Abstract:
Method and apparatus ( 121 ) for providing temperature flow rate compensation for a Coriolis flow meter. The described compensation compensates both flow calibration factor and the nominal time delay, commonly called “zero” in the art. After a Coriolis flow meter is installed into a process, whether for calibration or for actual process use, it need only be zeroed once over its lifetime following its installation. This is a significant improvement over prior Coriolis flow meters that may need to be re-zeroed after minor changes in pressure, temperature, or installation.

Description:
RELATED APPLIACTIONS 
     This is a National Stage entry of International Application No. PCT/US04/30492, with an international filing date of Sep. 17, 2004, the disclosure of which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     This invention relates to a compensation method and apparatus for a Coriolis flow meter. 
     Problem 
     It is known to use Coriolis effect mass flow meters to measure mass flow and other information of materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and Re. 31,450 to J. E. Smith of Feb. 11, 1982. Coriolis flow meters have one or more flow tubes, each having a set of natural vibration modes which may be of a simple bending, torsional, or twisting type. Each material filled flow tube is driven to oscillate at resonance in one of these natural modes. The natural vibration modes are defined in part by the combined mass of the flow tubes and the material within the flow tubes. Material flows into the flow meter from a connected pipeline on the inlet side. The material is then directed through the flow tube or flow tubes and delivered to a pipeline connected on the outlet side. 
     A driver applies force to oscillate the flow tube. When there is no flow through the Coriolis flow meter, all points along a flow tube oscillate with an identical phase. As the material begins to flow, Coriolis accelerations cause each point along the flow tube to have a different phase with respect to other points along the flow tube. The phase on the inlet side of the flow tube lags the driver; the phase on the outlet side leads the driver. Pick-off sensors on the flow tube produce sinusoidal signals representative of the motion of the flow tube. The phase difference between the two sensor signals is proportional to the mass flow rate of the material flowing through the flow tube or flow tubes. 
     It is known to use Coriolis flow meters having different flow tube configurations. Among these configurations are single tube, dual tubes, straight tube, curved tube, and flow tubes of irregular configuration. Most Coriolis flow meters are made of metal such as aluminum, steel, stainless steel and titanium. Glass flow tubes are also known. Most straight serial path Coriolis flow meters currently in the art are made out of metal, particularly Titanium. 
     It is known that as operational factors change, the characteristics of the Coriolis flow meter may be affected. These factors may be internal effects such as drift in electronic components associated with the Coriolis flow meter&#39;s transmitter or they can be external effects such as fluctuations in line pressure, density, viscosity, or changes in ambient temperature and drive frequency of the Coriolis flow meter. Much of the prior art has focused on compensating for these changes by upgrading or modifying circuit schemes or by making adjustments to the flow calibration factor. Examples of these methods are shown in U.S. Pat. No. 5,231,884 by Zolock and U.S. patent application Ser. No. 09/343,836 by Van Cleve et al. 
     Coriolis flow meters can be operated under controlled conditions in which pressure, density, and viscosity are maintained constant to eliminate the complexities of compensating for fluctuations in these parameters. However, this is not always feasible since it is often difficult to prevent fluctuations in the temperature of the process material or of the environment in which the Coriolis flow meter is operated. When pressure, density and viscosity can be held constant, temperature compensation of a Coriolis flow meter can be accomplished using a thermal sensor (commonly referred to as RTD) affixed to one or more portions of the Coriolis flow meter. The RTD measures the operating temperature of the flow meter portion to which it is affixed. The RTD information and drive frequency information is applied to meter electronics which generates a temperature compensated mass flow rate output. Flow tube pick-off signals generate an uncompensated mass flow signal which is applied to the meter electronics which alters the uncompensated received mass flow rate signal to produce a drive frequency compensated mass flow out put signal. 
     There are disadvantages associated with the use of RTD thermal sensors for temperature compensation. The first disadvantage is that the thermal sensor cannot be attached to the vibrating portion of the flow tube since the added weight of the sensor would alter the vibrational characteristics of the flow tube and degrade the accuracy of the output signal. The thermal sensor must therefore be attached elsewhere to the Coriolis flow meter or on the input or output line of the Coriolis flow meter. Because of this, the thermal sensor cannot be used to directly measure the temperature of the process material within the vibrating portion of the flow tube. Since accurate temperature compensation requires that the temperature of the material within the active portion of the flow tube be measured, the use of other mounting locations, such as those on the input or output line of the flow meter, results in a temperature that is different from the desired temperature. This causes inaccuracies in the temperature compensated flow rate output signal generated by the meter electronics. 
     A second disadvantage of the use of thermal sensors is that, since they are not mounted on the active portion of the vibrating flow tube, there is an inherent time difference between the time the thermal sensor detects a thermal change and the time the temperature of the flow tube material changes. This time difference results in further inaccuracies in the compensated mass flow rate output signal generated by the flow meter. 
     Solution 
     The present invention solves the above and other problems and achieves an advance in the art by the provision of a method and apparatus for temperature compensation for Coriolis flow meters which avoids the use of thermal sensors. The method and apparatus of the present invention monitors the drive frequency of the flow tube to provide temperature compensation. The Coriolis flow meter is operated in an environment in which all parameters, except temperature, which have an effect on the flow tube frequency, are held constant. These factors include such as stiffness or calibration factor sensitivity all of which can influence drive frequency. Other such parameters include material density, viscosity, and pressure. Under these controlled circumstances, changes in frequency of vibration must be due to changes in temperature which changes in Young modulus and the vibrating flow tube stiffness. 
     A Coriolis flow meter designed in accordance with the preferred embodiment of the invention has advantages in the area of compensation. The present invention compensates not only flow calibration but also adjusts the nominal time delay Δt 0 , commonly called “zero” in the art. This means that after a Coriolis flow meter is installed into a process, whether for calibration or for actual process use, it need only be zeroed once after installation. This is a significant improvement over Coriolis flow meters that may need to be re-zeroed after minor changes in pressure or temperature. 
     At the time of meter calibration the temperature effect on the flow tube is characterized by monitoring frequency and temperature changes as the material flow and actual temperature are varied. The calibration constants used to compensate flow are then determined and stored in the meter electronics. During operation, the material flow and the frequency of vibration of the flow tube are monitored. The resulting temperature changes and frequency information from the meter are applied to meter electronics which uses the stored calibration constants to calculate a temperature compensated mass flow rate {dot over (m)}. 
     The use of flow tube frequency for producing a temperature compensated mass flow is advantageous over the use of thermal sensors since changes in frequency are detected to produce a change in the compensated mass flow rate signal immediately upon the detection of the frequency change. The instantaneous change in flow tube frequency is applied to the meter electronics which generates a corrected compensated mass flow rate signal of increased accuracy corresponding to the change in flow tube temperature. 
     One aspect of the invention includes a method for providing thermal compensation for a Coriolis flow meter having at least one flow tube; said method comprising the steps of: 
     generating a first signal representing Coriolis deflections of said flow tube; 
     generating a second signal representing characteristics of said flow meter, wherein said characteristics include the drive frequency F of said Coriolis flow meter as well as an induced time delay Δt; and 
     characterized by the provision of meter electronics for using said first and said second signals to provide thermal compensation for said output signals of said Coriolis flow meter. 
     Preferably, the method further comprises said step of providing thermal compensation including the steps of: 
     receiving a calibrated mass flow rate from a master Coriolis flow meter, and 
     using said first and said second signals and said calibrated mass flow rate to provide said thermal compensation for said Coriolis flow meter. 
     Preferably, the method further comprises said step of providing thermal compensation includes the steps of: 
     using said first and said second signals and said calibrated mass flow rate to derive calibration constants for said Coriolis flow meter; and 
     using said calibration constants to provide said thermal compensation for said Coriolis flow meter. 
     Preferably, the method further comprises the further step of determining a thermally compensated flow rate for said Coriolis flow meter in response to said generation of said first and second signals and said provision of said thermal compensation for said Coriolis flow meter. 
     Preferably, the method further comprises the further steps of: 
     receiving a third signal representing calibration constants of said Coriolis flow meter; and 
     using said first and said second and said third signals and said calibration constants to determine a thermally compensated flow rate for said Coriolis flow meter. 
     Preferably, the method further comprises the further steps of: 
     determining a drive frequency F from said second signal; 
     deriving a linear frequency calibration constant α F  for zero; and 
     using said drive frequency F and said linear frequency calibration constant α F  for zero to derive said thermally compensated flow rate. 
     Preferably, the method further comprises compensated mass flow rate including further steps of: 
     deriving a coefficient α {dot over (m)}  for flow; and 
     using said drive frequency F and said linear drive frequency constant α F  for zero, and said constant α {dot over (m)}  for flow to derive a thermally compensated flow rate. 
     Preferably, the method further comprises the further steps of: 
     deriving a linear frequency (temp) constant α {dot over (m)}F  for flow; and 
     using said drive frequency F and said linear drive frequency constant α F  for zero and said constant α {dot over (m)}  for flow and said linear frequency (temp) constant α {dot over (m)}F  for flow to derive a thermally compensated mass flow rate. 
     Preferably, the method further comprises said step of deriving said calibration constants comprises the step of: 
     receiving a mass flow rate {dot over (m)} from a master Coriolis flow meter; and 
     using said received mass flow rate {dot over (m)} and said second signal for deriving said calibration constants of said Coriolis flow meter. 
     Preferably, the method further comprises said step of generating said calibration constants comprises the further steps of: 
     receiving said second signal to derive a flow induced time delay Δt and said drive frequency F of the Coriolis flow meter; and 
     using said first signal and said flow induced time delay Δt and said drive frequency F for deriving said calibration constants of said Coriolis flow meter. 
     Preferably, the method further comprises said step of generating said calibration constants comprises the further steps of: 
     deriving a linear drive frequency constant α F  for a nominal time delay Δt 0 ; and; 
     using said drive frequency F, and said linear drive frequency constant α F  for said nominal time delay Δt 0  to derive said calibration constants. 
     Preferably, the method further comprises said calibration constants are:
 
