Method and measuring apparatus for determining physical properties of gas

Abstract

A method to determine a physical property or a quantity of gas related to combustion including: flowing a gas from a reservoir through a critical nozzle and past a microthermal sensor wherein the mass flow of the gas through the critical nozzle is the same as the mass flow through the microthermal sensor; measuring the pressure drop in the reservoir as a function of time; deriving a first gas property factor based on a time constant of the pressure drop; determining a second gas property factor which depends from a flow signal generated by the microthermal sensor; determining a thermal conductivity of the gas; and determining the physical property or quantity based on a correlation between the physical property or quantity, and the first and/or second gas property factors and the thermal conductivity.

1 . A method for determining physical properties and/or quantities relevant to combustion of a gas and/or a gas mixture, the method comprising:
 flowing the gas and/or gas mixture from a gas reservoir through a critical nozzle and past a microthermal sensor, with the same mass flow is applied to the critical nozzle and the microthermal sensor;   measuring a pressure drop in the gas reservoir as a function of time;   determining a first gas property factor (Γ*), which is dependent on a first group of physical properties of the gas and/or gas mixture, on the basis of the measured pressure drop;   determining a second gas property factor (Γ), which is dependent on a second group of physical properties of the gas or gas mixture, from a flow signal generated by the microthermal sensor;   determining the thermal conductivity (λ) of the gas and/or gas mixture using the microthermal sensor; and   determining a physical property and/or quantity relevant to combustion from the first and/or second gas property factor (Γ*, Γ) and the thermal conductivity (λ) through correlation.   
     
     
         2 . The method according to  claim 1 , in which the starting point is an exponential decline of the measured pressure and the first gas property factor (Γ*) is derived from the time constant of the pressure drop, in which case the first gas property factor is formed in particular by measuring the temperature (T) and by omitting all gas-unrelated variables. 
     
     
         3 . The method according to  claim 1 , in which the second gas property factor (Γ) contains the quotient of heat capacity (c p ) divided by thermal conductivity (λ) of the gas or gas mixture, or is dependent on the same, and in which the second gas property factor is formed by measuring the temperature (T) additionally and by omitting all gas-unrelated variables. 
     
     
         4 . The method according to  claim 1 , wherein the gas property factors (Γ*, Γ) are validated by comparing the values for the total volume of released gas or gas mixture by:
 measuring the pressure and temperature in the gas reservoir at the start and end of the pressure drop reading and by determining the released standard volume by reference to the known volume of the gas reservoir; 
 summing the standard flow measured with the microthermal sensor during a time interval between the start and end of the pressure drop reading; 
 comparing the released standard volume to the summed up standard volume; and 
 in case of a discrepancy, by adjusting the first and/or the second gas property factor (Γ*, Γ) by adjusting the pressure signal or the standard flow variable of the microthermal sensor. 
 
     
     
         5 . A method for determining physical properties and/or quantities relevant to combustion of gas or gas mixtures, the method comprising:
 the gas or gas mixture flows under pressure through a critical nozzle and past a microthermal sensor into a gas reservoir, with the same mass flow being applied to the critical nozzle and the microthermal sensor;   measuring a pressure increase in the gas reservoir as a function of time;   determining a first gas property factor (Γ*), dependent on a first group of physical properties of the gas or gas mixture, by reference to the measured values of the pressure increase;   determining a second gas property factor (Γ), dependent on a second group of physical properties of the gas or gas mixture, from the flow signal of the microthermal sensor, with the second gas property factor containing, or depends on the heat capacity (c p ) of the gas or gas mixture;   determining a thermal conductivity (λ) of the gas or gas mixture using the microthermal sensor; and   determining a physical property or quantity relevant to combustion from the first and/or second gas property factor (Γ*,Γ) and the thermal conductivity (λ) through correlation.   
     
     
         6 . The method according to  claim 5 , where the starting point is a linear increase of the measured pressure and the first gas property factor (Γ*) is derived from a proportionality constant of the pressure increase, and in which case the first gas property factor is formed by measuring, for example, in addition the temperature (T) and the nozzle inlet pressure (pNozzle), and by omitting all gas-unrelated variables. 
     
     
         7 . The method according to  claim 5 , in which the second gas property factor (Γ) contains the quotient of heat capacity (c p ) divided by thermal conductivity (λ) of the gas or gas mixture, or is dependent on the same, and in which the second gas property factor is formed in particular by additionally measuring the temperature (T) and by omitting all gas-unrelated variables. 
     
