Abstract
Ultimate pressure vs input characteristics of a Hickman pump, containing Octoil-S and DC-705 as working fluids, have been observed as a function of an operating period and a backing pressure. It is shown that in both cases of using these two oils the characteristics vary markedly, in a way characteristic of each oil, as the pump is operated for several hundreds hours. It is confirmed that the important pressure-limiting factors are back-diffusion of gases from fore-vacuum and thermal decomposition of impurity or unstable component in the oil as well as an outgassing of the system. In case of DC-705, the lowest pressure is obtainable for an input where pumping of the oil occurs continuously. Pressure burst due to pumping is, however, not observed for each boiler except for a nonvolatile-reserving boiler over an entire region of input tested. The lowest ultimate pressure is not affected by changing a backing pressure from 10-3 to 10-6 torr as long as an input is optimum. The lowest pressure obtained is 4 × 10-1 torr without cooling a trap. Very good heat-resistant property of DC-705 has been suggested from these experiments.
