a system that controls and manages administration of a therapeutic gas , such as no , o 2 , or the like , to a patient is purged using a method that conserves purge gas . the system was fully disclosed in u . s . patent applications ser . no . 09 / 688 , 229 , filed on oct . 16 , 2000 and ser . no . 10 / 351 , 755 filed on jan . 27 , 2003 and includes an equalizing valve and a reservoir .