Abstract:
There is provided a method of calibrating a measurement system for measuring a position (h) of a meniscus of a liquid fuel included within a marine fuel tank and therefrom generating a corresponding content signal indicative of a proportion of the tank filled with fuel. The method includes steps of: (a) from a first time instance when the tank is substantially empty, filling during a calibration phase the tank with the fuel at a substantially constant rate whilst simultaneously recording corresponding values (h(t)) of a meniscus position indicative signal generated by the measurement system; (b) at a second time instance after said first time instance, terminating filling of the tank with fuel when the position indicative signal corresponds to the tank being substantially filled with fuel; and (c) subsequently, after the calibration phase, applying signal processing to the position indicative signal to generate the content signal indicative of the proportion of the tank filled with fuel, the signal processing accounting for the tank during the calibration phase being filled at the substantially constant rate.

Description:
BACKGROUND AND SUMMARY 
     The present invention relates to methods of calibrating measurement systems, for example to methods of calibrating measurement systems for measuring fuel levels in fuel tanks such as marine fuel tanks. Moreover, the present invention is concerned with measurements systems operable according to these methods. Furthermore, the present invention also concerns software executable on computing hardware for executing these methods. 
     Methods of calibrating measurement systems operable to measure liquid levels in containers and thereby determining a proportion of the container filled with liquid are known. For example, road vehicles such as automobiles contemporarily include fuel measurement systems for providing drivers of such vehicles with information regarding remaining fuel in fuel tanks of the vehicles. 
     Moreover, such vehicles are often mass produced items whose tanks are often of relatively simple and predictable shape and can be pre-characterized during initial design of such vehicles; namely, a mathematic relationship between a height of a meniscus of liquid fuel in such a tank to a volumetric proportion of the tank filled with fuel is known and predictable for road vehicles of a given design. 
     A problem arises in marine vessels on account of a lesser degree of standardization being encountered in comparison to products such as road vehicles which are subject to relatively large production runs of a given design of vehicle. Fuel and water tanks in marine vessels can be of highly complex and potentially variable shape as elucidated in a published United States patent application no. US2004/0149003. In this application, it is disclosed that a contained volume in a marine vessel tank is rarely uniform with respect to any axis, and especially with respect to a vertical axis along which conventional float-based fluid gauges operate. A consequence of such non-uniformity is that movement of the float gauge is rarely a direct indication of a quantity of liquid, for example fuel or water, input to or extracted from the marine vessel tank; such lack of accurate indication by such float-based fluid gauges is a technical problem that the present invention seeks to address. 
     The technical problem is further confounded when flexible bladder-style tanks are used on marine vessels, for example yachts, to fit within highly complex and irregular spaces provided within such marine vessels; such flexible tanks are able to assume a shape of the space into which they are introduced. 
     The aforementioned published patent application no. US2004/0149003 discloses a method of accommodating ascertainment of fill characteristics of fluid tanks on marine vessels. The method includes a first step of providing an experientially-based mapping of volume characteristics of a fluid tank positioned on a marine vessel, the correlation being correlated with respect to measurable fluid surface positions within the fluid tank. Moreover, the method includes a second step of outputting fluid fill condition information for operator use based on quantification of a parameter other than sensed fluid level in the fluid tank based on a function of that other parameter being compared to the experientially-based mapping function. In practice, the method makes use of a fuel dispenser, namely a volumetric measuring dispenser operable to measure and report a volume of fluid dispensed into an interior volume of the tank. 
     The method described in the aforesaid published patent application is found to be potentially awkward and inconvenient to employ in practice, especially if the tank is modified frequently by occasionally coupling one or more supplementary tanks thereto or by movement of the tank when implemented in flexible form to conform to given available space. 
     Thus, the present invention is concerned with providing an at least partial solution to the aforesaid technical problem by providing a more straightforward and readily-applicable method of calibrating measurement systems such as liquid level measurement systems for marine vessels. 
     It is desirable to provide a method of calibrating measuring systems such as liquid measuring systems. 