Δt 0 , α F , α {dot over (m)} , α {dot over (m)}F  
 
     Preferably, the method further comprises said step of deriving said calibration constants includes the step of solving the expression: 
                     [           Δ   ⁢           ⁢     t   0                 α   F               α     m   .                 α       m   .     ⁢           ⁢   F             ]     =       pinv   ⁡     (     [         1         (     F   -     F   0       )           m   .             m   .     ⁡     (     F   -     F   0       )             ]     )       ⁢   Δ   ⁢           ⁢   t           1.10             
Where
 
     Δt Flow induced time delay 
     Δt 0  Nominal time delay 
     {dot over (m)} Mass Flow Rate 
     F Drive Frequency 
     Fo Drive Frequency—nominal zero flow 
     α F  Linear Frequency constant for zero 
     α {dot over (m)}  Coefficient related to FCF 
     α {dot over (m)}F  Linear frequency (temp) constant 
     Preferably, the method further comprises said step of determining said thermally compensated mass flow rate comprises the step of solving the expression: 
                     m   .     =         Δ   ⁢           ⁢   t     -     Δ   ⁢           ⁢     t   0       -       (     F   -     F   0       )     ⁢     α   F           [       α     m   .       +       (     F   -     F   0       )     ⁢     α       m   .     ⁢           ⁢   F           ]             1.5             
Where:
 
