A mass flow controller having a feedback control gain, comprising: a sensor configured so as to sense the flow of fluid through the controller; a valve arranged so as to adjust the flow of fluid through the controller; and a processor (60, 104) configured so as to control the valve as a function of the flow of fluid sensed by the sensor; wherein the sensor and valve are arranged within a feedback system, and the processor is configured to update the feedback controller gain in real time based on the ratio of at least one calibration gas parameter to at least one operating gas parameter such that a closed loop transfer function of the feedback system remains substantially constant regardless of operating conditions so as to have a consistent control performance at different operating conditions from a calibration condition; wherein the at least one calibration gas parameter and the at least one operating gas parameter comprise (i) the molecular weight of the inlet gas M, (ii) the ratio of heat capacities of the inlet gas γ, (iii) the temperature of the inlet gas T, and (iv) the pressure of the inlet gas P, and excluding (v) valve displacement; a temperature sensor for sensing the inlet gas temperature T and a pressure sensor for sensing the inlet gas pressure P, wherein the processor updates the controller gain k in real time as a function of the following: k = k c a l ⋅ k v M c a l, γ c a l, P c a l, T c a l k v M, γ, T, P where k cal is the controller gain determined in the calibration condition, M cal the molecular weight of the calibration gas, γ cal the ratio of heat capacities of the calibration gas, T cal the temperature of the calibration gas, P cal the pressure of the calibration gas, and k v () the valve gain function which is dependent on the operation conditions including the molecular weight of the inlet gas M, the ratio of heat capacities of the inlet gas γ, the temperature of the inlet gas T, and the pressure of the inlet gas P. a sensor configured so as to sense the flow of fluid through the controller; a valve arranged so as to adjust the flow of fluid through the controller; and a processor (60, 104) configured so as to control the valve as a function of the flow of fluid sensed by the sensor; wherein the sensor and valve are arranged within a feedback system, and the processor is configured to update the feedback controller gain in real time based on the ratio of at least one calibration gas parameter to at least one operating gas parameter such that a closed loop transfer function of the feedback system remains substantially constant regardless of operating conditions so as to have a consistent control performance at different operating conditions from a calibration condition; wherein the at least one calibration gas parameter and the at least one operating gas parameter comprise (i) the molecular weight of the inlet gas M, (ii) the ratio of heat capacities of the inlet gas γ, (iii) the temperature of the inlet gas T, and (iv) the pressure of the inlet gas P, and excluding (v) valve displacement; a temperature sensor for sensing the inlet gas temperature T and a pressure sensor for sensing the inlet gas pressure P, wherein the processor updates the controller gain k in real time as a function of the following: k = k c a l ⋅ k v M c a l, γ c a l, P c a l, T c a l k v M, γ, T, P