     According to a first aspect of the invention, there is provided a method of calibrating a measurement system operable to measure a spatial position of a meniscus of a liquid included within a container and therefrom generating a corresponding liquid content signal indicative of a proportion of the container filled with the liquid, said method including steps of: 
     (a) from a first time instance when said container is in a first state including a first quantity of said liquid, progressively filling during a calibration phase said container with the liquid at a substantially constant rate of liquid delivery whilst periodically simultaneously recording corresponding generated values of a meniscus position indicative signal generated by said measurement system; 
     (b) at a second time instance after said first time instance during said calibration phase, terminating progressive filling of said container with said liquid when said position indicative signal corresponds to the container being in a second state including a second quantity of said liquid; and 
     (c) subsequently, in normal use after said calibration phase, applying signal processing to said position indicative signal provided from the measurement system to generate said corresponding liquid content signal indicative of said proportion of the container filled with the liquid, said signal processing accounting for said container during said calibration phase having been filled at the substantially constant rate of liquid delivery to the container. 
     The present invention is of advantage in that the measuring system is capable of being calibrated more straightforwardly and easily by the user. 
     Optionally, in the method, the first state corresponds to the tank being substantially empty and devoid of liquid therein, and the second state corresponding to the tank being in a full state substantially filled with liquid. 
     Optionally, in the method, said substantially constant rate of liquid delivery is a substantially constant volumetric rate of liquid delivery. Surprisingly, most practical liquid supply apparatus, for example contemporary fuel or water pumps, are operable to supply at a quasi-constant rate. 
     Optionally, in the method, said known rate of liquid delivery is determined from knowledge of a substantially full liquid capacity of the container and a time interval between said first and second time instances. The full capacity is conveniently known from a substantially maximum allowable height of the liquid in the container. 
     Optionally, in the method, said container is a fuel tank, and said liquid is a combustible fuel for including in said fuel tank. 
     Optionally, in the method, said measurement system operable to measure said spatial position of said meniscus of said liquid included in said container is operable to employ a float which is operable to float on said meniscus for measuring said position. 
     Optionally, in step (c) of the method, said signal processing is implemented in a processor operable to determine a mathematical relationship between a volumetric portion of the container filled with liquid and the corresponding meniscus position indicative signal and to apply substantially said mathematical relationship so as to derive in said normal use said container liquid content signal indicative of a proportion of the container filled with the liquid. More optionally, in the method, said mathematical relationship is a polynomial approximation derived from said periodically recorded generated values of said meniscus position indicative signal. 
     Optionally, the method is adapted for use with a marine vessel wherein said container is a fuel tank of said marine vessel. 
     Optionally, in the method, said container is a fuel tank arrangement susceptible to being supplemented with one or more additional fuel tanks in liquid communication with said fuel tank arrangement, said method being repeatedly applicable in response to said fuel tank arrangement being supplemented or de-supplemented with said one or more additional tanks, so that said container liquid content signal is indicative of a proportion of the fuel tank arrangement and its one or more supplementary fuel tanks coupled thereto that are filled with the liquid. 
     Optionally, in the method, the first time instance is recorded by the measurement system as commencing when a threshold rate of change of the meniscus position indicative signal occurs after user activation of a calibrate input of the measurement system. 
     According to a second aspect of the invention, there is provided a measurement system operable to measure a spatial position of a meniscus of a liquid included within a container and therefrom generating a corresponding liquid content signal indicative of a proportion of the container filled with the liquid, said measurement system being operable to being calibrated: 
     (a) from a first time instance when said container is in a first state including a first quantity of liquid by progressively filling during a calibration phase said container with the liquid at a substantially constant rate of liquid delivery whilst simultaneously recording corresponding generated values of a meniscus position indicative signal generated by said measurement system; 
     (b) at a second time instance after said first time instance during said calibration phase, by terminating progressive filling of said container with said liquid when said position indicative signal corresponds to the container being in a second state including a second quantity of liquid; and 
