Fluid flow measuring system using improved temperature compensation apparatus and method

Claim 1

A method for processing the electrical signal generated by a self-heated flow responsive thermal element in a fluid flow measuring apparatus to obtain therefrom a flow indicative output signal that is substantially invarient with respect to fluid temperature, said method comprising the steps of: exciting a fluid immersed thermal element with a signal generating constant current that is independent of fluid temperature and flow rate and selected to operate the element at a temperature above the fluid temperature; subtracting from said thermal element signal a constant offset signal that is greater than zero and smaller than the smallest magnitude of said thermal element signal during any condition to be measured and wherein said offset signal has the same value for all flow rates and temperaturEs of the fluid being measured and selected in accordance with the properties of the measuring apparatus and the flowing fluid, thereby forming a corrected signal; selecting a predetermined different fractional part of said corrected signal for use as said flow indicative output signal at each fluid temperature; whereby removing said constant offset signal prior to selecting a fractional part of the corrected thermal element signal for use as an output signal allows said output signal to be selected from said corrected signal independently of flow rate.

Claim 2

A method for processing the electrical signal generated by a self-heated flow responsive thermal element in a fluid flow measuring apparatus to obtain therefrom a flow indicative output signal that is substantially invarient with respect to fluid temperature, said method comprising the steps of: exciting a fluid immersed thermal element with a signal generating constant current that is independent of fluid temperature and flow rate and selected to operate the element at a temperature above the fluid temperature; subtracting from said thermal element signal a constant offset signal that is greater than zero and smaller than the smallest magnitude of said thermal element signal during any condition to be measured and wherein said offset signal has the same value for all flow rates and temperaturEs of the fluid being measured and selected in accordance with the properties of the measuring apparatus and the flowing fluid, thereby forming a corrected signal; selecting a predetermined different fractional part of said corrected signal for use as said flow indicative output signal at each fluid temperature; whereby removing said constant offset signal prior to selecting a fractional part of the corrected thermal element signal for use as an output signal allows said output signal to be selected from said corrected signal independently of flow rate.

Claim 8

A method for fitting the gain of a utilizing electrical apparatus to the characteristics of a thermal flow responsive element to achieve flow measurements which are independent of fluid temperature, said method comprising the steps of: measuring the voltage drop across a thermal flow responsive element at periodic fluid flow rate intervals and periodic fluid temperature intervals in the range to be measured by said flow responsive element while said element is mounted in a fluid stream and excited into the constant current self-heated operating mode, whereby a first periodic table of voltages corresponding to different fluid temperatures and different fluid flow rates is obtained; forming a first component by substracting from each of said voltages in said first periodic table a second component smaller than said first component at each fluid flow rate and each fluid temperature and constant for all values in said first periodic table, forming from said first component of voltage a second periodic table of voltages; normalizing the voltages for each flow rate in said second periodic table to the voltage at one selected temperature for that same flow rate, said normalizing including, for each flow rate, dividing the voltage corresponding to said selected temperature by the voltage corresponding to each other temperature, the results of said normalizing step forming a third periodic table of normalized voltage ratios, selecting from the voltage ratios corresponding to each temperature in said third periodic table a voltage ratio mathematically representative of all the ratios at that temperature, preforming said selecting step for each periodic temperature in said third periodic table whereby a family of voltage ratios is formed with each member of said family represEnting the gain needed at one fluid temperature in a fluid mass flow apparatus having uniform sensitivity over a large range of fluid temperatures; adjusting the gain vs. temperature response of a fluid temperature responsive variable gain electrical apparatus that is connected with said thermal flow responsive element to said family of voltage ratios; whereby removing said second component of voltage from the voltages in said first periodic table decreases the variation in said third periodic table voltages at each temperature.

