Measurement of gas composition using ultrasonic sensors
Creators: name: Wang, Sihe
Publisher: name: Imperial College London
Year: 2017

description: An improved experimental apparatus for measuring the speed of sound (SoS) of binary gas mixtures using a single acoustic sensor in an on-line mode is described in this thesis. The SoS sensor has a fixed acoustic path and is operated at a fixed frequency of 121362 Hz. A pair of piezoelectric transducers are used in the sensor as the ultrasound pulse transmitter and receiver, respectively. A single-pulse time of flight (ToF) based approach with a novel wave interpolation algorithm allowed the SoS to be resolved typically better than 1 part in 500,000 at one second measurement intervals. Suitability for precise rapid on-line measurement and simplicity are the two most important factors considered in the selection of SoS determination method. Real gas SoS is functionally dependent upon gas composition, temperature, pressure and acoustic frequency, which is the fundamental principle of the SoS technique for measuring the composition of binary gas mixtures. Pure reference gases including Ar, He and N2 were used in the experiment for analysing the SoS measurement uncertainty. For CO2/N2, the experimental CO2 mole fraction ranged from 0 to 0.3 and the uncertainty of the measured SoS results was estimated to be 0.1%. For any other tested binary gas mixture which consists of water vapour or an organic vapour, including n-hexane, n-heptane, n-octane, n-nonane, n-decane, toluene, cyclohexane, acetone, methyl ethyl ketone, ethyl acetate, methanol, ethanol, isopropanol, dichloromethane, chloroform, 1,2-dichloroethane, m-/o-/p-xylene, ethylbenzene, n-butylamine, limonene and α-pinene, as one component and dry air as the other, the mole fraction of the vapour component in the experiments was in the range from 0 to 95% of the saturated vapour mole fraction (0 to 95%SAT), and the SoS measurement uncertainty was estimated to be 0.14%. The experimental temperature range was from 5 to 55 °C, and each experiment was performed isothermally under atmospheric pressure. The temperature of the gas in the acoustic cavity of the SoS sensor can be measured with an uncertainty of 12.5 m°C by using the calibrated SoS sensor’s PT100. With the use of a barometric sensor mounted in the SoS sensor electronics board, a measurement of atmospheric pressure with 0.75 mbar uncertainty can be achieved. The carrier gas, N2 or dry air, was employed as the reference gas in each isothermal experiment for the calibration of the acoustic pathlength. The true gas composition of the binary gas mixtures was determined using a novel gas density measurement based on Archimedes’ Principle. The relationships between gas composition and density was established with an equation of state analysis using the Helmholtz equations stored in the NIST REFPROP software or the Van der Waals equation. Mixture density was measured with an uncertainty of 0.0015 kg/m3. A plot of measured SoS versus composition under different isothermal conditions is given for each investigated mixture. The deviation between the lossless real gas SoS determined using the NIST REFPROP software and the measured SoS was analysed for the mixtures with available thermodynamic data in the software including CO2/N2 and n-hexane, n-heptane, n-octane, n-nonane, n-decane, toluene, cyclohexane, water/dry air. Since CO2 is a strongly dispersive gas, the deviation for CO2/N2 is large and the maximum deviation reaches about 1.8%. For the other mixtures the deviation is below 0.5% and is reasonably small. By correlating the measured composition with SoS and temperature, SoS measurements allow rapid and real time gas composition determinations. Under atmospheric pressure and within the respective experimental composition ranges, for example for CO2/N2 at temperatures from 10 to 50 °C, SoS based technique is able to give compositions with an uncertainty of 0.0006 in mole fraction; for n-heptane/dry air mixtures at temperatures from 25 to 55 °C the uncertainty is between 0.16 and 0.40%SAT. This thesis also reports the most complete set of SoS data for organic vapour/dry air mixtures yet reported, including twenty-three different organic solvents across a range of temperatures, typically 5 to 55 °C. This study confirmed the versatility of SoS (ToF) ultrasonic measurements for the rapid and accurate composition measurement of binary gas mixtures for potential deployment in industrial or scientific instrumentation. Manufacture of the SoS sensor is practicable, and the difference in composition measurement uncertainty between a manufactured sensor and the prototype sensor used in this project should be negligible. In comparison with traditional gas composition measurement techniques, this technique is not only robust but also has the potential to be faster, simpler, more cost-effective, more precise and more accurate.