     (c) subsequently, in normal use after said calibration phase, by applying signal processing to said position indicative signal provided from the measurement system to generate said corresponding liquid content signal ( 410 ) indicative of said proportion of the container filled with the liquid, said signal processing accounting for said container during said calibration phase having been filled at the substantially constant rate of liquid delivery to the container. 
     Optionally, in the measurement system, said first state corresponds to the container being substantially in an empty state substantially devoid of said liquid, and said second state corresponds to the container being in a full state substantially filled with said liquid. 
     Optionally, in the measurement system, said container is a fuel tank and said liquid is a combustible fuel, said measurement system being adapted to measure fuel included within said fuel tank. More optionally, the measurement system is adapted for use in marine vessels. 
     According to a third aspect of the present invention, there is provided software on a data carrier, said software being executable on computing hardware for implementing a method according to the first aspect of the invention. 
     It will be appreciated that features of the invention are susceptible to being combined in any combination without departing from the scope of the invention as defined by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       By way of example only, embodiments of the present invention will now be described with reference to the accompanying drawings wherein: 
         FIG. 1  is a schematic illustration of an embodiment of the present invention, the illustration including a depiction of a marine vessel provided with a tank for accommodating a liquid, for example water or fuel, a measuring sensor included within the tank for measuring a meniscus height of the liquid included within the tank and generating a corresponding meniscus height output signal, a data processor for receiving the meniscus height output signal and processing the signal to generate a tank liquid-content indicative signal for user presentation on a display wherein the display provides the user with an indication of a volumetric proportion of the tank filled with the liquid; 
         FIG. 2  is a schematic graph of sensing characteristics of the measuring sensor illustrated in  FIG. 1 ; and 
         FIG. 3  is a schematic illustration of the tank of  FIG. 1  supplemented with an optionally additional tank. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , there is shown a marine vessel, for example a yacht or fishing boat, indicated generally by  10 . For purposes of implementing the present invention, the vessel  10  can be floating in water or alternatively supported on land. The vessel  10  includes a hull  20 , an upper deck  30  attached to the hull  20  at an upper region thereof, and a user control cabin indicated by  40  built onto the upper deck  30 . The control cabin  40  includes user operated controls  50 , for example a rudder wheel, for steering and controlling a speed of the vessel  10  when in operation in water. Moreover, the control cabin  40  includes a display  60 , for example a digital numerical display, a bar-graph display or a gauge-type dial display indicative in operation of a proportion of tank  100  of the vessel  10  filled with a quantity of a liquid  110 . The liquid  110 , by action of gravitational forces, collects at a lower region of the tank  100  and thereby forms an upper meniscus  120 . The tank  100  is provided with an exit pipe  130  for providing a path by which a quantity of the liquid  110  can be removed from the tank  100 , and an input pipe  140  for providing at least a partial degree of ventilation for the tank  100  and also for providing a path by which a quantity of the liquid  110  can be introduced into the tank  100 . 
     A measurement sensor  200  is included in substantially a central region of the tank  100  so as to render the sensor  200  less affected by roll, pitch or yaw of the vessel  10  when in operation floating on water; such roll, pitch and yaw is susceptible to affecting the meniscus  120 . The measurement sensor  200  can be implemented in various ways, for example: 
     (a) as a mechanical float-type gauge wherein a float  210  floats substantially on the meniscus  120  of the liquid  110 , the float  210  moving relative to a substantially vertically orientated float-guide to generate a signal representative of spatial position of the float relative to the guide and hence an indication of a height of the meniscus  120  in the tank  100 ; 
     (b) as a wavelength-based sensor such as those utilizes reflected light or sound, for example reflections from the meniscus  120 ; or 
     (c) an array sensor comprising a linear array of elements each operable to contribute to an output signal from the measurement sensor  200  and operable to exhibit a change of response from being immersed in the liquid  110  to being non-immersed in the liquid  110 , for example an array of thermal-conductively sensitive elements. 
     The measurement sensor  200  is operable to generate a meniscus height output signal  220  which is coupled to a data processor  400 . The data processor  400  can be implemented as digital hardware operable to execute software. Alternatively, the data processor  400  can be implemented using dedicated digital hardware, for example an application specific integrated circuit (ASIC). In operation, the data processor  400  generates a tank liquid-content indicative signal  410  for driving the aforementioned user display  60 . 
