of FIG. 5, which is similar to that shown in FIG. 1, a single lower plate  47  is located opposite five upper plates  48 . In the embodiment of FIG. 6 a rectangular section channel is defined having single plates  49  in upper and lower walls. 
     FIG. 7 illustrates the sensor structure in two adjacent plates  50  of the inclined plate separator  25  of FIG.  3 . One plate carries a continuous excitation source conductor  51  which faces an array of sensor electrodes  52 . FIG. 8 is a front view of the plate  50  carrying the electrodes  52 . Electrodes  52  are surrounded by a guard electrode  52   a  to obtain a uniform electrical field in front of the detecting electrodes  52 , between source (not shown in FIG. 8) and, detecting electrodes  52 . Assuming a flow of oil, oil-water emulsion and water in the direction of arrow  53 , the interface between the emulsion and oil may be located as indicated by broken line  54  and the interface between the emulsion and water may be located as indicated by broken line  55 . It will be appreciated that signals derived from sensors located adjacent the body of emulsion will be substantially different from signals derived from electrodes adjacent either the oil or water phases. 
     The controller  38  of FIG. 3 is set up to monitor changes in the location of the interfaces  54  and  55  as shown in FIG. 8 so as to prevent a substantial proportion of space between the inclined separator plates becoming filled with emulsion. 
     FIG. 9 shows one structural assembly which can be used to achieve sensing electrode arrays of the type described-with reference to FIG. 7 and 8. Each of the plates comprises an electromagnetic shield  56  and each of the spaced detection electrodes  52  is surrounded by a guard electrode  57 . The electrodes may be in the form of conductive areas supported by a printed circuit board and housed within an epoxy resin insert secured to the plates  50  by screws  38 . The plates could be formed from GRP. 
     FIG. 10 shows two vertical arrays of sensing electrodes  32  which are provided in the weir plate  26  of FIG.  3 . Each array may be independently monitored so as to provide comparative outputs between any vertically aligned pair and any vertically adjacent pair, thereby enabling the detection of any operational fault which might compromise the response of the system to the oil/water interface approaching either the top of the weir or the outlet  17 . 
     FIG. 11 illustrates in greater detail the structure of a sensor array of the type illustrated generally in FIG.  2  and used to define the sensor arrays  28 ,  29  and  30  in FIG.  3 . The illustrated assembly comprises an elongate support  59  incorporating a vertical array of pressure ports  60  each coupled to a piezoelectric pressure sensor (not shown). The support  59  also supports an array of detection electrodes  61  surrounded by guard electrode  62 . A source electrode  63  is also mounted on the support  59  spaced from the detection electrodes  61  such that the space between the electrodes is filled by the fluid within the separator. One possible location for an oil-emulsion interface is indicated by plane  64 , and one possible location for an emulsion-water interface is indicated by plane  65 . It will be appreciated that monitoring the outputs of the pressure and capacitance sensing transducers will enable the location of the interfaces  64  and  65  to be accurately determined and thereby enable the appropriate control of the overall process to ensure that the interfaces  64  and  65  are maintained within acceptable limits. 
     Referring now to FIG. 12, this shows a T-junction initial separator which may be connected to the separator illustrated in FIG. 3 upstream of the inlet  15 . An inlet  66  is coupled through a valve  67  to a T-junction  68  that is coupled by a valve  69  to an outlet  70  and is coupled directly to an outlet  71 . Flow sensors  72 ,  73  and  74  for example of the type describe with reference to FIG. 4 are located on the inlet to and the outlets from the T-junction  68 . Outputs from those sensors are processed in a signal processing unit  75  which controls the valves  67  and  69 . 
     It is known that supplying an oil/water mix to a T-junction as described in which the two outlets are directed in vertically opposite directions results in some separation of the two components, water tending to flow vertically downwards and oil tending to flow vertically upwards. The present invention enables this known effect to be optimised by controlling the flow to and back pressure within the T-junction  68  by appropriate modulation of the control applied to the valve  67  and  69 . 
     This document does not contain detailed formula linking inputs to the signal processing units and consequential valve control outputs. Detailed control algorithm will be required which will differ between different applications. To further explain the underlying design philosophy however, relevant conditions which might apply in a three phase separator such as that illustrated in FIG. 3 are discussed below. 
