A back pressure valve is a kind of relief valve to obtain a constant level of pressure in the first stage of valve operation. This type of valve makes it easier and speedier for the user to control pressure in the tank with a large buffer,purge a given quantity of mixed gas or to carry out such other operations which are rather hard to do with a second-stage control valve commonly in use. Much interest is being focused on this type of valve as a new concept of pressure control.