     In operation, when quantities of the liquid  110  are extracted from the tank  10  via the exit pipe  130  such that the meniscus  120  is at a lower height  300 , the tank  100  is deemed to be substantially empty, even if a relatively small residual of the liquid  110  is present below the lower height  300 . Conversely, when quantities of the liquid  110  are added to the tank  100  via the input pipe  140  such that the meniscus  120  is at an upper height  310 , the tank  100  is to be substantially full, even if a relatively small volume of space at an upper region of the tank  100  is not completely filled with the liquid  110 . In operation, for example when the sensor  200  is implemented as a mechanical float gauge, the float  210  spatially moves between the lower height  300  and the upper height  310  in response to a proportion of the tank  100  filled with the liquid  110 ; the tank  100  is susceptible to being employed in a manner such that the float  210  moves between the upper height  310  and the lower height  300 , although the float  210  may in normal circumstances move in a smaller range between these heights  300 ,  310  in response to tank filling operations and extractions of quantities of the liquid  110  from the tank  100 . 
     In operation, the tank  100  is potentially of a complex peripheral shape when installed in the marine vessel  10 . For example, as depicted in  FIG. 1 , the tank  10  is relatively wider at an upper portion thereof near to the upper height  310 , and tapers towards a narrower lower point on approach to the lower height  300 . In consequence of such a complex peripheral shape for the tank  100 , the meniscus height output signal  220  varies with a volume of the liquid  110  included within the tank  100  in a manner as depicted in  FIG. 2 .  FIG. 2  will now be elucidated in respect of a method of the present invention. 
     In  FIG. 2 , there is shown a graph comprising an abscissa axis  500  denoting increasing time from left to right, and an ordinate axis  510  denoting increasing meniscus height output signal  220  from bottom to top of the ordinate axis  510 . Values of the output signal  220  corresponding to the float  210  being at the lower and upper heights  300 ,  310  are denoted by “0%” and “100%” respectively. On the abscissa axis  500 , there is marked a first time instance denoted by “0”, and a second time instance denoted by “T”. 
     The method of the invention employs a calibration routine to calibrate the sensor  200 , the data processor  400  and the display  60  so that the display  60  provides a substantially accurate indication to the user (not shown) of a proportion of the tank  100  which is filled with the liquid  110 . The calibration routine commences at the first time instance at a time t=0 and terminates at the second time instance when t=T; a calibration period thereby exists between the first and second time instances. At the first time instance, the float  210  is arranged to be at a lower position; for example, the tank  100  is arranged to be substantially empty so that the meniscus  120  is substantially at the lower height  300 . The present invention is however not limited to the sensor  200  being calibrated between states of the tank  100  being substantially empty and substantially full; for example, the sensor  200  can be calibrated in a plurality of calibration steps whose calibration results are subsequently combined to provide calibration of the sensor  200  over a correspondingly larger range. 
     During the calibration period, a source of liquid, for example a nozzle of a hose of a fuel filling pump, is coupled to the input pipe  140 , and activated to fill the tank  100  so that the meniscus  120  rises from the aforesaid lower position, for example the lower height  300 , to an upper position, for example to the aforesaid upper height  310 , progressively during the calibration period. Of significance to the present invention is that, in practice, most fuel delivery systems or water delivery systems provided for marine vessels, for example in harbors, dispense at a substantially constant rate during the calibration period; in other words, the filling rate is quasi-constant during the calibration period. For implementing the method of the invention, it is not necessary to measure or know the rate of delivery in contradistinction to known contemporary approaches to measurement calibration. For implementing the present invention, it is merely necessary to know beforehand one or more of: 
     (a) a given volumetric capacity K of the tank  100  corresponding to the meniscus  210  being at the upper position, for example at the upper height  310 ; 
     (b) a duration of the calibration period, namely a period T, and a rate of delivery of the liquid  110  to the tank  100 ; the period T can be measured in a simple manner using a stopwatch or similar; and 
     (c) the position of the meniscus  120  at the lower and upper position over which calibration is desired. 
     For example, the tank  100  can be calibrated in option (c) above simply by monitoring the position of the meniscus and assuming that rate of liquid  110  to the tank  100  is quasi-constant during calibration. 
     A rate R of filling of the tank  100  can simply be derived from Equation 1 (Eq. 1): 
     
