1 April 2006

Hazardous Gas Detection for Homeland Security

By Tadeusz M. Drzewiecki, Joseph M. Iseman, and Nagaraja Yaddanapudi

Simultaneous, continuous, real time, measurement, monitoring, and control of gas mixtures are ubiquitous requirements in science and engineering and more recently in safety and homeland security applications. Property-based gas analysis for ternary gas mixtures (gases in groups of three) first saw applications in the 1990s. Since then, numerous applications (mainly in the medical field for measuring respiratory gases) are ternary, such as oxygen (O2), nitrogen (N2), and carbon dioxide (CO2).

Scientists handled the presence of water vapor, a fourth gas, as a known concentration constituent in a saturated state because we measure the temperature as part of the gas analysis scheme. However, we quickly realized a system that could assay four gases would have much broader applicability. We could thus monitor anesthetic agents, such as halogenated hydrocarbons in combination with nitrous oxide.

The four gases are oxygen, carbon dioxide, and the simultaneous two agents. Four-gas mixtures, as it turns out, also cover the great majority of cases industrial applications encounter. Typically, a hazardous gas will appear in the air with some water vapor content. In general, we can treat air as a single gas. Thus we can analyze air, water vapor (humidity), and the two other gases. We must treat air, however, as a three-gas mixture of oxygen, nitrogen, and carbon dioxide when analyzing gases in the presence of combustion or other reactive processes that can cause the oxygen and carbon dioxide levels to vary.

The continuing concern over expedient use of readily available chemicals in terrorist scenarios has raised the issue of hazardous gases monitoring to far greater levels of importance than just its economic and technical aspects. The main problem for conventional specific gas monitors, however, is knowing what sensors to implement for which hazardous gases. Most commercial-off-the-shelf (COTS) monitors may handle as many as six gas species, often with some overlap, but when the number of possibilities is in the hundreds, acquisition costs become prohibitive, either from the standpoint of the number of sensors used or the expense of total assaying devices, such as gas chromatographs.

Such a system-that could rapidly, accurately, and with appropriate sensitivity respond, detect, and warn of an occurrence of a chemical incident within enclosed spaces, such as buildings and transportation facilities-would be a valuable tool. Chemical threats in lower concentrations, such as those posed by chemical warfare agents that include blood, vesicant, nerve, choking, and blister agents, pose a resolution problem that the property-based approach is not geared for at the levels in which they may appear in the atmosphere. However, these gases may be concentrated by as much as a factor of 100, merely by removing oxygen and nitrogen from the sample, so now we can examine the 1ppm at concentration levels of 100 ppm.

A low-cost, property-based, universal gas sensor-analyzer system can be the basis for a threat detection and identification system that could meet government and industrial goals of both low-cost acquisition and operation and high performance by responding to concentration levels of gas species designated to present hazardous toxic doses to human potential that cause acute immediate danger to life and health and exceed permissible exposure limits.

About the Authors

Tadeusz M. Drzewiecki, Dr. Eng., is president and chief executive of Defense Research Technologies, Inc., a developer of fluidic sensors and control systems under government grants and contracts from the Department of Defense, National Institutes of Health, and Department of Energy, in Rockville, Md. Joseph M. Iseman, Ph.D., is senior scientist, and Nagaraja Yaddanapudi is senior mechanical engineer.

Ultrasonic Gas Measurement

The use of ultrasonic technology in gas measurement poses some challenges. Dampening of acoustic sound waves in gases is considerably higher as compared to liquids. High frequency sound waves are quickly absorbed by the medium. This issue is addressed with low frequency transducers, which operate at a frequency of 500 kHz. An additional difficulty lies in actually getting the sound waves from one transducer to the other and back again-the waves have to penetrate the pipe wall, enter the gas, and leave it again through the pipe wall. Because of the very different densities and sound velocities of metallic pipe walls and gaseous medium, a large portion of the sound wave is reflected at the transition surface between pipe wall and gas. Only a small portion is transmitted into the gas. The reflected part of the wave remains inside the pipe wall and propagates to the receiver as noise.

For correct flow measurement, only the sound wave propagating through the gas is relevant. Higher operating pressures reduce the difference in densities and thus also the reflected noise signals. In order to maximize the signal to noise ratio, a special multi-pulse signal transmission method was developed. Modern digital signal processing capabilities achieve this separation of noise from signal with high precision. The key lies in sensor specific filters and narrow band amplifiers. Even small signals of a few micro-volts are recognized and evaluated.

Clamp on ultrasonic measurement typically takes place on gas injection or natural gas operating systems in the oil and gas industry or on high-pressure gases in the production of synthetics (PP, PE) or the food industry (extraction, freeze drying) and also on low-pressure gases inside plastic pipelines.

SOURCE: John O'Brien of Flexim in New York (JOBrien@flexim.com)