Abstract: A vaporizer comprises gas inlet and outlet ports, gas inlet and outlet control means, one or more liquid reservoir chambers, at least one gas passage, at least one gas vaporization zone, a liquid filler body including filler control means wherein there is communication between the gas inlet control means, gas outlet control means and filler control means such thati) when one or both of the gas inlet control means and gas outlet control means allows gas to flow into and/or out of the liquid reservoir chamber the filler control means will not allow liquid to pass either to or from the liquid reservoir chamber through the filler body, andii) when one or both of the gas inlet control means and gas outlet control means allows gas to flow into and/or out of the liquid reservoir chamber the filler means cannot be moved to a position such that liquid could pass either to or from the liquid reservoir chamber through the filler body.