     The inlet flow sensor  19  provides information on the relative quantities of the various phases flowing at any one time into the separator. Control actions might be applicable if for example large slugs of water enter the vessel. 
     The vertically extending sensor arrays  28 ,  29  and  30  are distributed along the direction of flow through the separator, one ( 28 ) being between the inlet and the plate separator and enabling the efficiency of the flow distribution system to be monitored. If the flow distribution system becomes partially blocked by solids then the mass flux across the separator vessel would change and this could adversely affect the separation process in the rest of the unit. It would be possible to detect the development of such conditions from the upstream array  28 . The two downstream arrays  29  and  30  make it possible to monitor the position and depth of any heterogeneous layers such as emulsions between oil and water, and foam between gas and oil phases. These heterogeneous layers are sometimes transitory and build up locally in the vessel particularly towards the weir plate  26 . If an array of sensors is embedded in the weir plate itself as shown by electrodes  32  in FIG. 3, his gives additional information as to the position of various phase boundaries at the downstream end of the separation system. The outputs from the two vertical arrays  29  and  30  and the weir plate  26  enable optimised control of the water outlet valve  42 . Signals from the weir plate sensor electrodes  32  would be particularly useful to produce alarm signals should the heterogeneous liquid-liquid or indeed the produced water layer itself rise to be near the top of the weir plate. If this condition developed then entrainment of water in the separated oil would increase rapidly. The instrumentation downstream of the plate separator  25  would enable such conditions to be avoided. 
     Within the separator plate assembly  25 , the three vertically aligned arrays of electrodes  31  make it possible to obtain an indication of the mass flux distribution entering the plate system. It would also indicate if solids began to build up in the vessel upstream of the plate assembly and within the plate assembly itself. It will be appreciated that if solids start to build up between the separator plates separation efficiency will be rapidly degraded. The present invention provides real time measurements initiative of solids accumulation in the system. 
     With in the inclined plates separator, the axial separation of the three arrays of sensor electrodes  31  makes it possible to monitor the separation process in the axial direction. Two conditions which could develop and which would give rise to a loss of separation efficiency are choking, that is local build-up of one of the phases, and instability, that is instability of the interface between water and oil in the individual channels defined between adjacent plates. Both conditions give rise to rapid entrainment of the discontinuous phase and a fall in overall separation performance. These conditions can be monitored with the sensor electrode arrays as shown. By analysing the output of all the various sensor arrays, a control strategy can be developed to maintain operation within a pre-defined desirable envelope. 
     The incorporation of sensors in the otherwise conventional separator  27  which is provided on the gas outlet to eliminate mist, enables any build-up in liquid within the separator to be detected. If liquid builds up then the probability of mist entrainment in the gas outlet increases. Entrainment of liquid in the gas leaving the vessel can produce serious processing and safety conditions in downstream processes. 
     Although the controller  38  as described with reference to FIG. 3 is only used to control inlet and outlet valves, it will be appreciated that the controller may also be used to control the injection of chemicals to enhance phase separation, for example, by inhibiting the formation of emulsions and foams. 
     The sensor arrangement shown in FIG. 2, and as  28 ,  29  and  30  in FIG. 3, can be replaced by an alternative arrangement shown in FIG. 13. A series of sensors in this case are electrodes  76  for measuring capacitance. Contrary to the conventional arrangement of sources spaced apart from parallel detectors which allow fluid to flow between them, the electrodes  76  are arranged on a single surface  77 . This avoids the possibility of solid matter becoming trapped between the electrodes  76 . Each electrode  76  can be used as either a source (of an electric field) or a detector (of the electric field) by appropriate switching circuitry, and capacitance is measured between pairs of electrodes (probably adjacent electrodes, but not necessarily). 
     An alternative configuration of sensor array is shown in FIG.  14 . The electrodes of this array comprise a series of rings  78  disposed at regular intervals along a rod  79 . One possible mode of operation of the array is illustrated schematically, wherein each of the rings  78  labelled ‘s’ acts as a source and the ring  78  labelled ‘det’ acts as a detector. 
