Gas vaporizer for flashing liquid to vapor received from a source prior to introduction into a compressor or the like, such as in air conditioning or refrigeration systems. In certain embodiments the vaporize includes an adapter member for connection to a liquid source, a connector member having a plurality of flow passages for facilitating the transfer of heat to fluid present therein to vaporize the same, a body portion providing visual access such as via one or more sight glasses to an internal chamber therein for visual confirmation that liquid has been vaporized, and a hose connecting member for connection to a point of destination such as a compressor. In certain embodiments, the connector has an axial bore containing a high thermal conductive material.