     Δt Flow induced time delay 
     Δt 0  Nominal time delay 
     {dot over (m)} Mass Flow Rate 
     F Drive Frequency 
     F 0  zero drive frequency 
     α F  Linear Frequency constant for zero 
     α {dot over (m)}  constant related to FCF 
     α {dot over (m)}F  Linear FCF frequency (temp) constant 
     Another aspect of the invention comprises an apparatus that provides thermal compensation for a Coriolis flow meter having at least one flow tube; said apparatus comprises: 
     apparatus that generates a first signal representing Coriolis deflections of said flow tube; 
     apparatus that generates a second signal representing characteristics of said flow meter, wherein said characteristics include the drive frequency F of said Coriolis flow meter as well as an induced time delay Δt; and 
     characterized by the provision of meter electronics that uses said first and said second signals to provide thermal compensation for output signals of said Coriolis flow meter. 
     Preferably, said apparatus that provides thermal compensation includes: 
     apparatus that receives a calibrated mass flow rate from a master Coriolis flow meter, and 
     apparatus that uses said first and said second signals and said calibrated mass flow rate to provide said thermal compensation for said Coriolis flow meter. 
     Preferably, said apparatus that provides said thermal compensation further includes: 
     apparatus that uses said first and said second signals and said calibrated mass flow rate to derive calibration constants for said Coriolis flow meter; and 
     apparatus that uses said calibration constants and said first and said second signals and said calibrated mass flow rate to provide said thermal compensation for said Coriolis flow meter. 
     Preferably, apparatus that determines a thermally compensated flow rate for said Coriolis flow meter in response to said generation of said first and second signals and said provision of said thermal compensation for said Coriolis flow meter. 
     Preferably, apparatus that receives a third signal representing calibration constants of said Coriolis flow meter; and 
     apparatus that uses said first and said second and said third signals and said calibration constants to determine a thermally compensated flow rate for said Coriolis flow meter. 
     Preferably, apparatus that determines a drive frequency F from said second signal; 
     apparatus that derives a linear frequency calibration constant α F  for zero; 
     apparatus that derives a coefficient α {dot over (m)}  for flow; 
     apparatus that derives a linear frequency (temp) constant α {dot over (m)}F  for flow; and 
     apparatus that uses said drive frequency F and said linear drive frequency constant α F  for zero and said constant α {dot over (m)}  for flow and said linear frequency (temp) constant α {dot over (m)}F  for flow to derive a thermally compensated mass flow rate. 
     Preferably, said apparatus that derives said calibration constants comprises: 
     apparatus that receives a mass flow rate {dot over (m)} from a master Coriolis flow meter; 
     apparatus that receives said second signal to derive a flow induced time delay Δt and said drive frequency F of the Coriolis flow meter; 
     apparatus that derives a linear drive frequency constant α F  for a nominal time delay Δt 0 ; and 
     apparatus that uses said drive frequency F, and said linear drive frequency constant α F  for said nominal time delay Δt 0  and said mass flow rate {dot over (m)} to derive said calibration constants. 
     Preferably, said calibration constants are:
 
Δt 0 , α F , α {dot over (m)} , α {dot over (m)}F  
 
     Preferably, said apparatus that derives said calibration constants includes apparatus that solves the expression: 
                     [           Δ   ⁢           ⁢     t   0                 α   F               α     m   .                 α       m   .     ⁢           ⁢   F             ]     =       pinv   ⁡     (     [         1         (     F   -     F   0       )           m   .             m   .     ⁡     (     F   -     F   0       )             ]     )       ⁢   Δ   ⁢           ⁢   t           1.10             
Where
 
     Δt Flow induced time delay 
     Δt 0  Nominal time delay 
     {dot over (m)} Mass Flow Rate 
     F Drive Frequency 
     Fo Drive Frequency—nominal zero flow 
     α F  Linear Frequency constant for zero 
     α {dot over (m)}  Coefficient related to FCF 
     α {dot over (m)}F  Linear frequency (temp) constant Preferably, said apparatus that derives said thermally compensated mass flow rate solves the expression 
                     m   .     =         Δ   ⁢           ⁢   t     -     Δ   ⁢           ⁢     t   0       -       (     F   -     F   0       )     ⁢     α   F           [       α     m   .       +       (     F   -     F   0       )     ⁢     α       m   .     ⁢           ⁢   F           ]             1.5             
Where:
 