       
         
           
             
               
                 
                   R 
                   = 
                   
                     K 
                     T 
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     During the calibration period, a volume V of liquid in the tank  10  at a given instance t during the period can be computed from Equation 2 (Eq. 2): 
     
       
         
           
             
               
                 
                   
                     V 
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         ∫ 
                         0 
                         t 
                       
                       ⁢ 
                       
                         R 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           ⅆ 
                           t 
                         
                       
                     
                     = 
                     
                       t 
                       ⁢ 
                       
                         
                           ∫ 
                           0 
                           t 
                         
                         ⁢ 
                         
                           
                             K 
                             T 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             ⅆ 
                             t 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     On account of R being substantially constant during the calibration period, namely substantially quasi-constant, Equation 2 simplifies to Equation 3 (Eq. 3): 
     
       
         
           
             
               
                 
                   
                     V 
                     ⁡ 
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     Kt 
                     T 
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   3 
                 
               
             
           
         
       
     
     The meniscus height output signal  220  represented by h is related to the volume V(t) of liquid in the tank  100  by way of polynomial function P as provided in Equation 4 (Eq. 4):
 
 h ( t )= P[V ( t )]  Eq. 4
 
wherein the polynomial P is defined by Equation 5 (Eq. 5):
 
 P[V]=p   0   +p   1   V+p   2   V   2   +p   3   V   3 +  Eq. 5
 
     During the calibration period, the data processor  400  is operable to periodically record complementary sets of values of V(t) and h(t) and then at completion of the calibration period perform a numerical calculation, for example a least-squares computation, to determine the coefficients px of the polynomial P; during the calibration period, V(t) is calculable from Equation 3 (Eq. 3) and does not need to be measured in contradistinction to known approaches, namely only values h of the signal  220  as a function of time t need to be periodically recorded in the data processor  400  during the calibration period. 
     The polynomial P is effectively depicted in  FIG. 2  by a curve  520  which initially rises rapidly with increasing time along the abscissa axis  500  on account of the a lower tapering of the tank  100  and then substantially asymptotically approaches a value corresponding to the meniscus  210  being at the upper height  310 . For example at a time T/2 during the calibration period, the curve  520  is at a value P2 denoted by a line  610  whereas the tank  100  is only half full at T/2. Hypothetically, were the tank  100  to be of simple cubic form, the signal  220  would be a linear function of the volume V of liquid in the tank  100  and would give rise to a sensing characteristics as denoted by a line  530 ; in such case, the tank  100  being half full at a time T/2 would result in the signal  220  corresponding to a value Pi denoted by a line  600 . 
     After the calibration period as depicted in  FIG. 2 , the data processor  400  is operable to determine the coefficients px of the polynomial P and then determine a corresponding inverse function P′1. In normal operation after the calibration period, the meniscus  120  of the liquid  110  can be at some height h between the lower and upper heights  300 ,  310  respectively in response to the given instantaneous volume V of the liquid  110  in the tank  100 . Thus, In normal operation, the data processor receives the signal  220  indicative of height h of the float  210  and hence the meniscus  120  and applies the inverse function P″1 to generate the tank liquid-content indicative signal  410  denoted by U for driving the aforementioned user display  60  as described by Equation 6 (Eq. 6):
 
 U=P   −1 ( h )  (Eq. 6):
 
