Abstract:
A type of liquid level gauge for continuous measurement of boiler steam drum under full conditions and its method for calculation of liquid level, this liquid level gauge includes one metal measurement tube and one transmitter. The steam-side pipe connected to steam drum is fitted on the upper part of the metal measurement tube and the water-side pipe connected to steam drum is fitted on the lower part of the metal measurement tube, one insulated jacket pipe is available inside the metal measurement tube and  1 # probe,  2 # probe, and  3 # probe are mounted from down upwards inside the insulated jacket pope. The respective probes are mutually insulated and not conducted. The three probes are respectively connected with the transmitter through lead wires of the probes. This liquid level gauge is able to monitor the water and steam dielectric constants in a real-time manner, and the liquid level values can be outputted accurately in a continuous way with the liquid level gauge for boiler steam drum designed according to this invention in all the cases, no matter the boiler is under the startup, shutdown, or joint discharge status, etc, thus to achieve the function for a continuous measurement of liquid level under full conditions.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention involves the technical field of liquid level measurement, and especially involves a liquid level gauge with the function for continuous measurement of liquid level of boiler steam drum under full conditions and its method for calculation of liquid level. 
       BACKGROUND OF THE INVENTION 
       [0002]    As water and steam inside the boiler steam drum are both in the forms of high temperature and high pressure, and the changes in work conditions are complicated, the measurement of its liquid level has always been a difficult problem. The principle of measurement with balance container for most liquid level gauges of boiler steam drums used for adjustment and protection is adopted at present, while the measurement of steam drum with balance container is affected by work conditions, the measurement has often been inaccurate and failed for use during boiler startup and boiler shutdown, and additionally it may possibly give rise to failure of normal measurement when anomalous work condition is confronted during boiler operation, which has grievously affected the safe operation of boiler. While as only discontinuous measurement of liquid level can be applied to liquid level with electrical contact level gauges during boiler startup and shutdown or under anomalous work condition, namely the displayed liquid level can only be a range, therefore it cannot be used for automatic regulation of water level, and at the same time for the restriction of reliability, it is generally neither used for protection of liquid level. 
         [0003]    The pressure of boiler steam drum varies from 2 Mpa to 20 Mpa in general according to the size of unit, and the corresponding temperatures of saturated water and steam vary from 212° C. to 365° C. The dielectric constants of water and steam at normal temperature are 80 and 1, respectively, the dielectric constants of water and steam at 365° C. are 9.94 and 2.65, respectively, the change in dielectric constant of water is approximately 10 times, while the conventional capacitive level gauge can only be used on the occasions when the dielectric constant of medium measured is relative stable, and therefore, it can neither be used for liquid level measurement of boiler steam drums. 
       SUMMARY OF THE INVENTION 
       [0004]    The purpose of this invention is to provide a type of liquid level gauge for continuous measurement of boiler steam drum under full conditions, and with this level gauge, the value of liquid level can be calculated according to the water and steam dielectric constants monitored in a real-time manner. Whichever status the boiler is under such as boiler startup and boiler shutdown, or boiler joint discharge, etc, the values of liquid level can be continuously outputted reliably and accurately in all cases with this liquid level gauge, thus to achieve the function for a continuous measurement of liquid level under full conditions. 
         [0005]    Another purpose of this invention is to provide a calculation method with a type of liquid level of boiler steam drum through continuous measurement of all work conditions. 
         [0006]    The purpose of this invention is achieved in such a way: a type of liquid level gauge for continuous measurement of boiler steam drum under full conditions, and the liquid level gauge for boiler steam drum mentioned includes one metal measurement tube and one transmitter. The steam-side pipe connected to steam drum is fitted on the upper part of the metal measurement tube mentioned and the water-side pipe connected to steam drum is fitted on the lower part of the metal measurement tube mentioned, one insulated jacket pipe is available inside the metal measurement tube mentioned,  1 # probe,  2 # probe, and  3 # probe are mounted in turn from down upwards inside the insulated jacket pipe mentioned, and respective probes are mutually insulated and are not conducted. The three probes mentioned are respectively connected with the transmitter through lead wires. 
         [0007]    The lead wires for the probes mentioned are the shielded conducting wires, the core on one end of the shielded conducting wire is connected with probe, and the core on the other end of the shielded conducting wire mentioned and the shielding layer are connected to the transmitter: 
         [0008]    The three probes mentioned measure capacitance by turns, and both the other probes will stop measurement at the same time when any of the three probes is measuring the capacitance. 
         [0009]    The method for calculation of liquid level with the liquid level gauge for continuous measurement of boiler steam drum under full conditions, and the method mentioned for calculation of liquid level is: 
         [0010]    The parameters calibrated for the liquid level gauge are used, to measure the capacitance values of  1 # probe,  2 # probe, and  3 # probe during the work of the gauge, and  1 # probe and  3 # probe are used to respectively monitor the dielectric constants of water and steam. 
         [0011]    Firstly dA and dB are calculated, respectively: 
         [0012]    A) When the condition of 
         [0000]    
       
         
           
             
               
                 Cx 
                  
                 
                     
                 
                  
                 2 
               
               - 
               
                 Ce 
                  
                 
                     
                 
                  
                 2 
               
             
             &gt; 
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     3 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     3 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   3 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
           
         
       
     
         [0000]    is complied, dA is updated, 
         [0000]    
       
         
           
             dA 
             = 
             
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   1 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   1 
                 
               
               
                 H 
                  
                 
                     
                 
                  
                 1 
               
             
           
         
       
     
         [0013]    B) When the condition of 
         [0000]    
       
         
           
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     C 
                      
                     
                         
                     
                      
                     e 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   1 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
             &gt; 
             
               ( 
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   2 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   2 
                 
               
               ) 
             
           
         
       
     
         [0000]    is compliant, dB is updated, 
         [0000]    
       
         
           
             dB 
             = 
             
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   3 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   3 
                 
               
               
                 H 
                  
                 
                     
                 
                  
                 3 
               
             
           
         
       
     
         [0014]    C) When the condition is incompliant, then dA or dB updated for the latest time is used. 
         [0015]    D) The preset dA and dB are used when the liquid level gauge is just electrified. 
         [0016]    Then the liquid level value Hx is calculated: 
         [0017]    1) The temporary variable hx of liquid level is figured out without consideration about E 2  and E 3 : 
         [0000]    
       
         
           
             hx 
             = 
             
               
                 
                   
                     
                       
                         
                           ( 
                           
                             
                               Cx 
                                
                               
                                   
                               
                                
