A vaporizer comprising: a gas delivery unit which is configured to receive a first flow of gas and provide a second flow of gas containing a metered amount of a vaporized medium; a reservoir unit which contains a volatile medium and is configured to maintain a supply of the vaporized medium, wherein the reservoir unit is selectively fluidly connected to the gas delivery unit, wherein the reservoir unit comprises: a tank which contains the volatile medium in liquid form; a heater which is operative to heat the volatile medium to a vaporization temperature in order to vaporize the volatile medium and maintain an operating pressure in a headspace of the tank; and a valve assembly configured to meter the vaporized medium from the tank into the first gas flow, thereby generating the second gas flow; a first gas sensing unit configured to sense a flow rate and/or composition of the first gas flow; a vaporized medium sensing unit configured to sense a flow rate of the vaporized medium metered by the valve assembly; a manifold which includes flow paths for the vaporized medium and fluidly connects the reservoir unit and the vaporized medium sensing unit; and a control unit configured to control a flow rate of the first gas flow and an amount of the vaporized medium which is metered by the valve assembly from the tank into the first gas flow to generate the second gas flow; wherein the tank is formed of a thermally-conductive material; characterized in that: the tank provides a thermal mass which is sufficient to maintain the contained volatile medium at the vaporization temperature for a predetermined period of time of at least 3 minutes following de-actuation of the heater. a gas delivery unit which is configured to receive a first flow of gas and provide a second flow of gas containing a metered amount of a vaporized medium; a reservoir unit which contains a volatile medium and is configured to maintain a supply of the vaporized medium, wherein the reservoir unit is selectively fluidly connected to the gas delivery unit, wherein the reservoir unit comprises: a tank which contains the volatile medium in liquid form; a heater which is operative to heat the volatile medium to a vaporization temperature in order to vaporize the volatile medium and maintain an operating pressure in a headspace of the tank; and a valve assembly configured to meter the vaporized medium from the tank into the first gas flow, thereby generating the second gas flow; a first gas sensing unit configured to sense a flow rate and/or composition of the first gas flow; a vaporized medium sensing unit configured to sense a flow rate of the vaporized medium metered by the valve assembly; a manifold which includes flow paths for the vaporized medium and fluidly connects the reservoir unit and the vaporized medium sensing unit; and a control unit configured to control a flow rate of the first gas flow and an amount of the vaporized medium which is metered by the valve assembly from the tank into the first gas flow to generate the second gas flow; wherein the tank is formed of a thermally-conductive material; a tank which contains the volatile medium in liquid form; a heater which is operative to heat the volatile medium to a vaporization temperature in order to vaporize the volatile medium and maintain an operating pressure in a headspace of the tank; and a valve assembly configured to meter the vaporized medium from the tank into the first gas flow, thereby generating the second gas flow; a first gas sensing unit configured to sense a flow rate and/or composition of the first gas flow; a vaporized medium sensing unit configured to sense a flow rate of the vaporized medium metered by the valve assembly; a manifold which includes flow paths for the vaporized medium and fluidly connects the reservoir unit and the vaporized medium sensing unit; and a control unit configured to control a flow rate of the first gas flow and an amount of the vaporized medium which is metered by the valve assembly from the tank into the first gas flow to generate the second gas flow; wherein the tank is formed of a thermally-conductive material; characterized in that: the tank provides a thermal mass which is sufficient to maintain the contained volatile medium at the vaporization temperature for a predetermined period of time of at least 3 minutes following de-actuation of the heater.