Claim 9

A method for fitting the gain of a utilizing electrical apparatus to the characteristics of a thermal flow responsive element to achieve flow measurements which are independent of fluid temperature, said method comprising the steps of: measuring the voltage drop across a thermal flow responsive element at periodic fluid flow rate intervals and periodic fluid temperature intervals in the range to be measured by said flow responsive element while said element is mounted in a fluid stream and excited into the constant current self-heated operating mode, whereby a first periodic table of voltages corresponding to different fluid temperatures and different fluid flow rates is obtained; forming a first component by substracting from each of said voltages in said first periodic table a second component smaller than said first component at each fluid flow rate and each fluid temperature and constant for all values in said first periodic table, forming from said first component of voltage a second periodic table of voltages; normalizing the voltages for each flow rate in said second periodic table to the voltage at one selected temperature for that same flow rate, said normalizing including, for each flow rate, dividing the voltage corresponding to said selected temperature by the voltage corresponding to each other temperature, the results of said normalizing step forming a third periodic table of normalized voltage ratios, selecting from the voltage ratios corresponding to each temperature in said third periodic table a voltage ratio mathematically representative of all the ratios at that temperature, preforming said selecting step for each periodic temperature in said third periodic table whereby a family of voltage ratios is formed with each member of said family represEnting the gain needed at one fluid temperature in a fluid mass flow apparatus having uniform sensitivity over a large range of fluid temperatures; adjusting the gain vs. temperature response of a fluid temperature responsive variable gain electrical apparatus that is connected with said thermal flow responsive element to said family of voltage ratios; whereby removing said second component of voltage from the voltages in said first periodic table decreases the variation in said third periodic table voltages at each temperature.

Claim 21

A method for computing the magnitude of an offset signal to be subtracted from the output signal of a self-heated, constant current excited, thermal element immersed in a flowing fluid in processing said output signal to represent fluid flow rate independently of fluid temperature, said method comprising the steps of: measuring a first voltage across said self-heated fluid immersed thermal element while said element is subjected to a first fluid temperature and a first fluid flow rate; measuring a second voltage across said self-heated fluid immersed thermal element while said element is subjected to a second higher fluid temperature and said same first fluid flow rate; measuring a third voltage across said self-heated fluid immersed thermal element while said element is subjected to said first fluid temperature and a second higher fluid flow rate; measuring a fourth voltage across said self-heated fluid immersed thermal element while said element is subjected to said second fluid temperature and said second fluid flow rate; subtracting from said first, second, third and fourth voltages a single offset signal voltage having such magnitude that upon subtraction of said offset signal voltage the ratio of the remaining first voltage to the remaining second voltage equals the ratio of the remaining third voltage to the remaining fourth voltage.

Claim 22

A method for computing the magnitude of an offset signal to be subtracted from the output signal of a self-heated, constant current excited, thermal element immersed in a flowing fluid in processing said output signal to represent fluid flow rate independently of fluid temperature, said method comprising the steps of: measuring a first voltage across said self-heated fluid immersed thermal element while said element is subjected to a first fluid temperature and a first fluid flow rate; measuring a second voltage across said self-heated fluid immersed thermal element while said element is subjected to a second higher fluid temperature and said same first fluid flow rate; measuring a third voltage across said self-heated fluid immersed thermal element while said element is subjected to said first fluid temperature and a second higher fluid flow rate; measuring a fourth voltage across said self-heated fluid immersed thermal element while said element is subjected to said second fluid temperature and said second fluid flow rate; subtracting from said first, second, third and fourth voltages a single offset signal voltage having such magnitude that upon subtraction of said offset signal voltage the ratio of the remaining first voltage to the remaining second voltage equals the ratio of the remaining third voltage to the remaining fourth voltage.