                               1 
                             
                             + 
                             
                               Cx 
                                
                               
                                   
                               
                                
                               2 
                             
                             + 
                             
                               Cx 
                                
                               
                                   
                               
                                
                               3 
                             
                             - 
                             
                               Ce 
                                
                               
                                   
                               
                                
                               1 
                             
                             - 
                             
                               Ce 
                                
                               
                                   
                               
                                
                               2 
                             
                             - 
                             Ce3 
                           
                            
                           
                               
                           
                           ) 
                         
                         - 
                       
                     
                   
                   
                     
                       
                         
                           ( 
                           
                             
                               H 
                                
                               
                                   
                               
                                
                               1 
                             
                             + 
                             
                               H 
                                
                               
                                   
                               
                                
                               2 
                             
                             + 
                             
                               H 
                                
                               
                                   
                               
                                
                               3 
                             
                           
                           ) 
                         
                         × 
                         dB 
                       
                     
                   
                 
                 
                   dA 
                   - 
                   dB 
                 
               
               + 
               
                 E 
                  
                 
                     
                 
                  
                 1 
               
             
           
         
       
     
         [0018]    2) The position range of liquid level is judged according to the calculated hx value, and the actual liquid level Hx is calculated: 
         [0019]    A) At the time of hx&gt;E 1 +H 1 +H 2 , the liquid level is in the place of  3 # probe, 
         [0000]        Hx=hx+E 2 +E 3 
         [0020]    B) When Condition A is incompliant, and at the time of hx&gt;E 1 +H 1 , the liquid level is in the place of  2 # probe, 
         [0000]        Hx=hx+E 2 
         [0021]    C) When Condition B is neither compliant, it indicates that the liquid level does not exceed  1 # probe, and at this point there is no need for Hx to be changed, namely: 
         [0000]      Hx=hx 
         [0022]    of which: 
         [0023]    H 1 , H 2 , and H 3 : are respectively the physical heights of  1 # probe,  2 # probe, and  3 # probe. 
         [0024]    Ce 1 , Ce 2 , and Ce 3 : The capacitances of null values, and are the measured values for capacitances of  1 # probe,  2 # probe, and  3 # probe respectively when the medium is air. 
         [0025]    Cx 1 , Cx 2 , and Cx 3 : The capacitance values of  1 # probe,  2 # probe, and  3 # probe measured during the work of the liquid level gauge. 
         [0026]    E 1 : the distance from the lower end of  1 # probe to the centerline of water-side pipe of steam drum. 
         [0027]    E 2  and E 3 : are respectively the clearances between  1 # probe and  2 # probe as well as between  2 # probe and  3 # probe. 
         [0028]    L: the range of liquid level gauge, namely the center distance of the boiler water-side pipe and steam-side pipe. 
         [0029]    dA: the increment of capacitance between the measurement probes and the metal measurement tube aroused by water relative to air at unit height, and the change in dA has reflected the variation of water dielectric constant. 
         [0030]    dB: the increment of capacitance between the measurement probes and the metal measurement tube aroused by steam phase relative to air at unit height, and the change in dB has reflected the variation of steam dielectric constant. 
         [0031]    hx: the temporary variable of liquid level. 
         [0032]    Hx: the value of liquid level. 
         [0033]    The method for calculation of liquid level with a type of liquid level gauge for continuous measurement of boiler steam drum under full conditions, and the method mentioned for calculation of liquid level is: 
         [0034]    The parameters calibrated for the liquid level gauge are used, to measure the capacitance values of  1 # probe,  2 # probe, and  3 # probe during the work of the gauge, the  1 # probe and the  3 # probe are used to respectively monitor the dielectric constants of water and steam, and the capacitances of null value Ce 1 , Ce 2 , and Ce 3  as well as the capacitances of full value Cm 1 , Cm 2 , and Cm 3  are used to calculate and get the mapping heights of  1 # probe and  3 # probe relative to  2 # probe. 
         [0000]    
       
         
           
             
               H 
                
               
                   
               
                
               12 
             
             = 
             
               
                 
                   
                     Cm 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   
                     Cm 
                      
                     
                         
                     
                      
                     2 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     2 
                   
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
           
         
       
       
         
           
             
               H 
                
               
                   
               
                
               32 
             
             = 
             
               
                 
                   
                     Cm 
                      
                     
                         
                     
                      
                     3 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     3 
                   
                 
                 
                   
                     Cm 
                      
                     
                         
                     
                      
                     2 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     2 
                   
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
           
         
       
     
         [0035]    Firstly dA and dB are calculated, respectively: 
         [0036]    A) When the condition of 
         [0000]    
       
         
           
             
               
                 Cx 
                  
                 
                     
                 
                  
                 2 
               
               - 
               
                 Ce 
                  
                 
                     
                 
                  
                 2 
               
             
             &gt; 
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     3 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     3 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   32 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
           
         
       
     
         [0000]    is compliant, dA is updated, 
         [0000]    
       
         
           
             dA 
             = 
             
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   1 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   1 
                 
               
               
                 H 
                  
                 
                     
                 
                  
                 12 
               
             
           
         
       
     
         [0037]    B) When the condition of 
         [0000]    
       
         
           
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     C 
                      
                     
                         
                     
                      
                     e 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   12 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
             &gt; 
             
               ( 
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   2 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   2 
                 
               
               ) 
             
           
         
       
     
         [0000]    is compliant, dB is updated, 
         [0000]    
       
         
           
             dB 
             = 
             
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   3 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   3 
                 
               
               
                 H 
                  
                 
                     
                 
                  
                 32 
               
             
           
         
       
     
         [0038]    C) When the condition is incompliant, then dA or dB updated for the latest time is used. 
         [0039]    D) The present dA and DB are used when the liquid level gauge is just electrified. 
         [0040]    Then the liquid level value Hx is calculated: 
         [0041]    1) The temporary variable hx of liquid level associated with mapping heights of H 12  and H 32  is figured out without consideration about E 2  and E 3 : 
         [0000]    
       
         
           
             hx 
             = 
             
               
                 
                   
                     
                       
                         
                           ( 
                           
                             
                               Cx 
                                
                               
                                   
                               
                                
                               1 
                             
                             + 
                             
                               Cx 
                                
                               
                                   
                               
                                
                               2 
                             
                             + 
                             
                               Cx 
                                
                               
                                   
                               
                                
                               3 
                             
                             - 
                             
                               Ce 
                                
                               
                                   
                               
                                
                               1 
                             
                             - 
                             
                               Ce 
                                
                               
                                   
