AVS 58th Annual International Symposium and Exhibition
    Vacuum Technology Division Monday Sessions
       Session VT-MoM

Paper VT-MoM10
Direct Conductance Measurements of Laser-Drilled Pinhole Apertures

Monday, October 31, 2011, 11:20 am, Room 111

Session: Vacuum Measurement, Calibration & Primary Standards, Gas Flow and Permeation
Presenter: James Fedchak, National Institute of Standards and Technology
Authors: J.A. Fedchak, National Institute of Standards and Technology
D.R. Defibaugh, National Institute of Standards and Technology
Correspondent: Click to Email
A pinhole orifice with a known conductance can be used as a secondary flow standard. We are interested in using pinhole orifices to produce nitrogen gas flows into vacuum in the range of 10-11 mol/s to 10-6 mol/s (10-7 to 10-2 cm3/s; STP) for vacuum gauge calibrations because a flowmeter based upon an appropriate set of orifices is easy to operate and automate. Commercially available laser-drilled pinhole orifices with diameters from 1 μm to 50 μm can have molecular-flow conductances, C0, ranging from about 0.1 μL/s to 230 μL/s for N2 at 23 °C, and can be used to produce gas flows in the range of interest by applying an upstream pressure in the range of 10 Pa to 100 kPa (0.1 to 760 torr). Accurate measurements of the orifice conductance, C, as a function of pressure are required to use the pinhole orifice as a basis of a flowmeter. The NIST bellows flowmeter is a primary gas flow standard that was used to directly measure the conductance of a pinhole orifice to better than 0.2 % over the entire pressure range of interest. We present results of the conductance measurements for nitrogen and other gases. One might expect that the differences among the gases would be mainly due to their different thermal velocities, and that those differences would disappear when the normalized reduced flow rate (C/C0) is plotted as a function of inverse Knudsen number. However, this was not the case at higher pressures. For example, the reduced flow rate for nitrogen was smaller than for argon at the same inverse Knudsen number. Following a suggestion by Jitschen (Vacuum 76 (2004) 89-100), the effect of the heat capacity ratio on C/C0 was investigated.