Pressure Sensor at Barometric Levels Using Ionized Gas
Authors: Stewart Matthew C , Liu Xuehan , Jones John Dewey , Leung Albert 
Published: February 2020

This paper shows experimental evidence of the feasibility of using ionized gas pressure sensors at barometric pressures. Through building an experimental setup with millimeter scale gap length which represents the critical dimension of in defining the geometry of the sensing element, our research has verified function and linearity at relevant pressure. The electrical current in the sensing element itself is on the order of tens of pA. In combination with low power amplification circuitry, a device built based on the results of this experiment would be suitable for use in ultra-low power applications.

An ion-based pressure sensor uses a small alpha particle source, in our case Americium-241, to ionize a volume of gas. A gas at a higher pressure, i.e. higher molecule density, will be subject to more ionization interactions with alpha particles and produce a higher ion density. These ions can then be collected through the application of an electric field across the volume and measurement of electrical current collected at electrodes. Under certain conditions, the resulting current is linearly proportional to the gas pressure.
An experiment using some of these concepts was first conducted in 1946 [1]. Later, in 1996, a, experiment using a 20 mm gap length was built to explore this concept as a vacuum sensor for the harsh environment of Mars, establishing suitability at low pressures with experimental data focusing on pressures from 0-2 kPa [2]. A more thorough theoretical examination was conducted at that time. These sensors are theorized to be mechanically resilient and to minimize susceptibility to sensor drift due to material fatigue over time.
Using the setup shown in Figure 1, we conducted an experiment using a much smaller gap length. Our sensor has been constructed with a variable size and has been tested to gap lengths as short as 4 mm, representing a significant reduction. By examining the sensor dependence on the parameters of voltage and gap length, we are able to extrapolate to smaller structures and can verify that our design is scalable down to sub-millimeter structures.
We tested and collected data verifying function and linearity. Figure 2 shows experimental data over a pressure range from low vacuum to atmospheric pressure for gap lengths of 4 mm and 10 mm. This plot shows that both are linear and approach a zero reading at vacuum, making them suitable as absolute pressure sensors. Our study focused on an application as a small resilient barometer, so we collected the data shown in Figure 3 in the range of ambient atmospheric pressure ± 10 kPa with gap length of 4 mm. The linearity was verified to within the limits of our reference sensor.
References
[1] J. R. Downing and G. Mellen, “A sensitive vacuum gauge with linear response,” The Review of Scientific Instruments, vol. 17, no. 6, pp. 218-223, Jun. 1946.
[2] M. G. Buehler, L. D. Bell, and M. H. Hecht, “Alpha-particle gas-pressure sensor,” Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, vol. 14, pp. 1281-1287, May/Jun. 1996.

Figure 1