A method for determining a characteristic parameter of a fluid ( g), the fluid ( g) preferably being a burnable gas, said fluid ( g) flowing in a fluid flow through a sensor device, the sensor device comprising: a thermal flow sensor device with a heater element and first and second temperature sensors (2;4), the heater element being arranged between the first and second temperature sensors (2,4), a flow restrictor arranged between first and second positions (18,19) in said fluid flow, and a pressure sensor device (15;16,17) for determining a pressure difference (Δ p) in the fluid (g) between said first and second positions (18,19), the method comprising: (i) establishing the fluid flow through said sensor device with at least part of the fluid ( g) overflowing said thermal flow sensor device, the fluid flow having an unknown value; (ii) activating the heater element of said thermal flow sensor device and measuring at least one first temperature ( T 1) with said first temperature sensor and at least one second temperature ( T 2) with said second temperature sensor; (iii) measuring first and second absolute pressures ( p 1, p 2) in the fluid ( g) at the first and second positions (18,19), respectively, or measuring a differential pressure (Δ p) in the fluid (g) between the first and second positions (18,19) with said pressure sensor device (15;16,17); characterised in that the method further comprises: (iv) determining from the measured first and second temperatures ( T 1, T 2) a heat conductivity ( λ) of the fluid (g); (v) determining from the measured first and second temperatures ( T 1, T 2) and either the measured first and second absolute pressures (p1, p2) or the measured pressure differential (Δp) a microscopic force (ξ) of the fluid ( g), the microscopic force (ξ) being defined by the equation ξ = η 2 / ( ρ Pr 2 α), wherein η denotes dynamic viscosity, ρ denotes density, Pr denotes the Prandtl number Pr = c P η / λ, c P denotes specific heat, λ denotes heat conductivity, and α is a real number in the range 0 ≤ α ≤ 1; (vi) determining from said heat conductivity ( λ) and said microscopic force ( ξ) the characteristic parameter of the fluid ( g) with correlation functions, the characteristic parameter being a heat capacity c p ρ, a calorific value Hρ, a Wobbe index and/or a methane number. a thermal flow sensor device with a heater element and first and second temperature sensors (2;4), the heater element being arranged between the first and second temperature sensors (2,4), a flow restrictor arranged between first and second positions (18,19) in said fluid flow, and a pressure sensor device (15;16,17) for determining a pressure difference (Δ p) in the fluid (g) between said first and second positions (18,19), (i) establishing the fluid flow through said sensor device with at least part of the fluid ( g) overflowing said thermal flow sensor device, the fluid flow having an unknown value; (ii) activating the heater element of said thermal flow sensor device and measuring at least one first temperature ( T 1) with said first temperature sensor and at least one second temperature ( T 2) with said second temperature sensor; (iii) measuring first and second absolute pressures ( p 1, p 2) in the fluid ( g) at the first and second positions (18,19), respectively, or measuring a differential pressure (Δ p) in the fluid (g) between the first and second positions (18,19) with said pressure sensor device (15;16,17); characterised in that the method further comprises: (iv) determining from the measured first and second temperatures ( T 1, T 2) a heat conductivity ( λ) of the fluid (g); (v) determining from the measured first and second temperatures ( T 1, T 2) and either the measured first and second absolute pressures (p1, p2) or the measured pressure differential (Δp) a microscopic force (ξ) of the fluid ( g), the microscopic force (ξ) being defined by the equation ξ = η 2 / ( ρ Pr 2 α), wherein η denotes dynamic viscosity, ρ denotes density, Pr denotes the Prandtl number Pr = c P η / λ, c P denotes specific heat, λ denotes heat conductivity, and α is a real number in the range 0 ≤ α ≤ 1; (vi) determining from said heat conductivity ( λ) and said microscopic force ( ξ) the characteristic parameter of the fluid ( g) with correlation functions, the characteristic parameter being a heat capacity c p ρ, a calorific value Hρ, a Wobbe index and/or a methane number.