| AVS 58th Annual International Symposium and Exhibition | |
| Vacuum Technology Division | Monday Sessions |
| Session VT-MoM |
| Session: | Vacuum Measurement, Calibration & Primary Standards, Gas Flow and Permeation |
| Presenter: | Manuel Vargas, Institute of Mechanics - Bulgarian Academy of Sciences |
| Authors: | M. Vargas, Institute of Mechanics - Bulgarian Academy of Sciences M. Wüest, INFICON Ltd, Liechtenstein S.K. Stefanov, Institute of Mechanics - Bulgarian Academy of Sciences |
| Correspondent: | Click to Email |
The Capacitance Diaphragm Gauge (CDG) is one of the most widely used vacuum gauges in low and middle vacuum ranges. This device consists basically of a very thin ceramic or metal diaphragm which forms one of the electrodes of a capacitor. The pressure is determined by measuring the variation in the capacitance due to the deflection of the diaphragm caused by the pressure difference established across the membrane. In order to minimize zero drift, some CDGs are operated keeping the sensor at a higher temperature. This difference in the temperature between the sensor and the vacuum chamber makes the behavior of the gauge to be non-linear due to thermal transpiration effects. This effect becomes more significant when we move from the transitional flow to the free molecular regime (Kn > 0.1). Besides, CDGs may incorporate different baffle systems to avoid the condensation on the membrane or its contamination.
In this work, the thermal transpiration effect on the behavior of a rarefied gas and on the measurements in a CDG with a helicoidal baffle system is investigated by using the Direct Simulation Monte Carlo method (DSMC). This technique is based on the discretization of the number of particles, the space and the time domains, and it combines deterministic aspects for modelling the particle motion with statistical aspects for computing the collisions between particles. The study covers the behavior of the system under the whole range of rarefaction, from the continuum (Kn < 0.01) up to the free molecular limit (Kn > 100), for various temperature differences and different temperature gradient configurations (with radial and axial components). Moreover, in order to analyse the dynamic response of the system to a change in the sensor temperature from an initial isothermal configuration, some non-steady state calculations are performed. In this way the evolution of the macroscopic properties of the gas is studied from the initial moments until the steady state is achieved.