Jason R. Brown

Isothermal mass flow measurements in microfabricated rectangular channels over a very wide Knudsen range

Measurement and modeling of gas flows in microelectromechanical systems (MEMS) scale channels are relevant to the fundamentals of rarefied gas dynamics (RGD) and the practical design of MEMS-based flow systems and micropumps. We describe techniques for building robust, leak-free, rectangular microchannels which are relevant to micro- and nanofluidic devices, while the channels themselves are useful for fundamental RGD studies. For the first time, we report the isothermal steady flow of helium (He) gas through these channels from the continuum to the free-molecular regime in the unprecedented Knudsen range of 0.03–1000. On the high end, our value is 20-fold larger than values previously reported by Ewart et al (2007 J. Fluid Mech. 584 337–56). We accomplished this through a dual-tank accumulation technique which enabled the monitoring of very low flow rates, below 10−14 kg s−1. The devices were prebaked under vacuum for 24 h at 100 °C in order to reduce outgassing and attain high Kn. We devised fabrication methods for controlled-depth micro-gap channels using silicon for both channel ceiling and floor, thereby allowing direct comparisons to models which utilize this simplifying assumption. We evaluated the results against a closed-form expression that accurately reproduces the continuum, slip, transition, and free-molecular regimes developed partly by using the direct simulation Monte Carlo method. The observed data were in good agreement with the expression. For Kn > ~100, we observed minor deviations between modeled and experimental flow values. Our fabrication processes and experimental data are useful to fundamental RGD studies and future MEMS microflow devices with respect to extremely low-flow measurements, model validation, and predicting optimal designs.

Method for creating gas standards form liquid HFE-7100 and FC-72.

HFE-7100 and FC-72 fluorinert are two fluids used during weapon component manufacturing. HFE-7100 is a solvent used in the cleaning of parts, and FC-72 is the blowing agent of a polymeric removable foam. The presence of either FC-72 or HFE-7100 gas in weapon components can provide valuable information as to the stability of the materials. Therefore, gas standards are needed so HFE-7100 and FC-72 gas concentrations can be accurately measured. There is no current established procedure for generating gas standards of either HFE-7100 or FC-72. This report outlines the development of a method to generate gas standards ranging in concentration from 0.1 ppm to 10% by volume. These standards were then run on a Jeol GC-Mate II mass spectrometer and analyzed to produce calibration curves. We present a manifold design that accurately generates gas standards of HFE-7100 and FC-72 and a procedure that allows the amount of each to be determined.

Performance characteristics of cryofocusing GC/MS system at BWXT Pantex Plant.

Ensuring the reliability of all components within a weapon system becomes increasingly important as the stockpile ages. One of the most noteworthy surveillance techniques designed to circumvent (or take place alongside) traditional D&I operations is to collect a sample of gas from within the internal atmosphere of a particular region in a weapon. While a wealth of information about the weapon may be encoded within the composition of its gas sample, our access to that information is only as good as the method used to analyze the sample. It has been shown that cryofocusing-GC/MS offers advantages in terms of sensitivity, ease of sample collection, and robustness of the equipment/hardware used. Attention is therefore focused on qualifying a cryo-GC/MS system for routine stockpile surveillance operations at Pantex. A series of tests were performed on this instrument to characterize the linearity and repeatability of its response using two different standard gas mixes (ozone precursor and TO-14) at various concentrations. This paper outlines the methods used and the results of these tests in order to establish a baseline by which to compare future cryo-GC/MS analyses. A summary of the results is shown.