     Δt Flow induced time delay 
     Δt 0  Nominal time delay 
     {dot over (m)} Mass Flow Rate 
     F Drive Frequency 
     α F  Linear Frequency constant for zero 
     α {dot over (m)}  constant related to FCF 
     α {dot over (m)}F  Linear FCF frequency (temp) constant 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       These and other advantages and features of the present invention may be better understood in connection with a reading of the following detailed description thereof in connection of the drawings in which: 
         FIG. 1  discloses a perspective view of a first exemplary Coriolis flow meter embodying the invention. 
         FIG. 2  is a top view of the embodiment of  FIG. 1 . 
         FIG. 3  is a front view of the embodiment of  FIG. 1 . 
         FIG. 4  is a cross-sectional view taken along lines  4 - 4  of  FIG. 2 . 
         FIG. 5  is a graph showing the relationship between drive frequency and flow tube temperature. 
         FIG. 6  is a graph showing the relationship between drive frequency and material density. 
         FIGS. 7-9  are graphs showing the collection of data during calibration. 
         FIGS. 10-11  are flow charts of the method used to calibrate the Coriolis flow meter and to determine mass flow rate {dot over (m)}. 
         FIG. 12  illustrates the derivation of the calibration constants. 
         FIGS. 13 and 14  illustrate the comparative accuracies of the use of frequency changes versus RTD to provide thermal compensation. 
         FIG. 15  illustrates further details of Meter Electrons  121  of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Nomenclature 
     FCF Prior Art Flow Calibration Factor 
     α Prior Art FCF temperature coefficient 
     Δt Flow induced time delay 
     Δt 0  Nominal time delay at zero flow 
     {dot over (m)} Mass Flow Rate 
     F Operating Drive Frequency 
     F 0  Nominal Drive Frequency (under normal thermal conditions) as chosen by the manufacturer 
     ZERO(F) Term describing the effect Frequency has on Nominal Time Delay, Δt 0 . Equal to Δt 0 +(F−F 0 )α F    
     α F  Linear Frequency coefficient for zero 
     α {dot over (m)}  Proportionality Coefficient relating Δt to mass flow. Similar to FCF in the Prior Art. 
     α {dot over (m)}F  Linear frequency (temp) coefficient for α {dot over (m)} . Similar to α in the Prior Art. 
     FMUT Flow meter under test. 
     Description of  FIG. 1   
       FIG. 1  is a perspective view of a first possible exemplary embodiment of a Coriolis flow meter embodying the invention. It discloses a flow meter  100  having a flow tube  102  inserted through legs  117 ,  118  of base  101 . Pick-offs LP 0  and RP 0  and driver D are coupled to flow tube  102 . Flow meter  100  receives a process material flow from supply tube  104  and extends the flow through process connection  108  to flow tube  102 . Flow tube  102  is vibrated at its resonant frequency with material flow by driver D. The resulting Coriolis deflections are detected by pick-offs LP 0  and RP 00  which apply signals over conductors  112  and  114  to Coriolis flow meter electronics  121 . Coriolis flow meter electronics  121  receives the pick-off signals, determines the phase difference between, determines the frequency of oscillation and applies output information pertaining to the material flow over output path  122  to a utilization circuit not shown. Meter electronics  121  is shown in further detail on  FIG. 15 . 
     The material flow passes from flow tube  102  and through tube  106  which redirects the material flow through return tube  103  through process connection  107  to exit tube  105  which delivers the material flow to a user application. 
     Process connections  107 ,  108 ,  109  and  110  connect tubes  104 ,  105  and  106  to the ends of flow tube  102  and return tube. The process connections have a fixed portion  111  that includes threads  124 . Locking holes  130  receive set screws  411  to fixably connect element  111  to base  101  as shown in  FIG. 4 . The movable portion of process connections  107  through  110  are threaded onto male threads  124  to connect their respective tubes to the fixed body of the process connection of which the hexagonal nut portion  111  is a part. These process connections function in a manner similar to the well known copper tubing flared process connections to connect tubes  104 ,  105  and  106  to ends of flow tube  102  and return tube  103 . Details regarding the process connections are further shown in  FIG. 4 . 
     Description of  FIG. 2   
     In  FIG. 2  is a top view of flow meter  100  of  FIG. 1 . Pick-offs LP 0  and RP 0  and driver D each include a coil C. Each of these elements further includes a magnet which is affixed to the bottom portion of flow tube  102  as shown in  FIG. 3 . Each of these elements further includes a base, such as  143  for driver D, as well as a thin strip of material, such as  133  for driver D. The thin strip of material may comprise a printed wiring board to which coil C and its winding terminals are affixed. Pick-offs LP 0  and RP 0  also have a corresponding base element and a thin strip fixed to the top of the base element. This arrangement facilitates the mounting of a driver or a pick-off to be accomplished by the steps of gluing a magnet M to the underside of flow tube, gluing the coil C to a printed wiring board  133  (for driver D), positioning the opening in coil C around the magnet M, moving the coil C upwardly so that the magnet M fully enters the opening in coil C, then positioning base element  143  underneath the printed wiring board  133  and gluing these elements together so that the bottom of base  143  is affixed by glue to the surface of the massive base  116 . 
