A method for determining physical properties of combustion including: flowing a gas 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 a reservoir of gas flowing to the 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 first and/or second gas property factors and the thermal conductivity.