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 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 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 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.