     FIG. 15 illustrates a further alternative configuration of sensor array. A ‘V’ shape is cut into a rod  80 , and a source  81  is positioned in one face and a series of detectors  82  is spaced along the other face of the ‘V’. This sensor array is conventional in that it comprises a single source  81  which is not capable of acting as a detector and a series of detectors  82  spaced away from the source  81 , which detectors are not capable of acting as a source. This array configuration is advantageous over known conventional configurations in that solid material is unlikely to become trapped between the faces of the ‘V’. A typical angle subtended by the faces of the ‘V’ is 120 degrees. Screens  83  are located behind the sources  81  and the detectors  82 . 
     An array of electrodes which has minimal undesired “cross-talk” effect between electrodes and connections to electrodes is illustrated in FIG.  16 . The way comprises six layers of printed circuit board (PCB)  84 - 89  which are bonded on top of one another as shown in FIG.  17 . The dark (solid) regions in FIG. 16 correspond to conducting material. 
     A first of the layers  84  of the array comprises a series of electrodes  90  surrounded by a guard electrode  91 . A second layer  85  comprises a shielding conductor  92 , with electrical connections  93  leading to signal carrying connections in further layers of the array. 
     A third layer  86  contains signal carrying connections  94  and a shielding conductor ( 94   b ) which prevents “cross-talk” between connections  94  in the third layer  86 . In the present example, layer  86  allows connections from  10  electrodes  90  of layer  84 , via electrical connections  93  in the second layer  85  to “soldering points”  95  at an upper end of the third layer  86 . 
     A fourth layer  87  is essentially the same as layer  85 . It shields the third layer  86  from a fifth layer  88  but provides electrical connections between electrodes  90  and signal carrying connections  94  in the fifth layer  88 . 
     The fifth layer  88  is conceptually similar to the third layer  86  and provides connections from eight remaining electrodes  90  to the “soldering points”  95  at an upper end of the layer  88 . The signal carrying connections  94  are shielded from one another by conductor  94   b.  The sixth layer  89  serves as a shield between layer  88  and an external environment. 
     In general, for capacitance measuring electrodes, when the surface area of a connection to a given electrode becomes comparable to the surface area of that electrode, the measured capacitance will be influenced by the connection. When a conventional ‘dipstick’ arrangement of electrodes is immersed in a medium of given permittivity, each electrode should measure the same capacitance. However, the effect of the surface areas of the electrode connections is such that those electrodes near a lower end of the dipstick will measure a greater capacitance than those near an upper end of the dipstick. Tie array of electrodes illustrated in FIGS. 16 and 17 avoids this problem by shielding the electrodes  90  from the signal carrying connections  94 , and the signal carrying connections  94  from one another. Efficient shielding is provided by the conducting plates  92  of the second  85  and fourth  87  layers of the array, which prohibit cross-talk between the layers  84 ,  86  and  88 . 
     To obtain a desired performance of the sensors it is necessary to “hardwire” the following components: (i) guard electrode  91 , (ii) shielding conductor  92  in the second layer  85 , (iii) shielding conductor  94   b  in the third layer  86 , (iv) shielding conductor  92  in the fourth layer  87 , (v) shielding conductor  94   b  in the fifth layer  88 , (vi) shielding conductor comprising the sixth layer  89 , in as many places as possible. In the discussed example this has been achieved by the process of “through-hole plating”, while the sensors were manufactured using printed circuit board (PCB) technology. The connections are omitted in FIG.  16 . 
     Connections between electrodes  90 , connections  93  in layers  85  and  87  and signal carrying connections  94  in layers  86  and  88  were similarly made using “through-hole plating” technique. 
     The technique of shielding between individual layers in the “sandwich” described with reference to FIG. 16, a well as between individual signal carrying connections in layers  86  and  88  of FIG. 16 is advantageous because it allows both: 
     avoidance of cross-talk between signals from individual electrodes; as well as 
     the use of each electrode as a source or detector as described with reference to FIG.  13 . If the connections to the electrodes were not shielded then “cross-talk” between the sources and the detectors would be imposed on the measured signal, reducing the accuracy of measurement significantly. 
