Mechanical indicators for recording pressure changes in the working chambers of cylinders, turbines, and piston equipment provide adequately accurate and visual information for studying the state of equipment and for testing new machines. In order to eliminate certain deficiencies and enlarge the field of application of the device, the use has been suggested of a selsyn couple as a drive of the indicator drum, thereby providing a system with a minimum of mechanical joints. In the proposed apparatus, the standard drum and drive of a pressure indicator were replaced by a selsyn receiver with a drum fitted to its axis. The synchrotransmitter is joined to the drive shaft of the test machine. When using this design of indicator drum drive through a selsyn couple there is no need to introduce changes in the design of the indicator and it can easily be refitted to work with the standard mechanism. This setup considerably broadens the zone of investigations and provides fairly accurate and extremely visual information on processes occurring in all of the elements of test machines. Pressure diagrams in transitional processes and also comparative diagrams obtained by placing the indicator in different zones of the machine under study help describemore » effectively the state and functional relations of individual machine elements. Diagrams obtained by using recording drums of equal diameters have a constant length, and hence the diagrams could be processed using master forms to save time.« less