The Admiralty Cathode Ray Oscillograph Engine Indicator

Abstract

The Admiralty cathode ray oscillograph indicator was developed for use on compression-ignition engines of medium- and high-speed types. The size of the engine has not necessitated the development of the smallest possible unit. It consists of a pressure-sensitive device based on the magnetophone principle. The pressure to be recorded deflects a steel diaphragm and changes an air gap in a magnetic circuit. The magnet is energized by a high-frequency (20,000 cycles per sec.) alternating current flowing in an encircling coil. The pressure on the diaphragm modulates the high-frequency current, and the modulations are observed on the screen of a cathode ray oscillograph. The advantages of a high-frequency current in this connexion are: (1) all frequencies below about 2,000 cycles per sec. are faithfully reproduced; (2) steady pressures are recorded and calibration is effected with an ordinary pressure gauge; (3) amplifier and circuits have to transmit only a comparatively narrow band of frequencies. A disadvantage is that the upper limit of frequency (2,000 cycles per sec.) is not sufficiently high for the study of very rapid variations. The precautions necessary to ensure accuracy are discussed, and some examples of typical records obtained with the instrument are reproduced.