Abstract
We propose a millimeter-wave (mmWave) sensor to monitor gases and aerosols based on time-of-flight (ToF), thus, on the real refractive index. It utilizes an ultrawideband frequency-modulated continuous-wave (FMCW) radar operating from 72 to 92 GHz at a sampling rate of 500 Hz. Also, we introduce a model for simulation of the refractive index of arbitrary gases at mmWave frequencies using molecular spectroscopic data. Simulations then allow the identification of the respective gas under test. We characterized the performance of the sensor regarding the refractive index within various experiments. The precision is 0.027 ppm, and the 20-h long-term stability is better than 0.2 ppm. The linearity is at least 1.5 ppm. Due to sophisticated temperature compensation, the temperature drift is less than 10 ppm for 30 K. Since the sensor is accurate, precise, and robust, it is perfect for in-process real-time monitoring and classification of gases or aerosols in the industry.
Highlights
M ONITORING gases and aerosols is of great importance in the industry
As our system relies on the measurement of the ToF, we focus on the real refractive index n
Since only temperature-dependent dipole interactions occur in the microwave region and due to separable frequency ranges of the different polarization mechanisms, only αori and ξpar are contributing as frequency-dependent components to the refractive index
Summary
M ONITORING gases and aerosols is of great importance in the industry. Leakage of toxic, explosive, and radioactive substances can harm humans and pollute the environment; a lack of respiratory gases can thread the lives of workers; and unwanted emission of greenhouse gases contributes to global warming. Variations of the real refractive index of gases and aerosols using mmWave radar sensors have been investigated in numerous applications, e.g., detecting fluctuations of state variables of static gases [5], [6]; tracking of turbulences and marker gases in pipe flows [7], [8]; determination of velocity and volume fraction of bulk materials, such as coal dust [9]; characterization of carbon black and coffee powder in pipe streams [10], [11]; and monitoring the concentration of sodium chloride (NaCl) nanoparticles in exhaust pipes of flame reactors for nanopowder production [12] These sensors lack efficiency since they radiate electromagnetic waves in free-space mode, which causes interference and losses, decreasing the signal-to-noise ratio (SNR).
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