This Aeronautical Standard covers two basic types of instruments as follows: Type I ?C Range 35@000 feet. Barometric Pressure@ Scale ranges at least 28.1 -30.99 inches of mercury (946 ?C 1049 millibars). May include markers working in conjunction with the Barometric Pressure Scale to indicate pressure-altitude. Type II ?C Range 50@000 feet@ Barometric Pressure. Scale range at least 28.1 ?C 30.99 inches of mercury (946 ?C 1049 millibars). May include markers working in conjunction with the Barometric Pressure Scale to indicate pressure-altitude. PURPOSE: To specify minimum requirements for Pressure Actuated Sensitive Altimeters for use in aircraft@ the operation of which may subject the instrument to the environmental conditions specified in Section 3.3.