Claim 23

A method for computing the magnitude of an offset signal to be subtracted from the output signal of a thermal element which is surrounded by a flowing fluid, during the processing of said output signal to represent fluid flow rate independently of fluid temperature, said method comprising the steps of: measuring the output signal voltage across said thermal element at periodic temperature intervals and periodic flow rate intervals in the range to be measured by said thermal element and while said thermal element is mounted in a fluid stream and electrically excited by a constant current; organizing said periodic values of output signal voltage into a form characteristic of a family of curves each member of which represents variation in said output signal voltage with respect to fluid temperature and fluid velocity; equating the ratio fraction of a first and second curve slopes taken at a single first temperature and at first and second flow rates in said family of curves to the ratio fraction of a third and fourth curve slopes taken at a second temperature and at said same first and second flow rates in said family of curves, said step of equating including the act of diminishing the voltage factor in each of said first, second, third and fourth curve slopes by the value of said offset signal, with said offset signal being determined as a value which enables the equating of said ratio fractions.

Claim 24

A method for computing the magnitude of an offset signal to be subtracted from the output signal of a thermal element which is surrounded by a flowing fluid, during the processing of said output signal to represent fluid flow rate independently of fluid temperature, said method comprising the steps of: measuring the output signal voltage across said thermal element at periodic temperature intervals and periodic flow rate intervals in the range to be measured by said thermal element and while said thermal element is mounted in a fluid stream and electrically excited by a constant current; organizing said periodic values of output signal voltage into a form characteristic of a family of curves each member of which represents variation in said output signal voltage with respect to fluid temperature and fluid velocity; equating the ratio fraction of a first and second curve slopes taken at a single first temperature and at first and second flow rates in said family of curves to the ratio fraction of a third and fourth curve slopes taken at a second temperature and at said same first and second flow rates in said family of curves, said step of equating including the act of diminishing the voltage factor in each of said first, second, third and fourth curve slopes by the value of said offset signal, with said offset signal being determined as a value which enables the equating of said ratio fractions.

Claim 25

A method for fitting the gain of a flow indicating apparatus to the characteristics of a thermal flow responsive element to achieve flow measurements which are independent of fluid temperature, saiD method comprising the steps of: measuring the voltage drop across said thermal flow responsive element at periodic fluid flow rate increments and periodic fluid temperature increments in the range to be measured by said flow responsive element while said element is mounted in a fluid stream and excited in the constant current self-heated mode, whereby a first periodic table of voltages corresponding to different fluid temperatures and different fluid flow rates is obtained; dividing the difference between said thermal flow responsive element voltage at a first high flow rate and a second lesser flow rate both taken at a first low reference temperature by the difference between said thermal flow-responsive element voltage at said same high flow rate and said same second lesser flow rate both taken at the next temperature adjacent said reference temperature and at successive temperatures in said periodic table, said dividing steps providing a gain value relating flow responsive element voltages at each temperature in said periodic table to flow responsive element voltage at said reference temperature; multiplying the thermal flow responsive element voltage obtained during measurement of an unknown fluid flow rate by a value taken from said gain values and values interpolated from said gain values in accordance with the temperature of said unknown fluid.

Claim 26

A method for fitting the gain of a flow indicating apparatus to the characteristics of a thermal flow responsive element to achieve flow measurements which are independent of fluid temperature, saiD method comprising the steps of: measuring the voltage drop across said thermal flow responsive element at periodic fluid flow rate increments and periodic fluid temperature increments in the range to be measured by said flow responsive element while said element is mounted in a fluid stream and excited in the constant current self-heated mode, whereby a first periodic table of voltages corresponding to different fluid temperatures and different fluid flow rates is obtained; dividing the difference between said thermal flow responsive element voltage at a first high flow rate and a second lesser flow rate both taken at a first low reference temperature by the difference between said thermal flow-responsive element voltage at said same high flow rate and said same second lesser flow rate both taken at the next temperature adjacent said reference temperature and at successive temperatures in said periodic table, said dividing steps providing a gain value relating flow responsive element voltages at each temperature in said periodic table to flow responsive element voltage at said reference temperature; multiplying the thermal flow responsive element voltage obtained during measurement of an unknown fluid flow rate by a value taken from said gain values and values interpolated from said gain values in accordance with the temperature of said unknown fluid.