This Aeronautical Recommended Practice covers two types of two unit Pressure Ratio Instruments each of which consist of a Transducer and an Indicator. The Transducer computes the ratio of two pressures and converts this ratio to a synchro electrical signal which is transmitted to the Indicator. Type I - All material in this document applies to this classification only. Type II - Additional material for this classification will be added to superseding issues as the state of the art permits. Purpose: To recommend requirements for electrical Pressure Ratio Indicating Instruments for use in aircraft@ the operation of which may subject the instruments to the environmental conditions specified in Paragraph 3.3. This recommended practice is to be considered as being currently applicable and necessarily subject to revision from time to time due to rapid development in the aircraft industry.