Flowmeter

Abstract

A flowmeter for achieving a fluid flow rate value by using calibration curves on the basis of the output of a detection circuit using a bridge circuit ( 73 ) containing as constituent resistors respective temperature sensing elements of a flow rate detector containing a heating element ( 33 ) and a fluid temperature detector in an indirectly heated type flow rate sensor unit. The bridge circuit ( 73 ) varies the circuit characteristic value in plural steps by a multiplexer ( 731 ) for selectively connecting the output terminal and any one of the connection terminals between the in-series connected resistors. Plural calibration curves are provided in association with the steps of the circuit characteristic value, and any one of the plural calibration curves is selected in accordance with the step of the circuit characteristic value selected by the multiplexer ( 731 ). The flow rate range to be measured is set every calibration curve, and the multiplexer ( 731 ) is controlled in accordance with the fluid flow rate value thus achieved, and the calibration curve corresponding to the flow rate range to which the flow rate value belongs. According to this flowmeter, the flow rate can be measured with excellent precision over a board flow rate range.

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       7 . A thermal type flowmeter having a heating element and a flow rate detection circuit containing a temperature sensing element for flow rate detection disposed so as to be affected by the heating of the heating element and perform heat transfer from/to fluid, the heating of the heating element being controlled on the basis of a voltage applied to the heating element, the applied voltage to the heating element being controlled on the basis of the output of the flow rate detection circuit and the flow rate of the fluid being measured on the basis of the applied voltage, characterized in that the applied voltage to said heating element is equal to the total voltage of a base voltage the value of which is set every predetermined time period and invariable within each predetermined time period, and an addition voltage which has a fixed value and is variable in voltage applying time, a comparator for comparing the output of said flow rate detecting circuit with a reference value is provided to output a binary signal having a first level indicating that the heating of said temperature sensing element is insufficient and a second level indicating the other cases, the binary signal output from said comparator is sampled at a predetermined period to count the appearance frequency of the first level every predetermined time period and achieve the count value thereof every predetermined time period, the base voltage is adjusted so that when the count value is within a predetermined range, the value of the base voltage in a subsequent predetermined time period is not changed, when the count value is larger than the upper limit of the predetermined range, the value of the base voltage in the subsequent predetermined time period is increased by a predetermined step value and when the count value is smaller than the lower limit of the predetermined range, the value of the base voltage in the subsequent time period is reduced by the predetermined step value, and the addition voltage is applied during only a time period when the binary signal output from the comparator has the first level.  
   
   
       8 . The thermal type flowmeter as claimed in  claim 7 , wherein the addition voltage is set to two times to four times of the step value of the base voltage.  
   
   
       9 . The thermal type flowmeter as claimed in  claim 7 , wherein the predetermined range of the count value has a lower limit value which is smaller than a half of the sampling frequency within the predetermined time period and larger than zero, and has an upper limit value which is larger than a half of the sampling frequency within the predetermined time period and smaller than the sampling frequency.  
   
   
       10 . A thermal type flowmeter having a heating element and a flow rate detection circuit containing a temperature sensing element for flow rate detection disposed so as to be affected by the heating of the heating element and perform heat transfer from/to fluid, the heating of the heating element being controlled on the basis of a voltage applied to the heating element, the applied voltage to the heating element being controlled on the basis of the output of the flow rate detection circuit and the flow rate of the fluid being measured on the basis of the applied voltage, characterized in that the applied voltage to said heating element is equal to the total voltage of a base voltage the value of which is set every predetermined time period and invariable within each predetermined time period, and an addition voltage which has a fixed value and is variable in voltage applying time, a comparator for comparing the output of said flow rate detecting circuit with a reference value is provided to output a binary signal having a first level indicating that the heating of said temperature sensing element is insufficient and a second level indicating the other cases, the binary signal output from said comparator is sampled at a predetermined period to count the appearance frequency of the first level every predetermined time period and achieve the count value thereof every predetermined time period, the base voltage is adjusted so that when the count value is within a predetermined range, the value of the base voltage in a subsequent predetermined time period is not changed, when the count value is larger than the upper limit of the predetermined range, the value of the base voltage in the subsequent predetermined time period is increased by a predetermined step value and when the count value is smaller than the lower limit of the predetermined range, the value of the base voltage in the subsequent time period is reduced by the predetermined step value, the addition voltage is applied during only a time period when the binary signal output from the comparator has the first level, and a data interpolating calculation is made by using an instantaneous flow rate converting table comprising plural individual calibration curves which indicate the relationship between the applied voltage to said heating element and the instantaneous flow rate every discrete temperature value, thereby achieving an instantaneous flow rate value at an environmental temperature.  
   
   
       11 . The thermal type flowmeter as claimed in  claim 10 , wherein the individual calibration curves are created for discrete values of possible values of the voltage to be applied to said heating element, and when the instantaneous flow rate value is achieved, a data interpolation calculation is carried out to achieve the instantaneous flow rate value corresponding to a voltage value applied to said heating element.  
   