     The array of FIG. 16 is just one example of many possible ways of shielding the signal carrying connectors. A section through a generalised array of electrodes is shown in FIG.  17 . An uppermost surface and a lowermost surface of the array are formed from laminate  97 . A series of electrodes  98  are connected, through a series of shields  99  to a series of connectors  100 . The number of electrodes, shields and connectors may be tailored to any required purpose. 
     A generalised array of the type shown in FIG. 17 is shown in more detail in FIG. 17 a.  The cross-section of FIG. 17 a  is across a width of an array. Laminate  97  again forms the upper and lower surfaces of the array. A detection electrode  98  is provided with a guard electrode  98   a  at either side. A series of shielding layers  99  separate the detection electrode from a series of signal carrying connections  100 . Each signal carrying connection  100  is isolated from its neighbours by separation conductors  100   a.    
     FIG. 18 shows a source  101  and a detector  102  located on parallel faces of two parts  103  of a circular rod (middle section removed). A shield  104  is located behind both the source  101  and the detector  102 . The arrangement of detector/shield in the bottom half of FIG. 18 will be probably the generalised array from FIG. 16 and 17. The supports  103  illustrated are casings which may be constructed from metal or from dielectric material. When dielectric material is used, an electric field obtained from the source  101  is closer to being a uniform field than that obtained when a metal support  103  is used. 
     FIGS. 19 a  and  19   b  illustrate a source arid detector, of the form illustrated in FIG. 18, held in a metal casing. When the source and detector are located in air (FIG. 19 a ), the electric field obtained is close to parallel, but when they are immersed in water (FIG. 19 b ) the field degenerates significantly. Thus, as the permittivity of the media surrounding the detector and source is increased, the quality of electric field obtained is degraded. In contrast, when the source and detector are held in a dielectric casing (FIGS. 19 c  and  19   d ), the electric field obtained is close to parallel both in air (FIG. 19 c ) or in water (FIG. 19 d ). Thus, the dielectric casing is advantageous because it provides an electric field which is close to parallel when the detector and source are located in media of a range of permittivities. 
     FIG. 20 shows a cross-section of the separator vessel of FIG. 3, containing water oil arid air in three layers. An inclined plate separator  105  is provided with an array  106  of capacitance sensors. A section of the array  105  is illustrated in more detail in FIG.  21 . An upper electrode of the array is a source  107 , and the lower electrodes are a series of four detectors  108 . 
     The array may be calibrated to allow detection of the position of the oil/water interface to an accuracy better than the height of one electrode  108 . The calibration procedure comprises: 
     displacement of the interface, in a vertical direction, in small steps (at least a few steps per electrode height) along the entire length of the array  105 ; 
     storing the differences in readings from all adjacent pairs of electrodes (eg.: . . . C k−2 -C k−1 , C k−1 -C k , C k -C k+1 , C k+1 -C k+2 , . . .) 
     The above differences are zero (assuming that the electrodes are identical and far from “fringe” effects which occur close to ends of the array) if the pairs of adjacent electrodes  108  are immersed in the same medium (oil or water). However, when the two adjacent electrodes  108  are in different media, the difference between their readings reaches a maximum. All “intermediate” interface positions produce “intermediate” values C k -C k+1 , and these a used to identify the position of the interface with a high resolution. 
     FIG. 22 shows a number of capacitance profiles obtained for an array comprising a series of eighteen electrodes, while an interface level was traversed across the vessel. 
     FIG. 23 shows data from FIG. 22, read from two electrodes of the array. The data has been “transformed” into co-ordinates (C 10 -C 9 ) versus oil layer thickness (or an arbitrary position of the interface). This “spike-like” function is unique for a given media permittivity. By looking at differences between neighbouring electrodes the position of the interface can be established to an accuracy greater than the height of one electrode. 
     FIG. 24 shows a computer simulation where permittivity of distilled water (ε r = 80 ) was replaced by different values ( 50 ,  20  and  5 . 6 ). The character of the calibration curve remains unchanged but the maximum varies. This implies the need for separate calibrations for all combinations of media for which the sensor array is to be used.