                               
                                
                               2 
                             
                             - 
                             
                               Ce 
                                
                               
                                   
                               
                                
                               3 
                             
                           
                           ) 
                         
                         - 
                       
                     
                   
                   
                     
                       
                         
                           ( 
                           
                             
                               H 
                                
                               
                                   
                               
                                
                               12 
                             
                             + 
                             
                               H 
                                
                               
                                   
                               
                                
                               2 
                             
                             + 
                             
                               H 
                                
                               
                                   
                               
                                
                               32 
                             
                           
                           ) 
                         
                         × 
                         
                           d 
                            
                           B 
                         
                       
                     
                   
                 
                 
                   dA 
                   - 
                   dB 
                 
               
               + 
               
                 E 
                  
                 
                     
                 
                  
                 1 
               
             
           
         
       
     
         [0042]    2) The position range of liquid level is judged according to the calculated hx value, and the actual liquid level Hx is calculated: 
         [0043]    A) At the time of hx&gt;E 1 +H 12 +H 2 , the liquid level is in the place of  3 # probe, 
         [0000]    
       
         
           
             Hx 
             = 
             
               
                 H 
                  
                 
                     
                 
                  
                 1 
               
               + 
               
                 H 
                  
                 
                     
                 
                  
                 2 
               
               + 
               
                 E 
                  
                 
                     
                 
                  
                 1 
               
               + 
               
                 E 
                  
                 
                     
                 
                  
                 2 
               
               + 
               
                 E 
                  
                 
                     
                 
                  
                 3 
               
               + 
               
                 
                   
                     ( 
                     
                       hx 
                       - 
                       
                         E 
                          
                         
                             
                         
                          
                         1 
                       
                       - 
                       
                         H 
                          
                         
                             
                         
                          
                         12 
                       
                       - 
                       
                         H 
                          
                         
                             
                         
                          
                         2 
                       
                     
                     ) 
                   
                   
                     H 
                      
                     
                         
                     
                      
                     32 
                   
                 
                 × 
                 H 
                  
                 
                     
                 
                  
                 3 
               
             
           
         
       
     
         [0044]    B) When Condition A is incompliant, and at the time of hx&gt;E 1 +H 12 , the liquid level is in the place of  2 # probe, 
         [0000]        Hx=+E 2 −H 12 +H 1 
         [0045]    C) When Condition B is incompliant, and at the time of hx&gt;E 1 , the liquid level is in the place of  1 # probe, 
         [0000]    
       
         
           
             Hx 
             = 
             
               
                 
                   ( 
                   
                     hx 
                     - 
                     
                       E 
                        
                       
                           
                       
                        
                       1 
                     
                   
                   ) 
                 
                 × 
                 
                   
                     H 
                      
                     
                         
                     
                      
                     1 
                   
                   
                     H 
                      
                     
                         
                     
                      
                     12 
                   
                 
               
               + 
               
                 E 
                  
                 
                     
                 
                  
                 1 
               
             
           
         
       
     
         [0046]    D) When Condition C is incompliant, it indicates that the liquid level is below  1 # probe or at the same level of the bottom end of  1 # probe, 
         [0000]      Hx=E1 o    
         [0047]    Of which: 
         [0048]    H 1 , H 2 , and H 3 : are respectively the physical heights of  13  probe,  2 # probe, and  3 # probe. 
         [0049]    H 12  and H 32 : are respectively the mapping heights of  1 # probe and  3 # probe relative to  2 # probe, and under the ideal status of physical structure, H 12  is equal to H 1  and H 32  is equal to H 3 . 
         [0050]    Ce 1 , Ce 2 , and Ce 3 : the capacitances of null value, and are the measured values of capacitances for  1 # probe,  2 # probe, and  3 # probe respectively when medium is air. 
         [0051]    Cm 1 , Cm 2 , and Cm 3 : the capacitances of full value, and are the measured values for capacitances of  1 # probe,  2 # probe, and  3 # probe respectively when the medium is calibrated. 
         [0052]    Cx 1 , Cx 2 , and Cx 3 : the capacitance values of  1 # probe,  2 # probe, and  3 # probe measured during the work of the liquid level gauge. 
         [0053]    E 1 : the distance from the lower end of  1 # probe to the centerline of the water-side pipe of steam drum 
         [0054]    E 2  and E 3 : are respectively the clearances between  1 # probe and  2 # probe, as well as between  2 # probe and  3 # probe. 
         [0055]    L: range of the liquid level gauge, namely the center distance of the boiler water-side pipe and the steam-side pipe. 
         [0056]    dA: the increment of capacitance between the measurement probes and the metal measurement tube aroused by water relative to air at unit height, and the change in dA has reflected the variation of water dielectric constant. 
         [0057]    dB: the increment of capacitance between the measurement probes and the metal measurement tube aroused by steam relative to air at unit height, and the change in dB has reflected the variation of steam dielectric constant. 
         [0058]    hx: the temporary variable of liquid level. 
         [0059]    Hx: the value of liquid level. 
         [0060]    The beneficial effect of this invention is: As the changes in water and steam dielectric constants are only reflected for the situations of boiler startup, boiler shutdown, and boiler joint discharge, etc speaking from this liquid level gauge, while this liquid level gauge for boiler steam drum is able to monitor the dielectric constants of water and steam in a real-time manner, the liquid level values can be outputted reliably and accurately in a continuous way with the liquid level gauge for boiler steam drum designed according to this invention in all the cases, no matter the boiler is under the startup and shutdown, or joint discharge status, thus to achieve the function of continuous measurement of liquid levels under full conditions. 
         [0061]    Further description is made below about this invention in combination with figures attached and embodiments. 
     
