From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Leaflet for the Cossor-Dodds Pressure Unit Model 3361, an auxiliary apparatus for pressure recording.
| Identifier | ExFiles\Box 167\2\ img249 | |
| Date | 12th February 1940 guessed | |
| COSSOR-DODDS CATHODE RAY OSCILLOGRAPH PRESSURE RECORDING (AUXILIARY APPARATUS) COSSOR - DODDS PRESSURE UNIT. Model 3361 Of the pressure units at present available, the Cossor-Dodds Condenser type is probably the most useful for general pressure recording, being both robust and versatile. By using elements having diaphragms of suitable thickness, pressures ranging from a few pounds per square inch to several thousand pounds per square inch may be measured, and the unit is suitable for use with Internal Combustion and other engines operating at speeds above some 150 revs/min. Both the pressure unit and its associated amplifiers have been designed to respond to a rate of change of pressure equivalent to 30,000 c.p.s. and the equipment as a whole is therefore capable of application to problems involving the highest speeds of which reciprocating engines are normally capable. The frequency range is adequate for the study of detonation in supercharged aero engines, and also covers those frequencies of vibration commonly encountered in general engineering practice. The unit can be used to study cylinder or fuel line pressures on either a linear Time Base, or a Piston Displacement axis. In addition, it may be used for the measurement of rate of change of pressure, and for the delineation of movement, velocity and vibration. The illustrations show the only type of capacity pressure element which has been standardised. This screws into the normal 14 mm. sparking plug thread. The form of construction adopted enables the user to vary the sensitivity of the element by adjusting the air gap, but care must be taken to avoid bringing the two electrodes into contact. The slightly lower inherent sensitivity of this type of element when compared with the mica dielectric type is more than offset by its greater stability, linearity and permanence. It will be seen from the sectional view of the element that the form of construction adopted ensures that the fixed and moving plates of the condenser remain plane and parallel irrespective of the pressure to which the unit is subjected. In consequence the element performs in a linear manner at all pressures up to that at which the elastic limit of the diaphragms is exceeded. No other element at present available possesses this very valuable feature. The model 3361 unit may be relied upon to give an excellent pressure/time diagram which is free from super-imposed mechanical vibration under all but the most severe conditions. It is simple and robust from both the mechanical and thermal points of view, and the construction of the diaphragm unit is such that it cannot normally be destroyed by cylinder pressures. Solid Section shows "Perspex" Insulation DESCRIPTION The insulated centre electrode is carried by a steel sleeve which is screwed into the body of the unit and so adjusted as to leave an air dielectric of only one or two thousandths of an inch between this electrode and the moving plate. This latter is carried by two diaphragms having a combined thickness suited to the pressures to be measured. Two passages are drilled in the body of the unit in such a way as to communicate with the cavity between the two diaphragms. Short extension tubes are brought out from these passages, enabling a steady flow of cooling water to be passed through the element. In order that the insulating material in the element will remain undamaged it is essential to pass an adequate flow of cooling water through the unit, and of course this supply must not be cut off until the engine has cooled to below 70º C. By forming the diaphragms in the main body of the unit, changes in calibration dependent upon the extent to which the element is screwed into the test plant are avoided. On the few occasions when the nature of the experiments renders the 2 ccs. additional capacity of the cavity undesirable, a suitable plug may be inserted into the orifice such that the upper end of the plug clears the diaphragm by at least 0.02 inch. A suitable hole or holes should be drilled through the plug before it is inserted. The size of any such holes may be calculated having regard to the change in volume of the remaining cavity under the influence of the applied pressure, and the anticipated rate of pressure rise. Adaptors may be used for insertion of the Pressure unit in holes having a diameter of 18 mm. or more. Connection to the unit is effected by means of special screened cable, and it should be remembered that the overall sensitivity varies with the length of cable, so that recalibration should be resorted to whenever a change in connections has been made. LIST PRICES Model 3361 Pressure Unit ... £11-11-0 Model 3041 Fuel Line ,, ... £11-11-0 Special Screened Cable per 20-ft. ... £1- 0-0 COSSOR INSTRUMENTS Leaflet No. CL.291.A. | ||