Abstract:
The flow of a fluid of unknown composition is measured by leading the fluid over a first temperature sensor, a heater and a second temperature sensor. The temperature difference DTP between the temperature sensors is measured, as well as the temperature T of at least one of them. In addition, calibration data is used to store the temperature Tref of a known reference fluid. The offset T−Tref at a given temperature difference DTP is a direct measure of the composition of the fluid and allows to retrieve any parameter depending on the same.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of European patent application 08017691.0, filed Oct. 9, 2008, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a method for measuring a parameter depending on the composition of an unknown fluid by means of a flow sensor, in particular by means of a thermal flow sensor comprising a heater arranged between two temperature sensors. 
         [0003]    EP 1 426 740 describes a method for measuring a material-dependent parameter, such as a mixture ratio, of a fluid by means of a flow sensor. The fluid is led over a first temperature sensor, a heater and then a second temperature sensor. The temperature difference between the first and the second sensor as well as the temperature of the second sensor are measured. The mixture ratio of the fluid is then calculated by various, fairly complex mathematical transformations. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    Hence, it is a general object of the invention to provide a method of the type mentioned above that allows to measure a parameter depending on the composition of an unknown fluid that obviates the need for complicated mathematical transformations. 
         [0005]    This problem is solved by a method for measuring a parameter depending on the composition of an unknown fluid by means of a flow sensor, wherein said flow sensor comprises a heater arranged between a first and a second temperature sensor, said method comprising the steps of 
         [0006]    leading said unknown fluid over said first temperature sensor, said heater and said second temperature sensor, 
         [0007]    measuring a temperature difference DTP between a temperature TP 2  at said second temperature sensor and a temperature TP 1  at said first temperature sensor as well as a single temperature T, which single temperature is equal to k1.TP 1 +k2.TP 2  with k1≠−k2, 
         [0008]    retrieving first calibration data, which first calibration data was obtained from a calibration measurement carried out with a reference fluid of known composition, and which first calibration data is such that it allows to calculate the value of the single temperature that the reference fluid exhibited at a given temperature difference, 
         [0009]    using said first calibration data to calculate a reference temperature Tref(DTP) equal to the single temperature of said reference fluid at said temperature difference DTP, and 
         [0010]    deriving said parameter from a temperature offset D=T−Tref(DTP) between said single temperature T measured for said unknown fluid at said temperature difference DTP and the single temperature of said reference fluid at said temperature difference DTP. 
         [0011]    Accordingly, as in the prior art, the unknown fluid is led over the first temperature sensor, then the heater, and finally the second temperature sensor. The difference DTP=TP 2 −TP 1  between the temperature TP 2  at the second temperature sensor and the temperature TP 1  at the first temperature sensor is measured, as well as a “single temperature T”. The single temperature T is equal to k1.TP+k2.TP 2  with k1≠−k2. Advantageously T=TP 1  or T=TP 2  is used for computational simplicity, but any linear combination of the two temperatures can be used. 
         [0012]    Now, “first calibration data” is retrieved, e.g. from a memory. This first calibration data was obtained from a calibration measurement carried out with a reference fluid of known composition. The first calibration data is such that it allows to calculate the value of the single temperature that the reference fluid exhibited at a given temperature difference, and in particular at the temperature difference DTP measured for the unknown fluid. This single temperature of the reference fluid at DTP is called the “reference temperature Tref(DTP)”. 
         [0013]    In a next step, the “temperature offset” D=T−Tref(DTP) is calculated, i.e. the difference between the single temperature T measured for the unknown fluid and the single temperature of the reference fluid at the measured temperature difference DTP. 
         [0014]    As it has been found, the temperature offset D is a direct measure of the composition of the unknown fluid, independent of the temperature difference DTP and therefore independent of the flow of the fluid. Hence, the temperature offset D can be used to easily calculate the desired parameter, e.g. by means of a lookup-table that contains the data required for calculating the parameter from the temperature offset D. 
         [0015]    Since the temperature offset D does not depend on the flow but merely on the composition of the fluid, the present method can be used to determine the parameter at any non-zero flow as well as for zero flow without requiring any flow-dependent corrections. 
         [0016]    In a typical application, the unknown fluid is a mixture of two known fluids and the parameter to be determined from the temperature offset D is the mixture ratio of the two fluids. 
         [0017]    The present invention can be used for determining a composition-dependent parameter of any type of fluid, in particular of liquids as well as of gases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein: 
           [0019]      FIG. 1  is a top view of the heater and the temperature sensors of a flow sensor, 
           [0020]      FIG. 2  is a sectional view along line II-II of  FIG. 1 , 
           [0021]      FIG. 3  is a simplified block diagram of the flow sensor, 
           [0022]      FIG. 4  shows the dependence of the temperature difference DTP on the flow of the fluid for differing fluid compositions, 
           [0023]      FIG. 5  shows the dependence of the second temperature TP 2  on the temperature difference DTP, 
           [0024]      FIG. 6  shows the dependence of the offset D on the temperature difference DTP as calculated from  FIG. 5 , 
           [0025]      FIG. 7  shows the dependence of the first temperature TP 1  on the temperature difference DTP, and 