    
     
       ATTACHED FIGURES 
         [0062]      FIG. 1  is the schematic drawing of the liquid level gauge for continuous measurement of boiler steam drum under full conditions of this invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiment 1 
       [0063]    A type of liquid level gauge for continuous measurement of boiler steam drum under full conditions, and refer to  FIG. 1 , in the FIGURE, H 1 , H 2 , and H 3  are respectively the physical heights of  1 # probe,  2 # probe, and  3 # probe, E 1  is the distance from the lower end of  1 # probe to the centerline of the water-side pipe of steam drum, and E 2  and E 3  are the clearances between  1 # probe and  2 # probe as well as between  2 # probe and  3 # probe, respectively. L is the range of the liquid level gauge, namely the center distance of boiler water-side pipe and steam-side pipe. In this embodiment, H 1 =100 mm, H 2 =400 mm, H 3 =150 mm, E 1 =0, and E 2 =E 3 =3 mm. 
         [0064]    The liquid level gauge mentioned for boiler steam drum includes one metal measurement tube  4  and one transmitter  11 . The steam-side pipe  2  connected to steam drum is fitted on the upper part of the metal measurement tube mentioned and the water-side pipe  1  connected to steam drum is fitted on the lower part of the metal measurement tube mentioned, one insulated jacket pipe  8  is available inside the metal measurement tube mentioned,  1 # probe  5 ,  2 # probe  6 , and  3 # probe  7  are mounted in turn from down upwards inside the insulated jacket pipe mentioned, and respective probes are mutually insulated and not conducted, of which the  1 # probe is mainly used for monitoring the dielectric constant of water, the  3 # probe is mainly used for monitoring the dielectric constant of steam, and the  2 # probe is the main measurement probe. The three probes mentioned are respectively connected with the transmitter  11  through the lead wires  10 . 
         [0065]    In this embodiment, the lead wire  10  of the probe is the shielded conducting wire, the core on the one end of the shielded conducting wire is connected with probe and the core on the other end of the shielded conducting wire and the shielding layer are connected with the transmitter. 
         [0066]    In this embodiment, in order to prevent the mutual signal interference between the three measurement probes, the three probes undertake the measurement by turns. When  1 # probe is making measurement,  2 # and  3 # probe stop measurement, while  2 # probe is making measurement,  1 # and  3 # probes stop measurement, and when  3 # probe is making measurement,  1 # and  2 # probes stop measurement. 
         [0067]    In this embodiment, one insulated jacket  8  is available inside the metal measurement tube  4 , and both the upper place and the lower place of the metal measurement tube  4  are insulated. Inside the insulated jacket  8 , three measurement probes are available from down upwards:  1 # probe  5 ,  2 # probe  6 , and  3 # probe  7 , and these three probes are mutually insulated and not conducted. The core on the one end of the three shielded conducting wires is respectively connected to the three probes, and the core on the other end and the shielding layer are connected to the transmitter  11 , thus the three probes have formed three capacitances respectively with the metal measurement tube. In order to prevent the signal mutual interference between the three measurement probes, the three probes undertake measurement by turns, while  1 # probe is making measurement,  2 # and  3 # probes stop measurement, and in this analogy. The change in liquid level and variations of water and steam dielectric constants will give rise to corresponding changes in the values of these three capacitances. Transmitter  11  is used for measuring the three values of the capacitances, to calculate the liquid level values, to output the simulated current signals of liquid level and digital communication signals of liquid level. The transmitter  11  is built-in with single chip microcomputer, capacitance measurement, current transformation and transmission, digital communication interface, and other circuits. The microcomputer fulfills signal processing for capacitance measurement, calculation of the liquid level value, output of current value, digital communication, and other tasks. The transmitter calculates the liquid level value through measurement of the three capacitance values between the three probes and the metal measurement tube and outputs the simulated current signal of liquid level through current transmission circuit, for digital communication to be performed on digital communication interface and master computer. 
         [0068]    Two connecting pipes, namely the steam-side pipe  2  and the water-side pipe  1  are led out from the metal measurement tube, and the steam-side pipe  2  and the water-side pipe  1  are respectively connected to the steam-side pipe and the water-side pipe of the steam drum, to measure out the liquid level inside the measurement tube, to rightly get the liquid level inside the steam drum, according to the principle of connected vessel. Generally heat insulation measures shall be applied to outside of the measurement tube, so that the temperatures of water and steam inside the measurement tube are approximate to the temperatures of water and steam inside the steam drum, thus to reduce the error of liquid level aroused by density difference. 
       Embodiment 2 
       [0069]    Refer to  FIG. 1 , and in this embodiment, the method for calculation of liquid level with the liquid level gauge for continuous measurement of boiler steam drum under full conditions is: 
         [0070]    Calibration of following parameters is carried out before the liquid level gauge is delivered from the factory: 
         [0071]    H 1 , H 2 , and H 3 : are respectively the physical heights of  1 # probe,  2 # probe, and  3 # probe. 
         [0072]    E 1 : the distance from the lower end of  1 # probe to the centerline of the water-side pipe of steam drum. 
         [0073]    E 2  and E 3 : are respectively the clearances between  1 # probe and  2 # probe as well as between  2 # probe and  3 # probe. 
         [0074]    Ce 1 , Ce 2 , and Ce 3 : the capacitances of null value, and are the measured values of capacitances for  1 # probe,  2 # probe, and  3 # probe respectively when the medium is air. 
         [0075]    dA Presetting: the increment of capacitance between the measurement probes and the metal measurement tube aroused by water relative to air at unit height, and the change in dA has reflected the variation of water dielectric constant. 
         [0076]    dB Presetting: the increment of capacitance between the measurement probes and the metal measurement tube aroused by steam relative to air at unit height, and the change in dB has reflected the variation of steam dielectric constant. 
         [0077]    The liquid level gauge calibrated through the abovementioned parameters calculates the liquid level value Hx after one group of capacitance values (Cx 1 , Cx 2 , and Cx 3 ) is measured each time during the work of the gauge. 
         [0078]    Firstly dA and dB are calculated, respectively: 
         [0079]    A) When the condition of 
         [0000]    
       
         
           
             
               
                 Cx 
                  
                 
                     
                 
                  
                 2 
               
               - 
               
                 Ce 
                  
                 
                     
                 
                  
                 2 
               
             
             &gt; 
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     3 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     3 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   3 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
           
         
       
     
         [0000]    is compliant, dA is updated, 
         [0000]    
       
         
           
             dA 
             = 
             
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   1 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   1 
                 
               
               
                 H 
                  
                 
                     
                 
                  
                 1 
               
             
           
         
       
     
         [0080]    B) When the condition of 
         [0000]    
       
         
           
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   1 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
             &gt; 
             
               ( 
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   2 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   2 
                 
               
               ) 
             
           
         
       
     
         [0000]    is compliant, dB is updated, 
         [0081]    B) When the condition of 
         [0000]    
       
         
           
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   1 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
             &gt; 
             
               ( 
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   2 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   2 
                 
               
               ) 
             
           
         
       
     
         [0000]    is compliant, dB is updated, 
         [0000]    
       
         
           
             dB 
             = 
             
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   3 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   3 
                 
               
               
                 H 
                  
                 
                     
                 
                  
                 3 
               
             
           
         
       
