A method for determining a gas parameter (Pr;C H ;Hρ; I W ; N M ) of a gas (g), wherein said gas parameter ( Pr;C H ;Hρ; I W ; N M ) is a Prandtl number, a hydrogen concentration, or a combustion parameter of the gas (g), said method comprising: - operating a thermal sensor device being exposed to said gas (g) and comprising at least one temperature sensor (2;21,22) and at least one heater element ; - calculating from an output of said thermal sensor device : (i) a temperature (T) of said gas (g); (ii) a heat-transfer parameter (λ) of said gas (g), wherein said heat-transfer parameter (λ) of said gas (g) is determined from a stationary temperature distribution in said gas (g) caused by said at least one heater element ; (iii) a heat-capacity related parameter (D; c p ρ) of said gas (g), wherein said heat-capacity related parameter (D; c p ρ) of said gas (g) is determined from a time-varying heating scheme applied to said heater element ; characterized in that the method further comprises calculating from the output of said thermal sensor device : (iv) a natural-convection parameter 1 νD of said gas (g) related to a natural convection caused in said gas (g) by action of said at least one heater element ; and - calculating said gas parameter ( Pr;C H ;Hρ; I W ; N M ) of said gas (g) from said heat-transfer parameter (λ), said heat-capacity related parameter ( D; c p ρ ) , and said natural-convection parameter 1 νD . - operating a thermal sensor device being exposed to said gas (g) and comprising at least one temperature sensor (2;21,22) and at least one heater element ; - calculating from an output of said thermal sensor device : (i) a temperature (T) of said gas (g); (ii) a heat-transfer parameter (λ) of said gas (g), wherein said heat-transfer parameter (λ) of said gas (g) is determined from a stationary temperature distribution in said gas (g) caused by said at least one heater element ; (iii) a heat-capacity related parameter (D; c p ρ) of said gas (g), wherein said heat-capacity related parameter (D; c p ρ) of said gas (g) is determined from a time-varying heating scheme applied to said heater element ; characterized in that the method further comprises calculating from the output of said thermal sensor device : (iv) a natural-convection parameter 1 νD of said gas (g) related to a natural convection caused in said gas (g) by action of said at least one heater element ; and (i) a temperature (T) of said gas (g); (ii) a heat-transfer parameter (λ) of said gas (g), wherein said heat-transfer parameter (λ) of said gas (g) is determined from a stationary temperature distribution in said gas (g) caused by said at least one heater element ; (iii) a heat-capacity related parameter (D; c p ρ) of said gas (g), wherein said heat-capacity related parameter (D; c p ρ) of said gas (g) is determined from a time-varying heating scheme applied to said heater element ; characterized in that the method further comprises calculating from the output of said thermal sensor device : (iv) a natural-convection parameter 1 νD of said gas (g) related to a natural convection caused in said gas (g) by action of said at least one heater element ; and - calculating said gas parameter ( Pr;C H ;Hρ; I W ; N M ) of said gas (g) from said heat-transfer parameter (λ), said heat-capacity related parameter ( D; c p ρ ) , and said natural-convection parameter 1 νD . <EOS>