     The male threads  124  of process connections  107 - 110  are shown on  FIG. 2 . The inner details of each of these elements are shown on  FIG. 4 . Opening  132  receives conductors  112 ,  113  and  114 . Coriolis flow meter electronics  121  of  FIG. 1  is not shown on  FIG. 2  to minimize drawing complexity. However it is to be understood that the conductors  112 ,  113  and  114  extend through opening  132  and further extend over path  123  of  FIG. 1  to Coriolis flow meter electronics of  FIG. 1 . 
     Description of  FIGS. 3 and 4   
       FIG. 3  shows pick-offs LP 0 , RP 0  and driver D as comprising a magnet M affixed to the bottom portion of flow tube  102  and a coil C affixed to the base of each of elements LP 0 , RP 0  and driver D. 
       FIG. 4  is a sectional taken along line  4 - 4  of  FIG. 2 .  FIG. 4  discloses all the elements of  FIG. 3  and further details of process connections  108  and  109  and O-rings  430 . O-rings  430  couple flow tube  102  to base  401 .  FIG. 4  further discloses openings  402 ,  403  and  404  in base  101 . The top of each of these openings extends to the lower surface of the base of pick-offs LPO, RPO and driver D. The coil C and magnet M associated with each of these elements is also shown on  FIG.4 . Coriolis flow meter electronics  121  of  FIG. 1  is not shown on  FIGS. 3 and 4  to minimize drawing complexity. Element  405  in process connection  108  is the inlet of flow tube  102 ; element  406  in process connection  109  is the outlet of flow tube  102 . 
     The fixed portion  111  of process connection  108  includes male threads  409  which screw into mating threads in receiving hole  420  located in base  401  to attach fixed portion  111  to segment  401  of base  101 . The fixed portion of process connection  109  on the right is similarly equipped and attached by threads  409  into receiving hole  420  located in element  401  of base  101 . 
     Fixed element  111  of process connection  108  further includes a threaded portion  124  whose threads receive the movable portion  415  of process connection  108 . Process connection  109  is similarly equipped. Fixed element  111  of process connection  108  further includes on its left a conical stub  413  which together with movable element  415  acts as a flare fitting to force the right end of input tube  104  over the conical stub  413  of fixed portion  111 . This creates a compression fitting that sealably affixes the flared opening of supply tube  104  onto the conical stub portion  413  of fixed portion  111  of the process connection. The inlet of flow tube  102  is positioned in process connection fixed portion  111  and is flush with face  425  of stub  413 . By this means, the process material delivered by supply tube  104  is received by inlet  405  of flow tube  102 . The process material flows to the right through flow tube  102  to fixed portion  111  of process connection  109  where the outlet  406  of flow tube  102  is flush with face  425  of stub  413 . This sealably affixes the outlet of flow tube  102  to connector  109 . The other process connections  107  and  110  of  FIG. 1  are identical to those described for the details of process connections  108  and  109  on  FIG. 4 . 
     General Discussion of  FIGS. 5-12   
     The invention achieves temperature compensation of the mass flow output of a Coriolis flow meter by using drive frequency as an indicator of flow tube temperature change. Coriolis flow meters measure mass flow directly by calculating the time delay (Δt) between the inlet and outlet ends of the active portion of the vibrating flow tube of the Coriolis flow meter. An offset time delay at zero flow (Δt 0 ) is measured and subtracted from the calculated time delay during flow to produce a value that is directly proportional to mass flow via a proportionality constant α {dot over (m)}  which is termed calibration factor (FCF) in the prior art. 
     Fluid density also affects the drive frequency of the flow tube. To understand the sensitivity of frequency to temperature and density, the density effect must be characterized and compared to the temperature effect. The Coriolis flow meter of the present invention may be used with slurries whose specific gravities are quite limited. When frequency is to be used for temperature compensation, the affect of changing fluid density must also be evaluated and determined to determine mass flow. 
     The flow tube materials respond to changes in temperature. These changes must be considered to accurately calculate mass flow rate. Traditionally, RTD thermal sensors are used to directly measure the temperature. They are positioned on an inactive surface of the Coriolis tube. This is generally done outside of the brace bar(s). The disclosed Coriolis flow meter has a single-straight flow tube. There is no inactive section of the flow tube on which a temperature measurement representative of the flow tube can be made by a thermal sensor positioned on the flow tube without affecting output accuracy. 
     The present invention detects changes in flow tube temperature by monitoring drive frequency. Using drive frequency in accordance with the present invention to determine temperature changes and compensate for flow output raises the following issues. The prior art measures temperature using an RTD thermal sensor mounted on a secondary or inactive portion of the flow tube and infers the Coriolis flow tube temperature. 
     The method and apparatus of the present invention improves accuracy of the temperature determination and also improves the meter response time in detecting temperature changes. 
     When using frequency as a means to detect and compensate for temperature changes, a Taylor Series expansion on Δt isolates the effects of frequency and mass flow on Δt. The expansion is arranged to look like a flow equation and a least-squares pseudo-inverse problem is set up. Assume that the pick-off delay, Δt, is a function of mass flow and drive frequency.
 
