A method of forming servo data, and a self-servo write method using the same, the method including partial response maximum likelihood -coding servo data into m bits in n-bit units; multiplying each bit of the PRML-coded result by an o multiple; and converting a bitsream obtained in the multiplying to \u2212p) bits in mn-bit units. Catalyst material is selectively placed onto a plurality of prefabricated elevated structures on a wafer. The tubular body includes integrally formed threads for receiving a gas cap. As a result, the vapor purifier of the present invention can maintain the air to fuel stoichiometric ratio of the engine system.