     
         [0082]    C) When condition is incompliant, then dA or dB updated for the latest time is used. 
         [0083]    D) The preset dA and dB are used when the liquid level gauge is just electrified. 
         [0084]    Then the liquid level value Hx is calculated: 
         [0085]    1) The temporary variable hx of liquid level is figured out without consideration of E 2  and E 3 : 
         [0000]    
       
         
           
             hx 
             = 
             
               
                 
                   
                     
                       
                         
                           ( 
                           
                             
                               Cx 
                                
                               
                                   
                               
                                
                               1 
                             
                             + 
                             
                               Cx 
                                
                               
                                   
                               
                                
                               2 
                             
                             + 
                             
                               Cx 
                                
                               
                                   
                               
                                
                               3 
                             
                             - 
                             
                               Ce 
                                
                               
                                   
                               
                                
                               1 
                             
                             - 
                             
                               Ce 
                                
                               
                                   
                               
                                
                               2 
                             
                             - 
                             
                               Ce 
                                
                               
                                   
                               
                                
                               3 
                             
                           
                           ) 
                         
                         - 
                       
                     
                   
                   
                     
                       
                         
                           ( 
                           
                             
                               H 
                                
                               
                                   
                               
                                
                               1 
                             
                             + 
                             
                               H 
                                
                               
                                   
                               
                                
                               2 
                             
                             + 
                             
                               H 
                                
                               
                                   
                               
                                
                               3 
                             
                           
                           ) 
                         
                         × 
                         dB 
                       
                     
                   
                 
                 
                   dA 
                   - 
                   dB 
                 
               
               + 
               
                 E 
                  
                 
                     
                 
                  
                 1 
               
             
           
         
       
     
         [0086]    2) The position range of liquid level is judged according to the calculated hx value, and the actual liquid level Hx is calculated: 
         [0087]    A) At the time of hx&gt;E 1 +H 1 +H 2 , it indicates that the liquid level is in the place of  3 # probe, and at this point Hx shall be added with the clearance E 2  between  1 # probe and  2 # probe as well as the clearance E 3  between  2 # probe and  3 # probe, namely: 
         [0000]        Hx=hx+E 2 +E 3 
         [0088]    B) When Condition A is incompliant, and at the time of hx&gt;E 1 +H 1 , it indicates that the liquid level is in the place of  2 # probe, and at this point Hx shall be added with the clearance E 2  between  1 # probe and  2 # probe, namely: 
         [0000]        Hx=hx+E 2 
         [0089]    C) When Condition B is incompliant, it indicates that the liquid level does not exceed  1 # probe, and at this point there is no need for Hx to be changed, namely: 
         [0000]      Hx=hx 
         [0090]    Of which: 
         [0091]    H 1 , H 2 , and H 3 : are respectively the physical heights of  1 # probe,  2 # probe, and  3 # probe. In this embodiment, H 1 =100 mm, H 2 =400 mm, and H 3 =150 mm. 
         [0092]    Ce 1 , Ce 2 , and Ce 3 : the capacitances of null value, and are the measured values for the capacitances of  1 # probe,  2 # probe, and  3 # probe respectively when the medium is air. 
         [0093]    Cx 1 , Cx 2 , and Cx 3 : the capacitance values of  1 # probe,  2 # probe, and  3 # probe measured during the work of the liquid level gauge. 
         [0094]    E 1 : the distance from the lower end of  1 # probe to the centerline of the water-side pipe of steam drum. It is the negative value when the lower end of  1 # probe is lower than the center area of the water-side pipe, while it is the positive value when it is higher than the center area of the water-side pipe. In this embodiment, the distance from the lower end of  1 # probe to the centerline of the water-side pipe of steam drum is 0, namely to allow the lower end of  1 # probe and the centerline of the water-side pipe of steam drum to be aligned when it is made. 
         [0095]    E 2  and E 3 : are respectively the clearances between  1 # probe and  2 # probe as well as between  2 # probe and  3 # probe, 2˜3 mm in general, and in this embodiment, both the clearances between  1 # probe and  2 # probe as well as between  2 # probe and  3 # probe are 3 mm. 
         [0096]    L: range of the liquid level gauge, namely the center distance of the boiler water-side pipe and steam-side pipe. 
         [0097]    dA: the increment of capacitance between the measurement probes and the metal measurement lube aroused by water relative to air at unit height, and the change in dA has reflected the variation of water dielectric constant. 
         [0098]    dB: the increment of capacitance between the measurement probes and the metal measurement tube aroused by steam relative to air at unit height, and the change in dB has reflected the variation of steam dielectric constant. 
         [0099]    hx: The temporary variable of liquid level 
         [0100]    Hx: The value of liquid level 
         [0101]    When calibrated: Ce 1 =100 pF Ce 2 =180 pF Ce 3 =120 pF
       H 1 =100 mm H 2 =400 mm H 3 =150 mm   E 1 =0 E 2 =E 3 =3 dA Presetting=0.2, and dB Presetting=0       
 
         [0104]    The capacitance value is measured at a certain time: 
         [0105]    Cx 1 =125 pF Cx 2 =228 pF Cx 3 =125 pF 
         [0106]    The calculation process is as follows: 
         [0107]    Calculation of dA and dB: 
         [0108]    A) 
         [0000]    
       
         
           
             
               
                 Cx 
                  
                 
                     
                 
                  
                 2 
               
               - 
               
                 Ce 
                  
                 
                     
                 
                  
                 2 
               
             
             = 
             48 
           
         
       
       
         
           
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     3 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     3 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   3 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
             = 
             13.33 
           
         
       
     
         [0109]    When the condition of 
         [0000]    
       
         
           
             
               
                 Cx 
                  
                 
                     
                 
                  
                 2 
               
               - 
               
                 Ce 
                  
                 
                     
                 
                  
                 2 
               
             
             &gt; 
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     3 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     3 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   3 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
           
         
       
     
         [0000]    is compliant, dA is updated: 
         [0000]    
       
         
           
             dA 
             = 
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   1 
                 
               
               = 
               0.25 
             
           
         
       
     
         [0110]    B) 
         [0000]    
       
         
           
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   1 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
             = 
             100 
           
         
       
       
         
           
             
               
                 Cx 
                  
                 
                     
                 
                  
                 2 
               
               - 
               
                 Ce 
                  
                 
                     
                 
                  
                 2 
               
             
             = 
             48 
           
         
       
     
         [0111]    When the condition of 
         [0000]    
       
         
           
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   1 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
             &gt; 
             
               ( 
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   2 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   2 
                 
               
               ) 
             
           
         
       
     
         [0000]    is compliant, dB is updated: 
         [0000]    
       