Δ t =ƒ( {dot over (m)},F )  1.1
 
This can be expanded about an operating point with a Taylor Series:
 
                     Δ   ⁢           ⁢   t     =       f   ⁡     (         m   .     0     ,     F   0       )       +       (       m   .     -       m   .     0       )     ⁢       ∂   f       ∂   m         +       (     F   -     F   0       )     ⁢       ∂   f       ∂   F         +       1   2     ⁢     (       m   .     -       m   .     0       )     ⁢     (     F   -     F   0       )     ⁢         ∂   2     ⁢   f         ∂     m   .       ⁢     ∂   F           +   …           1.2             
Equation 1.2 is a full expansion and could be taken out to an infinite number of higher order terms. An optimal fit can be achieved by taking terms that effect zero flow and FCF linearly with temperature and terms that effect zero and FCF quadratically with temperature. However, over the operating temperature of the sensor (18-28 C) of the present invention, behavior is linear enough to use only frequency terms effecting FCF and Δt 0  linearly. Doing this and re-labeling the partial derivatives one obtains:
 
                     Δ   ⁢           ⁢   t     ≅       Δ   ⁢           ⁢     t   0       +       (       m   .     -       m   .     0       )     ⁢     α     m   .         +         (     F   -     F   0       )       ︸           zero   ⁢           ⁢   term   ⁢           ⁢   linear               in   ⁢           ⁢   frequency               ⁢     α   F       +           (       m   .     +       m   .     0       )     ⁢     (     F   -     F   0       )         ︸           flow   ⁢           ⁢   term   ⁢           ⁢   linear               in   ⁢           ⁢   frequency               ⁢     α       m   .     ⁢           ⁢   F                 1.3             
Now, rearrange the terms by grouping those associated with zero flow and those associated with flow:
 Δ t≅[Δt   0 +( F−F   0 )α F ]+( {dot over (m)}−{dot over (m)} )[α {dot over (m)} +( F−F   0 )α {dot over (m)}F ]  1.4 
For convenience choose nominal {dot over (m)} 0 =0, equation 1.4 can then be arranged to look like a flow equation:
 
                     m   .     =         Δ   ⁢           ⁢   t     -     Δ   ⁢           ⁢     t   0       -       (     F   -     F   0       )     ⁢     α   F           [       α     m   .       +       (     F   -     F   0       )     ⁢     α       m   .     ⁢           ⁢   F           ]             1.5             
A prior art flow equation is:
   {dot over (m)}=FCF (1−α T )(Δ t−Δt   0 )  1.6 
Drawing an analogy between equations 1.5 and 1.6, it can be seen that the portion of equation 1.5 relative to “zero” as a function of frequency is:
 ZERO( F )=[Δ t   0 +( F−F   0 )α F ]  1.7 
and the portion of equation 1.5 relating to “flow calibration factor” as a function of frequency is:
 
                     FCF   ⁡     (   F   )       =     1     [       α     m   .       +       (     F   -     F   0       )     ⁢     α       m   .     ⁢           ⁢   F           ]             1.8             
Rewriting equation 1.4 in the form of a vector equation sets up the Least-Squares Pseudo-Inverse problem:
 
                     Δ   ⁢           ⁢   t     =       [         1         (     F   -     F   0       )           m   .             m   .     ⁡     (     F   -     F   0       )             ]     ⁡     [           Δ   ⁢           ⁢     t   0                 α   F               α     m   .                 α       m   .     ⁢           ⁢   F             ]             1.9             
Assuming we can record Δt and drive frequency from the Coriolis flow meter while we record mass flow from a series connected master Coriolis flow meter, we can solve for the column vector in equation 1.4 by multiplying both sides by the pseudo-inverse of the row vector:
 
                     [           Δ   ⁢           ⁢     t   0                 α   F               α     m   .                 α       m   .     ⁢           ⁢   F             ]     =       pinv   ⁡     (     [         1         (     F   -     F   0       )           m   .             m   .     ⁡     (     F   -     F   0       )             ]     )       ⁢   Δ   ⁢           ⁢   t           1.10             
Equation 1.10 is the equation we need used by the present invention to optimally characterize the sensor&#39;s temperature dependence.
 
     As mentioned earlier, drive frequency has a linear dependence on temperature. Because of this, compensating flow for temperature effects using drive frequency should is advantageous. 
     Description of  FIG. 5   
       FIG. 5  illustrates the relationship between drive frequency and temperature for two flow tube sensors on lines  501  and  502 . The flow tube sensors were placed in an oven whose temperature was cycled between 15 and 35 C. A linear trend line was fit to each data set of lines  501  and  502  and the specified operating range for the sensor is shown between dotted lines  503  and  504 . Using the slope of each trend line  501  and  502 , we can estimate the sensitivity of drive frequency as 14 Hz per full scale temperature swing. 
     Description of  FIG. 6   
       FIG. 6  graphs drive frequency versus fluid specific gravity for the same two sensors on trend lines  601  and  602 . The sensors are specified to measure flow rate of slurries whose fluid densities are from 1.0 to 1.3 SG as shown for trend lines  603  and  604 . Three fluids were used to span the density operating range of the sensor. Three data points were taken for each sensor and a trend line was fit to the data. Using the slope of each trend line, we can estimate the sensitivity of drive frequency as 4 Hz per FS density swing. This seems significant, but is should be noted that each Coriolis flow meter is installed on a particular process using a particular fluid in this range. Further, once installed, users typically calibrate their process to the actual flow output, eliminating errors due to new process fluid in the device. 
     Description of  FIGS. 7-12   
       FIGS. 7-12  describe the method by which the calibration constants Δt 0 , α F , α {dot over (m)} , α {dot over (m)}F  of flow equation 1.5 are derived using the calibration procedure of the present invention. These calibration constants are derived using equation 1.10. Once derived, these are used in flow equation 1.5 together with the measured values of drive frequency F and Δt (Flow induced time delay) to determine the measured flow rate {dot over (m)}. This is described in detail in the following paragraphs. The calibration constants Δt 0 , α F , α {dot over (m)} , α {dot over (m)}F  are derived using a four level procedure as next described. 
     A master Coriolis flow meter and the Coriolis flow meter to be calibrated are connected in series and a test flow is applied. The flow data corresponding to Runs 1-4 of Table 1 below is used for the calibration. The Test Matrix for identifying mass flow of the master test meter and temperature/frequency contributions to Δt is shown in Table 1. The measured mass flow from the master Coriolis flow meter and the series connected tested Coriolis flow meter are shown in  FIG. 7 . 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Run 
                 Mass Flow (g/min) 
                 Fluid Temperature C. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 350 
                 28 
               
               
                 2 
                 50 
                 28 
               
               
                 3 
                 350 
                 18 
               
               
                 4 
                 50 
                 18 
               
               
                   
               
             
          
         
       
     