   
       12 . The thermal type flowmeter as claimed in  claim 11 , wherein the discrete values are set to the minimum values of values having the same high-order bit values of possible digital values of the voltage to be applied to said heating element, and when the data interpolation calculation is carried out, the instantaneous flow rate values corresponding to first discrete values having the same high-order bit values as the voltage value applied to said heating element and second discrete values whose high-order bit values are larger than that of the voltage value applied to said heating element by 1 are achieved by the individual calibration curves.  
   
   
       13 . The thermal type flowmeter as claimed in  claim 10 , wherein the addition voltage is set to two times to four times of the step value of the base voltage.  
   
   
       14 . The thermal type flowmeter as claimed in  claim 10 , wherein the predetermined range of the count value has a lower limit value which is smaller than a half of the sampling frequency within the predetermined time period and larger than zero, and has an upper limit value which is larger than a half of the sampling frequency within the predetermined time period and smaller than the sampling frequency.  
   
   
       15 . A flowmeter having an indirectly heated type flow rate sensor unit in which a flow rate detector containing a heating element and a temperature sensing element for flow rate detection is joined to a heat transfer member for flow rate detection, a fluid flow rate value being achieved with a calibration curve on the basis of the output of a detection circuit containing the temperature sensing element for flow rate detection, characterized in that the calibration curve comprises three portions corresponding to three areas of the output value of said detection circuit, and the three portions are represented by quaternary functions which use the output of said detection circuit as variables and are different in coefficient, a fluid flow rate value being achieved by using the portion of the calibration curve which corresponds to the area to which the output value of said detection circuit belongs.  
   
   
       16 . The flowmeter as claimed in  claim 15 , wherein the calibration curve is represented as follows:  
         f=a   1   V+b   1   v+c   1   v   2   +d   1   v+e   1 (0 <v<v   1 )    f=a   2   v   4   +b   2   v   3   +c   2   v   2   +d   2   v+e   2  ( v   1   <v<v   2 )    f=a   3   v   4   +b   3   v   3   +c   3   v   2   +d   3   v+e   3  ( v   2   ≦v )  
     wherein f represents the fluid flow rate, v represents the output of the detection circuit, and a 1 , b 1 , c 1 , d 1 , e 1 ; a 2 , b 2 , c 2 , d 2 , e 2 ; a 3 , b 3 , c 3 , d 3 , e 3  represent coefficients.  
   
   
       17 . The flowmeter as claimed in  claim 15 , wherein said detection circuit comprises a bridge circuit.  
   
   
       18 . The flowmeter as claimed in  claim 15 , wherein said flow rate sensor unit has a fluid temperature detector containing a fluid temperature detecting temperature sensing element and a fluid temperature detecting heat transfer member joined to said fluid temperature detector, and said detection circuit contains said fluid temperature detecting temperature sensing element.  
   
   
       19 . A thermal type flowmeter which includes a casing member having a fluid flow passage extending from a fluid flow-in port to a fluid flow-out port, a flow rate detecting unit which is secured to the casing member and varies in electrical characteristic value in accordance with the flow of the fluid in the fluid flow passage through the heat exchange between the flow rate detecting unit and the fluid in the fluid flow passage, and a fluid temperature detecting unit which is secured to the casing member and varies in electrical characteristic value in accordance with the temperature of the fluid through the heat exchange between the fluid temperature detecting unit and the fluid in the fluid flow passage, the flow rate of the fluid being also detected on the basis of the electrical characteristic value of the fluid temperature detecting unit, characterized in that a fluid residence area is formed at the upstream side, with respect to the flow of the fluid, of each of the position at which the heat exchange between the flow rate detecting unit and the fluid is carried out and the position at which the heat exchange between the fluid temperature detecting unit and the fluid is carried out, the fluid residence area having a flow cross section which is five times or more as large as the flow cross section at the position where the heat exchange between said flow rate detecting unit and the fluid is carried out or at the position where the heat exchange between said fluid temperature detecting unit and the fluid is carried out.  
   
   
       20 . The thermal type flowmeter as claimed in  claim 19 , wherein the flow cross section of the fluid residence area is ten times or more as large as the flow cross section at the position where the heat exchange between said flow rate detecting unit and the fluid is carried out or at the position where the heat exchange between said fluid temperature detecting unit and the fluid is carried out.  
   
   
       21 . The thermal type flowmeter as claimed in  claim 19 , wherein the volume of said fluid residence area is 50 times or more as large as the volume per unit length of the fluid flow passage in the fluid flow direction at the position where the heat exchange between said flow rate detecting unit and the fluid is carried out or at the position where said fluid temperature detecting unit and the fluid is carried out.  
   