         
           
             dB 
             = 
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     3 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     3 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   3 
                 
               
               = 
               0.0333 
             
           
         
       
     
         [0112]    Calculation of liquid level value Hx: 
         [0113]    1) The temporary variable hx of liquid level is figured out without consideration about E 2  and E 3 : 
         [0000]    
       
         
           
             
               
                 
                   hx 
                   = 
                     
                    
                   
                     
                       
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     Cx 
                                      
                                     
                                         
                                     
                                      
                                     1 
                                   
                                   + 
                                   
                                     Cx 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                   + 
                                   
                                     Cx 
                                      
                                     
                                         
                                     
                                      
                                     3 
                                   
                                   - 
                                   
                                     Ce 
                                      
                                     
                                         
                                     
                                      
                                     1 
                                   
                                   - 
                                   
                                     Ce 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                   - 
                                   
                                     Ce 
                                      
                                     
                                         
                                     
                                      
                                     3 
                                   
                                 
                                 ) 
                               
                               - 
                             
                           
                         
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     H 
                                      
                                     
                                         
                                     
                                      
                                     1 
                                   
                                   + 
                                   
                                     H 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                   + 
                                   
                                     H 
                                      
                                     
                                         
                                     
                                      
                                     3 
                                   
                                 
                                 ) 
                               
                               × 
                               dB 
                             
                           
                         
                       
                       
                         dA 
                         - 
                         dB 
                       
                     
                     + 
                     
                       E 
                        
                       
                           
                       
                        
                       1 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     260.0 
                      
                     
                         
                     
                      
                     mm 
                   
                 
               
             
           
         
       
     
         [0114]    2) The poison range of liquid level judged according to the calculated hx value, and the actual liquid level Hx is calculated: 
         [0115]    A) E 1 +H 1 +H 2 =500, incompliant to hx&gt;E 1 +H 1 +H 2   
         [0116]    B) E 1 +H 1 =100, compliant to hx&gt;E 1 +H 1 , then: 
         [0000]        Hx=hx+E 2=260.0+3=263.0 mm 
       Embodiment 3 
       [0117]    As certain errors exist with physical factors such as the diameters of three probes, the inner diameter of the measurement tube, and the thickness of insulated jacket, etc, during the production and manufacture of the liquid level gauge for boiler steam drum, for which the increment of capacitance aroused between the three probes by medium at unit height may possibly be inconsistent in the actual application of liquid level of boiler steam drum, it is hence required to set up the values of two mapping heights H 12  and H 32 , to respectively represent the mapping heights of  1 # probe and  3 # probe relative to  2 # probe, thereby to amend the impact of physical errors with the probe and measurement tube of the liquid level gauge on the liquid level value Hx, so that the liquid level gauge for boiler steam drum will be able to give out more accurate liquid level value Hx. 
         [0118]    Refer to  FIG. 1 , and in this embodiment, the method for calculation of liquid level with the liquid level gauge for continuous measurement of boiler steam drum under full conditions is: 
         [0119]    The calibration of following parameters is carried out before the liquid level gauge is delivered from the factory: 
         [0120]    H 1 , H 2 , and H 3 : are respectively the physical heights of  1 # probe,  2 # probe, and  3 # probe. 
         [0121]    E 1 : the distance from the lower end of  1 # probe to the centerline of the water-side pipe of steam drum. 
         [0122]    E 2  and E 3 : are respectively the clearances between  1 # probe and  2 # probe as well as between  2 # probe and  3 # probe. 
         [0123]    Ce 1 , Ce 2 , and Ce 3 : the capacitances of null value, and are the measured values for the capacitances of  1 # probe,  2 # probe, and  3 # probe respectively when the medium is air. 
         [0124]    Cm 1 , Cm 2 , and Cm 3 : the capacitances of full value, and are the measured values for the capacitances of  1 # probe,  2 # probe, and  3 # probe respectively when the medium is calibrated. Normally water is adopted to serve as this medium during calibration. 
         [0125]    H 12  and H 32 : are respectively the mapping heights of  1 # probe and  3 # probe relative to  2 # probe, and under the ideal status of physical structure, H 12  is equal to H 1 , and H 32  is equal to H 3  (This parameter may also not be calibrated while it is to be further calculated at the time of calculation). 
         [0126]    Its calculation method is: 
         [0000]    
       
         
           
             
               H 
                
               
                   
               
                
               12 
             
             = 
             
               
                 
                   
                     Cm 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   
                     Cm 
                      
                     
                         
                     
                      
                     2 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     2 
                   
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
           
         
       
       
         
           
             
               H 
                
               
                   
               
                
               32 
             
             = 
             
               
                 
                   
                     Cm 
                      
                     
                         
                     
                      
                     3 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     3 
                   
                 
                 
                   
                     Cm 
                      
                     
                         
                     
                      
                     2 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     2 
                   
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
           
         
       
     
         [0127]    dA Presetting: the increment of capacitance between the measurement probes and the metal measurement tube aroused by water relative to air at unit height, and the change in dA has reflected the variation of water dielectric constant. 
         [0128]    dB Presetting: the increment of capacitance between the measurement probes and the metal measurement tube aroused by steam relative to air at unit height. 
         [0129]    The liquid level gauge calibrated through abovementioned parameters calculates the liquid level value Hx after one group of capacitance values (Cx 1 , Cx 2 , and Cx 3 ) is measured each time during its work. 
         [0130]    Firstly dA and dB are calculated, respectively: 
         [0131]    A) When the condition of 
         [0000]    
       
         
           
             
               
                 Cx 
                  
                 
                     
                 
                  
                 2 
               
               - 
               
                 Ce 
                  
                 
                     
                 
                  
                 2 
               
             
             &gt; 
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     3 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     3 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   32 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
           
         
       
     
         [0000]    is compliance, dA is updated, 
         [0000]    
       
         
           
             dA 
             = 
             
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   1 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   1 
                 
               
               
                 H 
                  
                 
                     
                 
                  
                 12 
               
             
           
         
       
     
         [0132]    B) When the condition of 
         [0000]    
       
         
           
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   12 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
             &gt; 
             
               ( 
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   2 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   2 
                 
               
               ) 
             
           
         
       
     
         [0000]    is compliant, dB is updated, 
         [0000]    
       
         
           
             dB 
             = 
             
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   3 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   3 
                 
               
               
                 H 
                  
                 
                     
                 
                  
                 32 
               
             
           
         
       
