A method for controlled dosing of a gas with supply pressure (P Supply) of said gas, said method being carried out by using an apparatus comprising: - a control volume with a known volume (V CV); - one or more storage units (1; 1a) containing storage material (3; 3a) capable of reversible absorption, or adsorption, and desorption of the gas and said unit being equipped with heating (2;2a) to release the gas by desorption, - a supply valve that controls the fluid communication between a source of the gas and the control volume; - a dosing valve that controls the fluid communication between the control volume and a process whereto the gas is being dosed; - a pressure sensor and a temperature sensor that measure pressure (P CV) and temperature (T CV) of the gas in the control volume; - a controller that is programmed to control the heating of the one or more storage units (1;1a) to obtain a supply pressure (P Supply) within a desired interval between am minimum supply pressure (P MIN) and a maximum supply pressure (P MAX), upstream of the control volume and furthermore to carry out this method for controlled dosing; characterized in that the supply pressure (P Supply) of the gas is a fluctuating supply pressure, the method comprising: - raising the control-volume pressure (P CV) by closing the dosing valve and opening the supply valve; - dosing of the gas through the dosing valve while the supply valve is closed and recording the decrease in control-volume pressure (P CV) until a lower critical pressure level (P CRIT) is reached; - calculating the amount of the dosed gas based on the known volume (V CV) of the control volume and the change in control-volume pressure (P CV) in the period where the supply valve is closed - comparing the amount of dosed gas with a target or set-point to adjust or regulate the subsequent dosing period or dosing event. - a control volume with a known volume (V CV); - one or more storage units (1; 1a) containing storage material (3; 3a) capable of reversible absorption, or adsorption, and desorption of the gas and said unit being equipped with heating (2;2a) to release the gas by desorption, - a supply valve that controls the fluid communication between a source of the gas and the control volume; - a dosing valve that controls the fluid communication between the control volume and a process whereto the gas is being dosed; - a pressure sensor and a temperature sensor that measure pressure (P CV) and temperature (T CV) of the gas in the control volume; - a controller that is programmed to control the heating of the one or more storage units (1;1a) to obtain a supply pressure (P Supply) within a desired interval between am minimum supply pressure (P MIN) and a maximum supply pressure (P MAX), upstream of the control volume and furthermore to carry out this method for controlled dosing; characterized in that the supply pressure (P Supply) of the gas is a fluctuating supply pressure, the method comprising: - raising the control-volume pressure (P CV) by closing the dosing valve and opening the supply valve; - dosing of the gas through the dosing valve while the supply valve is closed and recording the decrease in control-volume pressure (P CV) until a lower critical pressure level (P CRIT) is reached; - calculating the amount of the dosed gas based on the known volume (V CV) of the control volume and the change in control-volume pressure (P CV) in the period where the supply valve is closed - comparing the amount of dosed gas with a target or set-point to adjust or regulate the subsequent dosing period or dosing event. - one or more storage units (1; 1a) containing storage material (3; 3a) capable of reversible absorption, or adsorption, and desorption of the gas and said unit being equipped with heating (2;2a) to release the gas by desorption, - a supply valve that controls the fluid communication between a source of the gas and the control volume; - a dosing valve that controls the fluid communication between the control volume and a process whereto the gas is being dosed; - a pressure sensor and a temperature sensor that measure pressure (P CV) and temperature (T CV) of the gas in the control volume; - a controller that is programmed to control the heating of the one or more storage units (1;1a) to obtain a supply pressure (P Supply) within a desired interval between am minimum supply pressure (P MIN) and a maximum supply pressure (P MAX), upstream of the control volume and furthermore to carry out this method for controlled dosing; - raising the control-volume pressure (P CV) by closing the dosing valve and opening the supply valve; - dosing of the gas through the dosing valve while the supply valve is closed and recording the decrease in control-volume pressure (P CV) until a lower critical pressure level (P CRIT) is reached; - calculating the amount of the dosed gas based on the known volume (V CV) of the control volume and the change in control-volume pressure (P CV) in the period where the supply valve is closed - comparing the amount of dosed gas with a target or set-point to adjust or regulate the subsequent dosing period or dosing event.