   
       22 . The thermal type flowmeter as claimed in  claim 19 , wherein said fluid flow passage comprises a first flow passage part intercommunicating with said fluid flow-in port, and a second flow passage part intercommunicating with said fluid flow-out port, at which the heat exchange between said flow rate detecting unit and the fluid is carried out and the heat exchange between said fluid temperature detecting unit and the fluid is carried out, said fluid residence area is located between said first flow passage part and said second flow passage part, and the flow cross section of said first flow passage part is smaller than the flow cross section of said fluid residence area.  
   
   
       23 . The thermal type flowmeter as claimed in  claim 22 , wherein said second flow passage part has a part extending in parallel to said fluid residence area at the position where the heat exchange between said flow rate detecting unit and the fluid is carried out and at the position where the heat exchange between said fluid temperature detecting unit and the fluid is carried out.  
   
   
       24 . The thermal type flowmeter as claimed in  claim 22 , wherein a filter is interposed at the intercommunication portion between said fluid residence area and said second flow passage part.  
   
   
       25 . The thermal type flowmeter as claimed in  claim 19 , wherein said casing member is formed of metal.  
   
   
       26 . The thermal type flowmeter as claimed in  claim 19 , wherein said flow rate detecting unit is designed so that a heating element, a flow rate detecting temperature sensing element and a flow rate detecting heat transfer member extending into said fluid flow passage are arranged so as to perform heat transfer thereamong, and said fluid temperature detecting unit is designed so that a fluid temperature detecting temperature sensing element and a fluid temperature detecting heat transfer member extending into said fluid flow passage are arranged so as to perform heat transfer therebetween.  
   
   
       27 . A thermal type flowmeter which includes a casing member having a fluid flow passage extending from a fluid flow-in port to a fluid flow-out port, a flow rate detecting unit which is secured to the casing member and varies in electrical characteristic value in accordance with the flow of the fluid in the fluid flow passage through the heat exchange between the flow rate detecting unit and the fluid in the fluid flow passage, and a fluid temperature detecting unit which is secured to the casing member and varies in electrical characteristic value in accordance with the temperature of the fluid through the heat exchange between the fluid temperature detecting unit and the fluid in the fluid flow passage, a fluid-temperature-compensated flow rate of the fluid being detected by a detection circuit containing the flow rate detecting unit and the fluid temperature detecting unit, characterized in that a fluid residence area is formed at the upstream side, with respect to the flow of the fluid, of each of the position at which the heat exchange between said flow rate detecting unit and the fluid is carried out and the position at which the heat exchange between said fluid temperature detecting unit and the fluid is carried out, the flow velocity of the fluid at said fluid residence area being equal to ⅕ or less of the flow velocity of the fluid at the position where the heat exchange between the flow rate detecting unit and the fluid is carried out or at the position where the heat exchange between the fluid temperature detecting unit and the fluid is carried out.  
   
   
       28 . The thermal type flowmeter as claimed in  claim 27 , wherein said fluid residence area is formed so that the flow velocity of the fluid is equal to 1/10 or less of the flow velocity of the fluid at the position where the heat exchange between the flow rate detecting unit and the fluid is carried out or at the position where the heat exchange between the fluid temperature detecting unit and the fluid is carried out.  
   
   
       29 . The thermal type flowmeter as claimed in  claim 27 , wherein the volume of said fluid residence area is 50 times or more as large as the volume per unit length of the fluid flow passage in the fluid flow direction at the position where the heat exchange between said flow rate detecting unit and the fluid is carried out or at the position where said fluid temperature detecting unit and the fluid is carried out.  
   
   
       30 . The thermal type flowmeter as claimed in  claim 27 , wherein said fluid flow passage comprises a first flow passage part intercommunicating with said fluid flow-in port, and a second flow passage part intercommunicating with said fluid flow-out port, at which the heat exchange between said flow rate detecting unit and the fluid is carried out and the heat exchange between said fluid temperature detecting unit and the fluid is carried out, said fluid residence area is located between said first flow passage part and said second flow passage part, and the flow cross section of said first flow passage part is smaller than the flow cross section of said fluid residence area.  
   
   
       31 . The thermal type flowmeter as claimed in  claim 30 , wherein said second flow passage part has a part extending in parallel to said fluid residence area at the position where the heat exchange between said flow rate detecting unit and the fluid is carried out and at the position where the heat exchange between said fluid temperature detecting unit and the fluid is carried out.  
   
   
       32 . The thermal type flowmeter as claimed in  claim 30 , wherein a filter is interposed at the intercommunication portion between said fluid residence area and said second flow passage part.  
   
   
       33 . The thermal type flowmeter as claimed in  claim 27 , wherein said casing member is formed of metal.  
   
   
       34 . The thermal type flowmeter as claimed in  claim 27 , wherein said flow rate detecting unit is designed so that a heating element, a flow rate detecting temperature sensing element and a flow rate detecting heat transfer member extending into said fluid flow passage are arranged so as to perform heat transfer thereamong, and said fluid temperature detecting unit is designed so that a fluid temperature detecting temperature sensing element and a fluid temperature detecting heat transfer member extending into said fluid flow passage are arranged so as to perform heat transfer therebetween.