     
         [0133]    C) When condition is incompliant, then dA or dB updated for the latest time is used. 
         [0134]    D) The preset dA and dB are used when the liquid level gauge is just electrified. 
         [0135]    Then the liquid level value Hx is calculated: 
         [0136]    1) The temporary variable hx of liquid level associated with mapping heights H 12  and H 32  is figured out without consideration about E 2  and E 3 : 
         [0000]    
       
         
           
             hx 
             = 
             
               
                 
                   
                     
                       
                         
                           ( 
                           
                             
                               Cx 
                                
                               
                                   
                               
                                
                               1 
                             
                             + 
                             
                               Cx 
                                
                               
                                   
                               
                                
                               2 
                             
                             + 
                             
                               Cx 
                                
                               
                                   
                               
                                
                               3 
                             
                             - 
                             
                               Ce 
                                
                               
                                   
                               
                                
                               1 
                             
                             - 
                             
                               Ce 
                                
                               
                                   
                               
                                
                               2 
                             
                             - 
                             
                               Ce 
                                
                               
                                   
                               
                                
                               3 
                             
                           
                           ) 
                         
                         - 
                       
                     
                   
                   
                     
                       
                         
                           ( 
                           
                             
                               H 
                                
                               
                                   
                               
                                
                               12 
                             
                             + 
                             
                               H 
                                
                               
                                   
                               
                                
                               2 
                             
                             + 
                             
                               H 
                                
                               
                                   
                               
                                
                               32 
                             
                           
                           ) 
                         
                         × 
                         dB 
                       
                     
                   
                 
                 
                   dA 
                   - 
                   dB 
                 
               
               + 
               
                 E 
                  
                 
                     
                 
                  
                 1 
               
             
           
         
       
     
         [0137]    2) The position range of liquid level judged according to the calculated hx value, and the actual liquid level Hx is calculated: 
         [0138]    A) At the time of hx&gt;E 1 +H 12 +H 2 , it indicates that the liquid level is in the place of  3 # probe, and at this point Hx is namely the sum of height of  1 # probe H 1 , the height of  2 # probe H 2 , as well as E 1 , E 2 , and E 3  and the height of liquid level at  3 # probe, namely: 
         [0000]    
       
         
           
             Hx 
             = 
             
               
                 H 
                  
                 
                     
                 
                  
                 1 
               
               + 
               
                 H 
                  
                 
                     
                 
                  
                 2 
               
               + 
               
                 E 
                  
                 
                     
                 
                  
                 1 
               
               + 
               
                 E 
                  
                 
                     
                 
                  
                 2 
               
               + 
               
                 E 
                  
                 
                     
                 
                  
                 3 
               
               + 
               
                 
                   
                     ( 
                     
                       hx 
                       - 
                       
                         E 
                          
                         
                             
                         
                          
                         1 
                       
                       - 
                       
                         H 
                          
                         
                             
                         
                          
                         1 
                          
                         
                             
                         
                          
                         2 
                       
                       - 
                       
                         H 
                          
                         
                             
                         
                          
                         2 
                       
                     
                     ) 
                   
                   
                     H 
                      
                     
                         
                     
                      
                     32 
                   
                 
                 × 
                 H 
                  
                 
                     
                 
                  
                 3 
               
             
           
         
       
     
         [0139]    B) When Condition A is incompliant, and at the time of hx&gt;E 1 +H 12 , it indicates that the liquid level is in the place of  2 # probe, and at this point Hx shall be added with the clearance E 2  between  1 # probe and  2 # probe, while at the same time H 12  shall be converted back to the physical height H 1 , namely: 
         [0000]        Hx=+E 2 −H 12 +H 1 
         [0140]    C) When Condition B is incompliant, and at the time of hx&gt;E 1 , it indicates that the liquid level is in the place of  1 # probe, and at this point the liquid level in the place of  1 # probe shall be calculated back to the physical height according to the proportion of H 12  and H 1 , namely: 
         [0000]    
       
         
           
             Hx 
             = 
             
               
                 
                   ( 
                   
                     hx 
                     - 
                     
                       E 
                        
                       
                           
                       
                        
                       1 
                     
                   
                   ) 
                 
                 × 
                 
                   
                     H 
                      
                     
                         
                     
                      
                     1 
                   
                   
                     H 
                      
                     
                         
                     
                      
                     12 
                   
                 
               
               + 
               
                 E 
                  
                 
                     
                 
                  
                 1 
               
             
           
         
       
     
         [0141]    D) When Condition C is incompliant, it indicates that the liquid level is below  1 # probe or at the same level of the bottom end of  1 # probe, namely: 
         [0000]      Hx=E1 o    
         [0142]    Of which: 
         [0143]    H 1 , H 2 , and H 3 : are respectively the physical heights of  1 # probe,  2 # probe, and  3 # probe. 
         [0144]    In this embodiment, H 1 =100 mm, H 2 =400 mm, and H 3 =150 mm. 
         [0145]    H 12  and H 32 : are respectively the mapping heights of  1 # probe and  3 # probe relative to  2 # probe, and under the ideal status of physical structure, H 12  is equal to H 1 , and H 32  is equal to H 3 . 
         [0146]    Ce 1 , Ce 1 , and Ce 3 : the capacitances of null value, and are the measured values for capacitances of  1 # probe,  2 # probe, and  3 # probe respectively when the medium is air. 
         [0147]    Cm 1 , Cm 2 , and Cm 3 : the capacitances of full value, and are the measured values for capacitances of  1 # probe,  2 # probe, and  3 # probe respectively when the medium is calibrated. Normally water serves as this medium during calibration, and in this embodiment, water is adopted to serve as this medium. 
         [0148]    Cx 1 , Cx 2 , and Cx 3 : the capacitance values of  1 # probe,  2 # probe, and  3 # probe measured during the work of the liquid level gauge. 
         [0149]    E 1 : the distance from the lower end of  1 # probe to the centerline of the water-side pipe of steam drum. It is the negative value when the lower end of  1 # probe is lower than the center area of the water-side pipe and it is the positive value when the lower end of  1 # probe is higher than the center area of the water-side pipe, while in this embodiment the distance from the lower end of  1 # probe to the centerline of the water-side pipe of stream drum is 0, namely the lower end of  1 # probe is aligned with the centerline of the water-side pipe of steam drum. 
         [0150]    E 2  and E 3 : are respectively the clearances between  1 # probe and  2 # probe as well as between  2 # probe and  3 # probe. In this embodiment, the both clearances between  1 # probe and  2 # probe as well as between  2 # probe and  3 # probe are 3 mm. 
         [0151]    L: the range of the liquid level gauge, namely the center distance of the boiler water-side pipe and steam-side pipe. 
         [0152]    dA: the increment of capacitance between the measurement probes and the metal measurement tube aroused by water relative to air at unit height, and the change in dA has reflected the variation of water dielectric constant. 
         [0153]    dB: the increment of capacitance between the measurement probes and the metal measurement tube aroused by steam relative to air at unit height, and the change in dB has reflected the variation of steam dielectric constant. 
         [0154]    hx: the temporary variable of liquid level. 
         [0155]    Hx: the value of liquid level. 
         [0156]    When calibrated: Ce 1 =100 pF Ce 2 =180 pF Ce 3 =120 pF
       Cm 1 =123 pF Cm 2 =270 pF Cm 3 =159 pF   H 1 =100 mm H 2 =400 mm H 3 =150 mm   E 1 =0 E 2 =E 3 =3 dA Presetting=0.2 dB Presetting=0       
 