     
         8 . The method according to  claim 5 , where the gas property factors (Γ*,Γ) are validated by comparing the values for the total volume of the gas or gas mixture fed into the gas reservoir by:
 measuring the pressure and temperature in the gas reservoir at the start and end of the pressure increase reading and by determining the released standard volume fed into the gas reservoir by reference to the known volume of the gas reservoir; 
 summing up the standard flow measured with the microthermal sensor during the time interval between the start and end of the pressure increase reading; 
 comparing the standard volume fed into the gas reservoir to the summed up standard volume and based on the comparison determining if a discrepancy exists; and in response to the determination of a discrepancy, adjusting the first and/or the second gas property factor by adjusting the pressure signal or a flow variable of the microthermal sensor. 
 
     
     
         9 . The method according to  claim 5 , wherein the method includes calibrating the flow signal of the microthermal sensor, by:
 calibrating the flow signal of the microthermal sensor for a specific calibration gas or gas mixture;   determining a ratio (Γ/Γ*) of the second gas property factor, to the first gas property factor for an unknown gas or gas mixture on the basis of the flow signal of the microthermal sensor; and   comparing the standard volume values from the reading of the pressure drop or pressure increase and the reading of the summed up standard flow of the microthermal sensor, which are then used to adjust a ratio of the second gas property factor to the first, and to adapt the value for the second gas property factor (Γ).   
     
     
         10 . The method according to  claim 1  where the physical property is a density or the thermal conductivity or the heat capacity or the viscosity of the gas or gas mixture, and/or where the quantity relevant to combustion is the energy content or the calorific value or the Wobbe index or the methane number or the air requirement of the gas or gas mixture. 
     
     
         11 . The method according to  claim 1 , where the certain physical property or the quantity relevant to combustion (Q) is determined by aid of a correlation function which is:
     f   corr (Γ, Γ*, λ)=const·Γ r ·Γ* s ·λ t ,
   wherein r, s and t are exponents, and const is a constant.   
     
     
         12 . The method according to  claim 1 , where the pressure in the gas reservoir is higher at the start of the pressure drop reading than the critical pressure (perit) of the critical nozzle and the external pressure downstream of the critical nozzle is less than half of the critical pressure, or where the pressure in the gas reservoir at the start of the pressure increase reading is lower than half of the critical pressure (perit) of the critical nozzle and the pressure upstream of the critical nozzle is higher than the critical pressure. 
     
     
         13 . A measuring apparatus for determining a physical property and/or quantity relevant to combustion of gas and/or gas mixtures comprising:
 an analyzer unit configured to carry out a procedure in accordance with  claim 1  and   a gas reservoir equipped with a pressure sensor, a critical nozzle and a microthermal sensor to measure the flow and thermal conductivity, in which case the gas reservoir is connected to the critical nozzle and the microthermal sensor.   
     
     
         14 . A method to use a gas reservoir and a critical nozzle for determining physical properties and/or quantities relevant to combustion of gas or gas mixtures, the method comprises:
 flowing the gas or gas mixture under pressure from the gas reservoir through the critical nozzle;   measuring a pressure drop in the gas reservoir as a function of time;   determining a gas property factor (Γ*), dependent on a physical property of the gas or gas mixture, based on the measured values of the pressure drop; and   determining a desired physical property or quantity relevant to combustion based on the gas property factor (Γ*) through correlation.   
     
     
         15 . A method to determine a physical property and/or a quantity relevant to a combustion gas or gas mixture using a gas reservoir and a microthermal sensor, calibrated for a specific calibration gas or gas mixture, the method comprises:
 flowing the gas or gas mixture under pressure from the gas reservoir past the microthermal sensor;   determining a flow rate (vx) of the gas or gas mixture using the microthermal sensor;   determining a summed-up volume flow (vx·A) based on the flow rate (vx);   comparing the summed up volume flow to a gas volume released from the gas reservoir;   determining a gas property factor (S/v′x), dependent on the physical properties of the gas or gas mixture, based on the comparison, wherein the quantity (v′x) represents a flow rate based on the released gas volume, and wherein the gas property factor comprises, for example, the heat capacity (c p ) of the gas or gas mixture or is dependent on the same; and   a desired physical property or quantity relevant to combustion is determined on the basis of the gas property factor through correlation.   
     
     
         16 . A method to determine a physical property of combustion comprising:
 flowing a gas from a reservoir through a critical nozzle and a microthermal sensor wherein the mass flow of the gas through the critical nozzle is the same mass flow through the microthermal sensor;   measuring a pressure drop in the reservoir as the gas flows through the critical nozzle;   determining a first gas property factor based on the measured pressure drop;   determining a second gas property factor based on a flow signal generated by the microthermal sensor;   determining a thermal conductivity of the gas using the microthermal sensor, and   determining a physical property of the combustion based on a correlation of the thermal conductivity and at least one of the first and second gas property factors.