Paper VT-WeM4
Role of Rotor Surface Conditions on Calibration Constant of Spinning Rotor Gauges

Wednesday, October 24, 2018, 9:00 am, Room 203B

In spinning rotor gauges, measured pressure is inversely proportional to the tangential momentum accommodation coefficient (TMAC), a quantity which is affected by interactions between gas molecules and the rotor surface. This is why control over surface conditions of the rotor is extremely important, in order to accurately determine pressure. In our study, surface of rotor spheres were altered by various treatment steps, and changes in the TMAC with respect to the initial condition. For this purpose three different treatment steps were taken, each done over 24 hours. First the rotor sphere was vacuum baked at 300 °C. Next, the rotor sphere was heated again at 300 °C in oxygen at 1e-2 Pa. Finally, it was heated at 300 °C in pure hydrogen at 1e-2 Pa. In order to examine reproducibility of the method, two rotor spheres were simultaneously treated, while one rotor sphere was used as a control. TMAC of the untreated spheres and after each treatment step was determined using primary static expansion calibration system which was developed at Institute of Metals and Technology. Additionally, in order to determine the dependence of TMAC on the gas used, measurements were repeated for six pure gases; helium, methane, neon, nitrogen, argon and krypton. Results obtained showed a relative change in TMAC of up to 7% after heating in oxygen. Furthermore, a strong dependence on the gas used was observed, where largest change of the TMAC was observed for helium and neon.