Biofuel ethanol adulteration detection using an ultrasonic measurement method

Hydrous ethanol is a worldwide used biofuel. According to Brazilian regulations, the concentration of ethanol in hydrous ethanol can be accepted at a maximum concentration of 93.8% and a minimum of 92.6% by mass. The aim of this study is to identify the possible changes in hydrous ethanol fuel using ultrasonic attenuation and propagation velocity. The experiments were performed in the Laboratory of Ultrasound of the Brazilian National Institute of Metrology (Inmetro). The experiments and uncertainties in the methodology were evaluated according to the Guide to the Expression of Uncertainty in Measurement, JCGM 100:2008. The test samples used in this study were mixtures of ethanol and water with ethanol concentrations varying from 89.84% to 93.71% by mass; and a commercial fuel ethanol bought from a local distributor. The correlation coefficient between ethanol concentrations and ultrasonic propagation velocity was 0.99 (in modulus), and the maximum combined uncertainty was 0.60 m s−1. Considering attenuation, the correlation coefficient was 0.97, and the maximum combined uncertainty was 0.085 dB cm−1. However, its signal is not stable resulting an unreliable parameter. Within the tested concentration range, the highest concentration that is statistically different (p < 0.002, α = 5%) from 92.60% is 92.25%, considering propagation velocity as parameter. To validate the methodology, a commercial ethanol fuel was tested using the proposed method as well as the gas chromatography analytical method (gold standard). Result was statistically identical for propagation velocity when compared to the gold standard.