           [0026]      FIG. 8  shows the dependence of the offset D on the temperature difference DTP as calculated from  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Definitions 
       [0027]    The term “unknown fluid” designates the fluid whose composition is not known. For example, the unknown fluid is a mixture of two known fluids with unknown mixture ratio. 
         [0028]    The term “fluid temperature TF” designates the temperature of the fluid in the absence of any heating contribution by the heater, e.g. a temperature measured at a fairly large distance before the heater. 
         [0029]    Note Regarding  FIGS. 4-8 : 
         [0030]    Each of the diagrams of  FIGS. 4-8  shows seven different curves. These curves correspond to measurements with a fluid of varying composition. In the specific example shown here, the fluid was air mixed with C4H10. The bottommost curve of each diagram corresponds to a measurement where the content of C4H10 was 0%, the second curve from the bottom corresponds to a fluid with a C4H10-content of 10%, the third curve to a fluid with a C4H10-content of 20%, etc., with the topmost curve corresponding to a fluid with a C4H10-content of 60%. 
         [0031]    The temperatures in  FIGS. 4-8  are in arbitrary units. In  FIG. 4 , DTP=3.25 corresponds to a temperature difference of 0° C. 
         [0032]    The Sensor: 
         [0033]      FIGS. 1 and 2  show an example of a thermal flow sensor comprising a heater  1  arranged between a first temperature sensor  2  and a second temperature sensor  3 . In the present embodiment, the temperature sensors are thermopiles, albeit the invention can also be carried out with other types of temperature sensors, such as resistive temperature sensors. The flow sensor further comprises a substrate  4 , such as a semiconductor substrate, wherein the heater  1 , the temperature sensors  2 ,  3  as well as further components are integrated on a surface thereof. An opening or recess  5  in substrate  4  has been manufactured e.g. by anisotropic etching and is spanned by a membrane  6 . The temperature sensors  2 ,  3  as well as the heater  1  are arranged at least partially on the membrane  6  for good thermal insulation. 
         [0034]    This type of flow sensor is e.g. described in EP 1 426 740 and WO 01/98736. To measure the flow of a fluid, the fluid is led over first temperature sensor  2 , then heater  1  and finally second temperature sensor  3 . Heater  1  is heated by an electric current, advantageously to a temperature that lies at a fixed offset above the temperature of substrate  4 . Thermal conductance through membrane  6  as well as through the fluid leads to a temperature increase at the inner contacts of the temperature sensors  2 ,  3 , while the outer contacts remain at the bulk temperature of substrate  4 . In the presence of a non-zero flow, however, the temperature distribution is asymmetric and the temperature TP 1  measured by first temperature sensor  2  will generally be lower than the temperature TP 2  measured by second temperature sensor  3 . 
         [0035]    The temperature difference DTP=TP 2 −TP 1  between the second and the first temperatures TP 2 , TP 1  is a measure of the flow and can be used to determine the flow. 
         [0036]    A simple block diagram of the flow sensor is shown in  FIG. 3 . As can be seen, it comprises a control unit  7 , which e.g. comprises analogue circuitry, such as amplifiers, an A/D-converter as well as digital circuitry. It controls heater  1  and measures the signals from the temperature sensors  2 ,  3 . It processes the signals by accessing lookup tables LUT 1 , LUT 2  and LUT 3  in a manner described below, and has an interface  8  through which it can communicate with external circuitry. 
         [0037]    Control unit  7  is also connected to a temperature sensor  9  measuring the fluid temperature TF. 
         [0038]    Advantageously, all or at least part of the electronic components shown in  FIG. 3  are integrated on semiconductor substrate  4 , but part or all of these components may also be implemented as external circuitry. 
         [0039]    Flow Measurement: 
         [0040]    To measure the flow of the fluid, as mentioned, the temperature difference DTP is determined. As can be seen from  FIG. 4 , the relationship between the flow and the temperature difference DTP is non-linear and depends on the mixture ratio of the fluid. Therefore, control unit  7  accesses a first lookup-table LUT 1 , which has two input values, namely the temperature difference DTP as well as the mixture ratio of the fluid, and which provides the flow as an output value. Interpolation of the output values of lookup-table LUT 1  allows to calculate the flow for any temperature difference DTP and mixture ratio. 
         [0041]    The mixture ratio required for reading LUT 1  can be obtained by the measurement procedure described in the next section. 
         [0042]    Measuring the Composition: 
         [0043]      FIG. 5  shows the dependence of temperature TP 2  at second temperature sensor  3  on the temperature difference DTP for different fluid compositions. As can be seen, the curves are mutually parallel (except for deviations at high temperature differences DTP, i.e. at high flow values, where the flow starts to become turbulent). 
         [0044]    For example, the bottommost curve, which was measured for pure air, is roughly at an offset of 0.014 (arbitrary units) below the curve above it, which was measured for a mixture of 90% air and 10% C4H10, with the offset being independent of DTP. And the third curve (measured for 80% air and 20% C4H10) is roughly 0.027 above the bottommost curve. 
         [0045]    Hence, in the present example, the offset D=TP 2   x (DTP)−TP 2 air between the temperature TP 2   x  measured for a fluid of unknown mixing ratio between air and C4H10 and the temperature TP 2 air measured for pure air at the same temperature difference DTP depends on the mixing ratio only, but not on DTP (i.e. not on the flow). 
         [0046]    This is illustrated by  FIG. 6 , which shows the offset D for the curves of  FIG. 5  in respect to an earlier calibration measurement carried out with pure air. (Note: The vertical axis of  FIG. 6  is scaled in 10,000 times the units of the vertical axis of  FIG. 5 .) As can be seen, the percentage x of C4H10 can be directly derived from the offset D using a table as follows 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 D 
                 x 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                  0% 
               
