104 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



range from to 12,000 kg/cm 2 can be found in Table 6, column 3. 

 The resistance variation follows the pressure variation without time 

 lag or hysteresis. However, if the pressure is transmitted to the 

 transducer by a liquid, a time lag may arise because of the increased 

 viscosity of the liquid at high pressures. 



A practical pressure transducer has been described by Bridgman. 1 

 It consists of a ring-shaped insulating core (bone) 1 cm in diameter 

 and 0.5 cm thick; an insulated manganin wire (no. 38, approximately 

 30 ohms/ra) is wound in a noninductive fashion around the core. The 

 total transducer resistance is 100 ohms. The contact resistance 

 should be small and should not change with pressure. Of course, the 

 influence of the contact resistance is small if the resistance of the 

 transducer itself is large. 



The wire must be "seasoned" by successive pressure applications. 

 Even after continued use, Bridgman observed a zero point shift (i.e., 

 a change of resistance at 1 atm pressure) of \ per cent over the period 

 of a month of continued usage. Periodic calibration is advised. 

 Bridgman estimated an error of yo P er cent in measuring pressure 

 from to 13,000 kg/cm 2 and in the temperature range from to 50°C 

 caused primarily by the reading error of the instruments. 



The advantages of the resistive pressure transducer over ?a\y 

 mechanical gauge are its simplicity and ruggedness. The introduc- 

 tion of insulated leads into the pressure chamber causes considerable 

 technical difficulties at very high pressures. 



1-52. Capacitive Pressure Transducers 



The dielectric constants of gases, liquids, and solids vary with the 

 pressure and can be measured by capacitive systems: A simple 



arrangement for capacitive pres- 

 sure measurements in benzene has 

 been described by Trendelnburg 2 

 and is shown in Fig. (1-5)2. It con- 

 sists of a cylindrical capacitor suit- 

 able to withstand the pressure. In 

 the range between atmospheric 

 pressure and 1 00 atm, the dielectric 

 constant changes by \ per cent. The 

 static transfer function (change 

 of capacitance versus pressure) is nonlinear, as expected from theo- 

 retical considerations (Clausius-Mosotti equation). With the help of 



2 P. W. Bridgman, Proc. Am. Acad. Arts Sci., 47, 321 (1911). 

 2 F. Trendelnburg, Z. tech. Physik, 11, 465 (1930). 



Fig. (1-5)2. Capacitive pressure trans- 

 ducer based upon the variation with 

 pressure of the dielectric constant. 