     The measured mass flow from the master Coriolis flow meter and the series connected test Coriolis flow meter using the four test runs of Table 1 are shown in  FIG. 7 . The four runs are shown of  FIG. 7  together with the corresponding values of temperature, drive frequency and {dot over (m)} for the master Coriolis flow meter are used. Line  703  represents the flow rate of the master Coriolis flow meter. 
       FIG. 8  illustrates the data obtained for the four tests runs of  FIG. 7 . This is the data profile that is used, as subsequently described for equation 1.10, to derive the values of the calibration constants Δt 0 , α F , α {dot over (m)} , α {dot over (m)}F . Line  803  represents the flow rate of the master Coriolis flow meter. 
       FIG. 9  shows the results of using both frequency and temperature (RTD) compensation for steady flow states. Both methods give a good data fit for calibration of the Coriolis flow meter during steady state flow conditions.  FIG. 9  shows drive frequency on line  901 , temperature on line  902 , flow rate {dot over (m)} for each run on line  903  and the error rate on line  905 . The results of both methods of compensation are represented by line  905  since they are both congruent for steady state flows. This accuracy is indicated by zero error for both methods across line  905  all four test runs. 
     Meter electronics  121 , shown on  FIGS. 1 and 15 , executes the data processing functions required to implement the operations shown on  FIGS. 7-9 . 
     Description of  FIGS. 10 and 11   
       FIG. 10  is a block diagram  1000  describing how the method and apparatus of the present invention derives the calibration constants.  FIG. 10  discloses a plurality of processing or program steps each representing one or more program instructions stored in memory  1502  of Coriolis flow meter electronics  121 . The instructions are executed by processor  1502  of the Coriolis flow meter electronics  121  with the results either being stored in memory  1501  of the flow meter electronics or being outputted to a user over path  122 . The process of  FIG. 10  derives the calibration constants Δt 0 , α F , α {dot over (m)} , α {dot over (m)}F . The derived calibration constants are inputted to  FIG. 13  where they are used to derive a temperature compensated mass flow rate {dot over (m)}. 
     On  FIG. 10 , element  1002  receives the input pick-off signals and from LPO and RPO of  FIG. 1  and applies them to element  1004  which uses the received information to derive the detected Δt and the drive frequency F of the flow meter under test. Element  1006  receives from element  1003  the signals representing the measured flow rate {dot over (m)} of the series connected reference flow meter. Element  1006  receives the above indicated information from elements  1004  and  1003  and uses them to derive the calibration constants Δt 0 , α F , α {dot over (m)} , α {dot over (m)}F  by solving equation 1.10. 
     The derived calibration constants are extended from element  1006  to element  1106  of  FIG. 11 . Element  1102  receives the input pick-off signals from pick-offs LPO and RPO of  FIG. 1  for the flow meter under tests. This information is extended to element  1104  which derives the Δt and the drive frequency F for the flow meter under test and extends them to element  1106 . Element  1106  receives the outputs of elements  1104  and  1006  to derive the temperature compensated mass flow rate of the flow meter under test by using the equation 1.5 indicated in element  1106 . The terms Δt and Δt 0  are derived by element  1006 . The expression F−F 0  in both the numerator and dominator of equation 1.5 are obtained by element  1106  from element  1104 . The term α F  is obtained by the element  1004 . The terms α {dot over (m)}  and α {dot over (m)}F  are obtained from element  1104 . 
     Element  1106  employs all of the terms on the right side of equation 1.5 to provide a temperature compensated mass flow rate output to a user over output  122  of  FIG. 1  from meter electronics  121 . 
     Description of  FIG. 12   
       FIG. 12  shows the method the present invention employs to derive the calibration constants Δt 0 , α F , α {dot over (m)} , α {dot over (m)}F  using equation 1.10. The derivation of the calibration constants Δt 0 , α F , α {dot over (m)} , α {dot over (m)}F  of the Flow Meter Under Test (FMUT) requires:
         The actual flow measurement from a series connected reference flow meter connected in series with the Coriolis flow meter under test.   Temperature measurement from the FMUT.   Drive Frequency measurement from the FMUT.
 
These measurements are made while the FMUT is subjected to process conditions shown in table 1 as shown below.
       
                                           TABLE 1                   Ideal Reference   Ideal Fluid       Run   Mass Flow (g/min)   Temperature C.                                1   350   28       2   50   28       3   350   18       4   50   18                    
The Actual Reference Mass Flow and Fluid Temperature may differ from the Ideal Reference Mass Flow and the Fluid Temperature due to limitations in the controllability of the flow stand on which the FMUT is being calibrated.
 
     For each run of table 1, multiple measurements of Actual Mass Flow, the FMUT temperature and the FMUT drive frequency are made. Let:
     M=# of measurements of the three aforementioned quantities during Run 1.   N=# of measurements of the three aforementioned quantities during Run 2.   O=# of measurements of the three aforementioned quantities during Run 3.   P=# of measurements of the three aforementioned quantities during Run 4.   

