A New Gas Density and Binary Concentration Meter

A unique gas density meter that is compatible with light gases at low pressures now available. Gas density and concentration test data over pressure for air, hydrogen, nitrogen, argon, methane, carbon dioxide, sulfur hexafluoride and gas mixtures are presented in this white paper. The measurement of hydrogen density at low pressure (101KPa to 400KPa) is demonstrated. Coupling binary gas concentration and density measurements of low pressure, light gases like hydrogen, natural gas and methane opens up new sensing applications like fuel cells, energy management, chemical and semiconductor processing. INTRODUCTION A number of gas concentration sensors have been developed and include thin film electrochemical, infrared [1], metal oxide semiconductor [2,3], acoustic [4,5] and ultrasonic [6]. While some can resolve part per million concentration differences there have been problems with hysteresis, selectivity, drift, cross sensitivity. Steel [7-10], glass and even micromachined silicon [11-12] resonating tubes have been used to measure density and mass flow of liquids and highly compressed gases via the Coriolis effect [7-10]. Unlike steel tube meters which are fabricated one at a time; MEMS-based sensors employ wafer fabrication enables hundreds of micromachined silicon Coriolis mass flow tubes and even assembled subsystems to be produced with one wafer stack. This batch fabrication method reduces the manufacturing costs enabling a wider use of density sensor technology. This paper shows how ISSYS has developed resonating micromachined silicon tube technology for use in density and binary gas concentration measurements. Special attention is given to light, low pressure gases like hydrogen and nitrogen which have not been measured with conventional metal Coriolis mass flow and density meters due to the high material density and thickness of the metal tubes. TEST RESULTS Since gas is a compressible fluid the first step in developing a gas sensor is generally to characterize its performance over pressure. The silicon tube resonant frequency varied with pressure for nitrogen at room temperature. As the pressure increases, the density increases and hence the resonant frequency decreases due to the increase in total mass. A water filled tube will have a resonant frequency of 23.65 KHz, while an air filled tube will have a frequency of 28.7 KHz. To accurately measure the density of a compressible gas using the frequency of a resonating tube requires knowledge of the gas temperature and pressure, which are related via the ideal gas law. Two White Paper ‐ A low pressure, light gas concentration and density sensor www.mems-issys.com options are available; making measurements at a known, fixed pressure or continuously monitoring the pressure of the gas. For this study an absolute capacitive pressure sensor (top) was used in series with the density meter (bottom) as shown in Fig. 1. The pressure sensor output if fed directly into the density meter via a cable. The density sensor was designed as an intrinsically safe device, meaning that the voltage, current and capacitance values were kept low to prevent a flame igniting spark from being generated in case of unforeseen conditions causing shorting of adjacent wires, components or circuit board traces. The density sensor design including the external pressure sensor were certified intrinsically safe (UL, ATEX, Class 1 Div 1) for the industrial environment, enabling them to be utilized for measuring gases like hydrogen and methane. During data gathering an additional pressure sensor was employed on the test stand to insure high pressure accuracy of the measurements. A number of gas density data plots were generated using both the resonating microtube and capacitive pressure sensor. Compressed gas cylinders, with 4 digits of accuracy using NIST traceable standard, were used in the calibration and testing of the device. All date was taken for these figures at temperatures between 23 and 24°C. Fig. 2 gives a pictorial comparison on the density of several light gases versus pressure such as hydrogen, methane, nitrogen, air, argon and carbon dioxide. Fig. 3 expands the low pressure range of hydrogen gas density data. This low pressure region is one of the unique capabilities of this technology. The linear fit for the hydrogen density data, R value of 0.9989, closely matches that of the ideal gas reference. The density of silicon is 2.33 Fig.2. The variation of several light gases Fig. 1. The density meter (bottom) and pressure sensor (top). White Paper ‐ A low pressure, light gas concentration and density sensor www.mems-issys.com gm/cc versus 8 gm/cc for stainless steel or 4.54 gm/cc for titanium. The use of conventional metal tube Coriolis mass flow and density meters have been restricted to relatively high pressures [7-10]. The silicon microtube walls are only a few tens of microns thick versus millimeters thick for conventional metal Coriolis mass flow and density meter tubes. While the measurement of the density of a low pressure light gas like hydrogen is extremely difficult for conventional metal tubes, it can be accomplished using the less dense, micromachined silicon resonating tubes. It is of industrial interest to measure the density of heavier gases in applications such as semiconductor processing, welding, refrigeration and insulating electrical switches. The fabrication of integrated circuits, MEMS and the MEMS microtubes used in this density sensor utilize sulfur hexafluoride and other gases as an etchants in the deep reactive ion etching or plasma etching processes [13,14]. Fig. 4 plots the density of sulfur hexafluoride SF6 versus pressure. As a comparison the density of carbon dioxide is also plotted on the same graph. Both gases are relatively non ideal at higher pressures due to their molecular complexity and as a result the density variation deviates from linearity at high pressures. It should be noted that the density of liquids with densities ranging from 0.7 to 1.5 gm/cc have been measured with the silicon microtubes of the same dimensions [11,12]. The density of fluids can be used to determine the concentration of binary solutions if the two different components have significantly differing densities. Premixed nitrogen-hydrogen cylinders were tested and the results plotted along with that of pure Density of Heavier Gases Versus Pressure 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0 50