     In implementing the present invention in practice, the user firstly needs to ensure that the tank  100  is in a first state whereat the float  210  is at the aforesaid lower position, for example tank  100  is substantially empty. Next, the user needs to key into the data processor, for example via a key-pad of the data processor  400 , one or more of the following: 
     (a) a measure of the capacity of the tank  100  between the aforementioned upper and lower positions of the float  210 ; 
     (b) a rate of delivery of liquid  110  into the tank  100 ; and 
     (c) details regarding the upper and lower positions of the float  210  over which calibration is to be executed. 
     Thereafter, the user presses a switch of the data processor to the define commencement of the calibration period simultaneously with commencing supplying liquid to the tank  100 . When the tank  100  has been filled to a desired extent such that the float  210  is at the aforementioned upper position, for example the tank  100  is full whereat the signal  220  reaches a maximum corresponding to h being substantially equal to the upper height  310 , the data processor  400  takes such a situation to be an end of the calibration period and optionally provides the user with an optical or audio warning that the tank  100  is filled to a desired extent so that the user ceases supply of liquid to the tank  100 . The data processor  400  then proceeds to compute coefficients of the polynomial P and therefrom corresponding coefficients of the inverse polynomial P′1, or alternatively computes coefficients of the inverse polynomial P″1 directly also by numerical analysis such as least-squares analysis, and thereby is in normal operation operable to implement the computation denoted by Equation 6 (Eq. 6) and thereby provide on the display  60  a substantially accurate measure of a volume of liquid  110  included at any instance after the calibration period in the tank  100 . 
     In certain circumstances, the user desires to supplement the tank  100  with one or more subsidiary tanks, for example to carry more fuel on board the marine vessel  10  to execute a longer sailing journey than normal at a sacrifice of available living space onboard the vessel  10 . Thus, as depicted in  FIG. 3 , there is optionally included in the vessel a subsidiary tank indicated generally by  700 . The subsidiary tank  700  includes an outer wall  720  for include a quantity of liquid denoted by  730 . The tank  700  is provided at a lower region thereof with a first coupling pipe  800  including therealong a first control valve  810  and a first part of a detachable coupler  820 . Similarly, the tank  100  includes at a corresponding lower region thereof a second coupling pipe  840  including therealong a control valve  830  and a second part of the detachable coupler  820 , When the tanks  100 ,  700  are not mutually coupled, the first and second valves  810 ,  840  are in a closed state and the first and second part of the detachable coupler  820  are spatially detached. When coupling the tanks  100 ,  700  together, the two parts of the detachable coupler  820  are mated and then the first and second valves  810 ,  820  are opened to enable liquid communication between the tanks  100 ,  700 . Optionally, the valves  810 ,  820  can be integrated into the detachable coupler  820  so that they are opened to enable liquid communication between the tanks  100 ,  700  only when the parts of the coupler  820  are mated together; the valves  820  would then be closed when the parts of the coupler  800  are detached from one another. 
     Coupling of the tanks  100 ,  700  together affects calibration of the data processor  400  which is beneficially recalibrated to reflect a combined volumetric capacity of the tanks  100 ,  700 . On account of the tanks  100 ,  700  potentially being of mutually different shape, the polynomial P is potentially considerably modified. The method of the invention enables the user to readily calibrate the data processor  400  for a first situation of the tank  100  alone being included in the vessel  10 , and a second situation of both of the tanks  100 ,  700  being present in mutual liquid communication in the vessel  10 . Optionally, the data processor  400  is provided with memory for storing calibration data corresponding to first and second situations so that the user can merely select therebetween by using switches on the data processor  400  when subsequently, after performing calibrations for the two situations, coupling and de-coupling the subsidiary tank  700  from the tank  100 . The display  60  is thereby capable of providing accurate measurement results for the first and second situations. 
     The present invention can be applied virtually irrespective of complexity of shape of the tanks  100 ,  700  and without specific knowledge of geometries of the tanks  100 ,  700 . Moreover, the present invention can be conveniently implemented during normal refueling or refilling operations, either in service or during initial manufacturing of the vessel  10 . 
     Optionally, during the calibration period, the user firstly depresses a switch on the data processor  400 , for example a “calibrate” switch, when the user is desirous to perform a calibration routine. The data processor  400  then waits and only assumes commencement of the calibration period, namely t=0, when the signal  220  begins to start increasing, namely when the user has had time to walk from the data processor  400  to apply a refueling hose to the input pipe  140  and pull a trigger of the hose when actually refilling commences. Such an intelligent recognition of commencement of the calibration period renders the data processor  400  especially straightforward for the user to use during calibration. Thus, the data processor  400  can be optionally implemented with simple controls, for example with a keypad for indicating a full capacity of the tank  100 , or the tanks  100 ,  700  as appropriate, for example a maximum value of the signal  220  “h” corresponding to the upper height  310 , a “calibrate” switch and optionally one or more configuration switches for coping with various tank configurations. The data processor  400  is thus potentially relatively easy for the user to operate, namely is “user friendly”. 
     Optionally, the data processor  400  is included as part of an electronic data control system included on the vessel  10 . Such a data control system can also provide other functions such a vessel engine management, environmental comfort control on board the vessel  10 , a safety warning, a collision hazard warning just to mention a few examples. Moreover, the control system can be included within the vessel  10  by way of an associated CAN bus coupled to various sensors, actuators, valves, sounders, displays and so forth included on the vessel  10 . 
     In the foregoing, it will be appreciated that the present invention relies on supply of fluid  110  to the tank being quasi-constant during filling of the tank  100 . This characteristic has not been hitherto appreciated and utilized effectively for calibration, thereby distinguishing the present invention from known contemporary approaches. 
     Modifications to embodiments of the invention described in the foregoing are possible without departing from the scope of the invention as defined by the accompanying claims. 
     Expressions such as “including”, “comprising”, “incorporating”, “consisting of, “have”, “is” used to describe and claim the present invention are intended to be construed in a non-exclusive manner, namely allowing for items, components or elements not explicitly described also to be present. Reference to the singular is also to be construed to relate to the plural. 
     Numerals included within parentheses in the accompanying claims are intended to assist understanding of the claims and should not be construed in any way to limit subject matter claimed by these claims.