               
                   
                 140 
                 10% 
               
               
                   
                 270 
                 20% 
               
               
                   
                 380 
                 30% 
               
               
                   
                 470 
                 40% 
               
               
                   
                 540 
                 50% 
               
               
                   
                 620 
                 60% 
               
               
                   
                   
               
             
          
         
       
     
         [0047]    Hence, the mixing ratio of an unknown fluid composition can be measured by the following steps: 
         [0048]    First, a reference measurement is made for varying flows of a fluid of known composition. This reference measurement can be used to derive the dependence of TP 2 ref on the temperature difference DTP. This dependence is stored as the “first calibration data”, e.g. in a lookup table LUT 2  of the sensor ( FIG. 3 ). 
         [0049]    Then, the unknown fluid is measured at a certain flow, i.e. at a certain temperature difference DTP, and the second temperature TP 2 (DTP) is measured. The calibration data in LUT 2  is used to calculate the second temperature TP 2 ref(DTP) that the reference fluid had (or would have had) at the same temperature difference DTP, and the offset D=TP 2 (DTP)−TP 2 ref(DTP) is calculated. 
         [0050]    From table I above, which may e.g. be stored as “second calibration data” in a third lookup table LUT 3  of the sensor (or of a device external to the sensor), it is now possible to calculate the composition x using interpolation. 
         [0051]    In  FIGS. 5 and 6 , the second temperature TP 2  has been used as “single temperature” T in the sense defined above. However, it must be noted that the same procedure can also be carried out with the first temperature TP 1 , as illustrated in  FIGS. 7 and 8 . As can be seen, the curves in  FIG. 7 , which show the first temperature TP 1  as a function of the temperature difference DTP for air with increasing C4H10 additions, are again parallel (except for small deviations at high flow values where turbulences set it). 
         [0052]    Hence, the same kind of calculation can be carried out for the first temperature TP 1 . 
         [0053]    As mentioned above, the “single temperature” T can not only be TP 1  or TP 2 , but also any linear combination thereof, in particular TP 1 +TP 2 . (Using T=TP 1 −TP 2 , however, makes little sense since, in this case, the offset D would be constantly 0). 
         [0054]    Experimental evidence shows that the offset D also depends, to some slight degree, on the fluid temperature TF. Hence, advantageously, the “second calibration data” relates the temperature offset D as well as the fluid temperature TF to the mixing ratio x. For example, lookup-table LUT  3  can be a two-dimensional table having offset D and fluid temperature TF as input values. 
         [0055]    The present method can also be used for measurements on other types of fluids, not only mixtures of air and C4H10. For example, it can be used to measure the mixture ratio of other gas compositions, as well as of liquid compositions. 
         [0056]    Also, the parameter to be measured can be any value depending on the composition, not only the mixing ratio, by storing suitable “second calibration data”. 
         [0057]    Notes: 
         [0058]    As mentioned above, the mixing ratio x is required for selecting the appropriate part of lookup-table LUT 1  when measuring the flow of the fluid. As can be seen know, this mixing ratio can be obtained from the offset D, or, in other words, offset D may be used as an input value for retrieving the flow from lookup-table LUT 1 . 
         [0059]    The lookup-tables LUT 1 , LUT 2 , LUT 3  can, some or all of them, also be arranged outside the flow sensor, in external circuitry. In a particularly advantageous embodiment the offset D accessible through interface  8  such that a user of the flow sensor can perform composition-dependent operations in external circuitry. 
         [0060]    As it has been mentioned, deviations from the described behavior start to occur when the flow of the fluid starts to become turbulent. Hence, advantageously, the temperature difference DTP and the single temperature T are measured for laminar flows. 
         [0061]    While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practised within the scope of the following claims.