         [0160]    The capacitance values are measured at a certain time: 
         [0161]    Cx 1 =125 pF Cx 2 =228 pF Cx 3 =125 pF 
         [0162]    Calculation of the mapping heights H 12  and H 32 : 
         [0000]    
       
         
           
             
               H 
                
               
                   
               
                
               12 
             
             = 
             
               
                 
                   
                     
                       Cm 
                        
                       
                           
                       
                        
                       1 
                     
                     - 
                     
                       Ce 
                        
                       
                           
                       
                        
                       1 
                     
                   
                   
                     
                       Cm 
                        
                       
                           
                       
                        
                       2 
                     
                     - 
                     
                       Ce 
                        
                       
                           
                       
                        
                       2 
                     
                   
                 
                 × 
                 H 
                  
                 
                     
                 
                  
                 2 
               
               = 
               
                 102.22 
                  
                 
                     
                 
                  
                 mm 
               
             
           
         
       
       
         
           
             
               H 
                
               
                   
               
                
               32 
             
             = 
             
               
                 
                   
                     
                       Cm 
                        
                       
                           
                       
                        
                       3 
                     
                     - 
                     
                       Ce 
                        
                       
                           
                       
                        
                       3 
                     
                   
                   
                     
                       Cm 
                        
                       
                           
                       
                        
                       2 
                     
                     - 
                     
                       Ce 
                        
                       
                           
                       
                        
                       2 
                     
                   
                 
                 × 
                 H 
                  
                 
                     
                 
                  
                 2 
               
               = 
               
                 173.33 
                  
                 
                     
                 
                  
                 mm 
               
             
           
         
       
     
         [0163]    Calculation of dA and dB: 
         [0164]    A) 
         [0000]    
       
         
           
             
               
                 Cx 
                  
                 
                     
                 
                  
                 2 
               
               - 
               
                 Ce 
                  
                 
                     
                 
                  
                 2 
               
             
             = 
             48 
           
         
       
       
         
           
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     3 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     3 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   32 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
             = 
             11.54 
           
         
       
     
         [0165]    When the condition of 
         [0000]    
       
         
           
             
               
                 Cx 
                  
                 
                     
                 
                  
                 2 
               
               - 
               
                 Ce 
                  
                 
                     
                 
                  
                 2 
               
             
             &gt; 
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     3 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     3 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   32 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
           
         
       
     
         [0000]    is compliant, dA is updated. 
         [0000]    
       
         
           
             dA 
             = 
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   12 
                 
               
               = 
               0.24457 
             
           
         
       
     
         [0166]    B) 
         [0000]    
       
         
           
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   12 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
             = 
             97.8 
           
         
       
       
         
           
             
               
                 C 
                 × 
                 2 
               
               - 
               
                 Ce 
                  
                 
                     
                 
                  
                 2 
               
             
             = 
             48 
           
         
       
     
         [0167]    When the condition of 
         [0000]    
       
         
           
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   12 
                 
               
               × 
               H 
                
               
                   
               
                
               2 
             
             &gt; 
             
               ( 
               
                 
                   Cx 
                    
                   
                       
                   
                    
                   2 
                 
                 - 
                 
                   Ce 
                    
                   
                       
                   
                    
                   2 
                 
               
               ) 
             
           
         
       
     
         [0000]    is compliant, dB is updated. 
         [0000]    
       
         
           
             dB 
             = 
             
               
                 
                   
                     Cx 
                      
                     
                         
                     
                      
                     3 
                   
                   - 
                   
                     Ce 
                      
                     
                         
                     
                      
                     3 
                   
                 
                 
                   H 
                    
                   
                       
                   
                    
                   32 
                 
               
               = 
               0.0288 
             
           
         
       
     
         [0168]    Calculation of liquid level value Hx: 
         [0169]    1) Calculation of temporary variable hx: 
         [0000]    
       
         
           
             
               
                 
                   hx 
                   = 
                     
                    
                   
                     
                       
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     Cx 
                                      
                                     
                                         
                                     
                                      
                                     1 
                                   
                                   + 
                                   
                                     Cx 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                   + 
                                   
                                     Cx 
                                      
                                     
                                         
                                     
                                      
                                     3 
                                   
                                   - 
                                   
                                     Ce 
                                      
                                     
                                         
                                     
                                      
                                     1 
                                   
                                   - 
                                   
                                     Ce 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                   - 
                                   
                                     Ce 
                                      
                                     
                                         
                                     
                                      
                                     3 
                                   
                                 
                                 ) 
                               
                               - 
                             
                           
                         
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     H 
                                      
                                     
                                         
                                     
                                      
                                     12 
                                   
                                   + 
                                   
                                     H 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                   + 
                                   
                                     H 
                                      
                                     
                                         
                                     
                                      
                                     32 
                                   
                                 
                                 ) 
                               
                               × 
                               dB 
                             
                           
                         
                       
                       
                         dA 
                         - 
                         dB 
                       
                     
                     + 
                     
                       E 
                        
                       
                           
                       
                        
                       1 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     271.3 
                      
                     
                         
                     
                      
                     mm 
                   
                 
               
             
           
         
       
     
         [0170]    2) The position range of liquid level is judged according to the calculated hx value, and the actual liquid level Hx is calculated: 
         [0171]    A) E 1 +H 12 +H 2 =502.22 mm, incompliant to hx&gt;E 1 +H 12 +H 2   
         [0172]    B) E 1 +H 12 =102.22 mm, compliant to hx&gt;E 1 +H 12 , then: 
         [0000]        Hx=hx+E 2 −H 12 +H 1=271.3+3−102.22+100=272.1 mm