Pressure Measurements in the BET Method of Measuring the Surface Area of Powder Samples

Sources of the irreproducibility in measuring pressure by means of the BET adsorption apparatus with the zero-manometer are described. These have been examined experimentally and found to be caused by the contamination in the zero-manometer (ΔPe), the vibration of mercury in the manometer (ΔPv) and the differences in shape of mercury meniscus between the right and left tubes of the manometer (ΔPm). In this paper, the new method of indirectly determining these systematic errors of pressure is proposed. The maximum values of the above errors, (ΔPe) max, (ΔPv) max and (ΔPm) max were (4.7±0.5) Torr, ±0.7 Torr and ±0.2 Torr, respectively. In order to examine the effects of the above systematic errors on the surface area S, the simulation on the processes of S-measurement has been carried out using the computer, NEAC 3200 model 50 system. According to the results of simulation, the values of S for the samples, which were below 100 m2, were found to be scattered with the error of several tens per cent. On the basis of these results, the surface area S can be measured with the accuracy of ±1%, and the technique of S-measurement can be mastered for about 10 days.