Thermochemical Measurements with a Capacitance Manometer R. P. Turcotte and T.D. Chikalla Battelle Memorial Institute, Pacijic Northwest Laboratories, Richland, Wash. 993.52
L. Eyring Arizona State University, Tempe, Ariz. 9.5281 RECENTLY THE OXYGEN dissociation pressures over a 1.22-mg sample of berkelium-249 oxide have been determined at high temperatures using a capacitance manometer system ( I ) . It is the purpose of this note to relay other experiments which demonstrate the high accuracy and extremely high sensitivity of such a system and the types of phenomena to which the technique can be applied. During the past decade and particularly during the past few years, a large variety of pressure transducers have become commercially available which vary widely in concept and possible application. As a research tool, several types offer the desirable linear electrical output as a function of pressure along with high sensitivity. The advantages of a transducer over more conventional methods of pressure measurement include : small volume within the measurement device, linearity over longer ranges, negligible change in volume over the whole range of pressures, and, most important, pressure may be continuously recorded either absolutely or in a differential sense. If pressure measurements are to be made in a closed system as a function of temperature, some small corrections may have to be made to subtract intrinsic system effects (e.g., those due to the hot zone) from real reaction with the sample. A versatile all-metal gas handling system has been constructed as shown in Figure 1. The illustrative reactions reported here were studied in a similar but unsymmetrical system with only the single sample tube in the hot zone. The fully symmetrical system offers large advantages in control of temperature effects both within the thermostated region and those due to the hot zone. Theoretically, a zero pressure differential will be obtained as a function of furnace temperature if the ratio of hot zone volume to total volume on the reference and sample sides of the manometer are made equal. In order to obtain this condition as well as to minimize absolute pressure changes, a minimum hot zone volume is desirable and is best achieved through use of precision ground sample tubes, filler rods, and crucibles. Alternatively, small bore quartz or other refractory oxide tubing (1-2 mm i d . ) has been successfully employed, although this restricts the sample size. For the system described, with a total system volume of approximately 12 cc and hot zone volume of
