An apparatus for providing a test gas to a gas monitoring instrument having a window for admitting gas to a gas sensor therein, is formed from a mechanical adaptor open at a portion thereof and having walls constructed and arranged such that the mechanical adaptor removably fits over the window to form a substantially gas tight chamber of predefined volume defined by the walls of the mechanical adaptor and the instrument, means disposed in the mechanical adaptor for receiving a vial of test gas and breaking a seal in the vial to admit the test gas to the chamber. The invention is also directed to a kit including the vial of test gas, and a method for testing a gas detection instrument utilizing the apparatus and a vial of test gas.