     During the calibration Runs of  FIG. 8 , the reference mass flow {dot over (m)},the FMUT temperature and the FMUT drive frequency are each be measured M+N+O+P times. It is required that M+N+O+P be at least equal to the number of parameters being solved using Equation 1.10. However, it is recommended that the sum M+N+O+P be much greater than the number of parameters being solved for. This is the definition of a least-squares problem, which is what we are trying to solve. 
     To eliminate confusion, let&#39;s assume M=N=O=P, so that the total number of measurements of reference Mass Flow, the FMUT temperature and the FMUT frequency is 4 times M. Dimensionally, the calibration problem shown in Equation 1.10, dictated by table 1 and depicted in  FIG. 8  is also shown in  FIG. 12 . 
     Once the data of  FIG. 8  is taken and the equation 1.10 is solved, four values are obtained, namely Δt 0 , α F , α {dot over (m)} , α {dot over (m)}F . These are the desired calibration constants. The term Δt 0  is the Zero offset of the FMUT. This value comes from zeroing the meter in the traditional sense (i.e. pushing the zero button). The term α F  is the linear Frequency (hence temperature) effect on zero offset of the FMUT. The term α {dot over (m)}  is the Flow Cal Factor (FCF) of the FMUT. 
     The term α {dot over (m)}F  is the linear frequency (hence temperature) effect on FCF of FMUT. These calibration constants can now be used in Equation 1.5, to calculate FMUT mass flow compensated for frequency (hence temperature) effects. 
     Meter electronics  121 , shown on  FIGS. 1 and 15 , executes the data processing required to implement the operations shown on  FIG. 12 . 
     Description of  FIG. 13   
       FIG. 13  shows the results of using frequency determination to achieve thermal compensation as compared to the use of RTD thermal sensors to monitor flow tube frequency and provide thermal compensation.  FIG. 13  clearly shows the merit of frequency based temperature compensation as compared to the use of the RTD. Line  1303  represent the mass flow provided by the master Coriolis flow meter. Solid line  1308  portrays the results obtained using RTD sensors. Dotted line  1309  illustrates the use of frequency monitoring as provided in accordance with the present invention. 
     Line  1303  portrays the drive frequency. Line  1302  portrays the temperature. Line  1301  portrays the resultant mass flow rate. In the straight portions of line  1303 , the results of the master, the RTD and the use of frequency overlay one another the results diverging only in the portions represented by portions  1305 ,  1306 , and  1307  which represent sudden changes in mass flow rate. Element  1305  contains lines  1303 ,  1308 , and  1309 . Line  1303  represents the mass flow rate of the master. Line  1308  represents the response of the RTD. Line  1309  represents the response using frequency monitoring. It can be seen that the line  1303  and  1309  responses are essentially the same in the period of transition represented by element  1305 . In can also be seen that the response  1308  of element  1305  indicates that the response of the RTD diverges considerably from that compared to that of the master. The same observation may be made with respect to elements  1306  and  1307  where the results obtained using frequency compensation in portrayed by line  1309  follow more closely the response of the master as portrayed by line  1303  then does the results portrayed by the line  1308  representing the results obtained using RTD thermal compensation. 
     The mass flow rate {dot over (m)} 1 ,{dot over (m)} 2 ,{dot over (m)} 3 , and {dot over (m)} 4  on  FIG.13  correspond to the correspondingly designated elements on  FIGS. 7 ,  8 , and  9 . 
     The conclusion to be reached from an analysis of  FIG. 13  is that the use of frequency monitoring to provide thermal compensation is far superior to that of the use of RTD when accuracies is desired during transient conditions of the mass flow rate. 
     Meter electronics  121 , shown on  FIGS. 1 and 15 , executes the data processing required to implement the operations shown on  FIG. 13 . 
     Description of  FIG. 14   
       FIG. 14  discloses the comparative accuracies of the response using frequency monitoring as compared to the use of RTD. Line  1401  portrays the drive frequency, line  1402  portrays the temperature, and line  1403  portrays the mass flow rate. Line  1414  portrays the compensation error produced by the use of frequency monitoring as compared to the use of RTD. It can be seen, use of RTD versus frequency monitoring produces comparable results when the mass flow rate is essentially constant. However, the elements  1405 ,  1406 , and  1407  indicate conditions where the mass flow rate is undergoing transients. As shown for the element  1405 , the response of using frequency monitoring  1415  corresponds to the mass flow rate  1414  of the master more closely than does the response  1405  of the RTD. 
     Meter electronics  121 , shown on  FIGS. 1 and 15 , executes the data processing required to implement the operations shown on  FIG. 14 . 
     Description of  FIG. 15   
     As shown on  FIG. 15 , meter electronics  121  includes a processor  1501  and a memory  1502 . 
     Processor  1501  may comprise a conventional CPU. Alternatively it may comprise a special or a general-purpose processor or a DSP. Memory  1502  may comprise any type of memory system, such as flash memory or a conventional ROM and RAM, for the storing information on both a long-term and short-term basis. 
     Meter electronics  121  receives input information over path  123  and applies its output information over path  122  to a user. Meter electronics  121  receives input signals over path  123  from pickoffs LPO and RPO. These pickoff signals are shown on  FIG. 10  in element  1002 . The pickoffs signals are applied from element  1002  to element  1004  which performs the indicated functions and outputs its information to element  1006 . Elements  1002  and  1004  are duplicated on  FIG. 11  as elements  1102  and  1104 . Element  1006  on  FIG. 10  receives the outputs of elements  1004  and  1003  and derives the indicated calibration constants. Memory  1502  and processor  1501  of  FIG. 15  are used for this function. 
     Element  1106  on  FIG. 11  receives the outputs of element  11006  and element  1104  and derives the indicated the mass flow rate by solving equation 1.5. These functions are performed by the memory  1502  and the processor  1501  of  FIG.15 . 
     Memory  1502  and processor  1501  of meter electronics  121  are also used to perform the various computations and functions shown in on  FIGS. 5 ,  6 ,  7 ,  8 ,  9 ,  12 ,  13  and  14 . 
     It is to be expressly understood that the claimed invention is not to be limited to the description of the preferred embodiment but encompasses other modifications and alterations within the scope and spirit of the inventive concept. While specific relationships and equations have been described in connection with the invention, it is to be understood that the invention includes and may be practiced using modifications of the disclosed equations and relationships. Also although the method and apparatus is shown in conjunction with a Coriolis flow meter, it should be under stood that the method and apparatus of the invention can be used with a Coriolis flow meter of any type including a Coriolis flow meter having metal, plastics or glass flow tube(s).