The quantitative determination of ethanol concentration in petrol is important for fuel-quality control and for monitoring ethanol-blended fuels. In this study, a poly (methyl methacrylate) (PMMA) optical-fiber evanescent-wave sensor is investigated as a simple method for determining ethanol concentration based on changes in refractive index. Petrol–ethanol blends of E₀, E₁₀, E₂₀, E3₀, E₅₀, E₈₅, and E₁₀₀ were studied. The refractive index of the prepared blends was measured using an Abbe refractometer, and the corresponding output of the optical-fiber sensor was recorded at 25, 40, and 60 °C. The experimental results show that the refractive index decreases continuously with an increase in ethanol concentration, while the sensor output increases correspondingly. Linear calibration equations were obtained at the three temperatures. At 25 °C, the calibration equation was V = 0.07392C + 0.900 with R² = 0.9799. At 40 °C and 60 °C, the equations were V = 0.07104C + 1.929 with R² = 0.9709 and V = 0.07332C + 2.692 with R² = 0.9549, respectively. Here, V is the sensor output in mV and C is the ethanol concentration in % v/v. The corresponding sensitivities were 0.07392, 0.07104, and 0.07332 mV/% ethanol at 25, 40, and 60 °C, respectively. The similar values of the calibration slopes indicate that the sensitivity of the sensor towards ethanol concentration remains comparatively stable over the investigated temperature range. However, the change in intercept indicates that the sensor baseline is affected by temperature. Therefore, temperature-specific calibration is required for accurate quantitative determination of ethanol concentration. The available experimental data contain calibration points and regression values, but replicate measurements, instrument-resolution information, and an independent validation dataset are not available. Hence, numerical values for measurement uncertainty, limit of detection, and recovery cannot be reliably established from the present data alone. Further experiments using replicate measurements and an independent validation set are recommended to establish the complete analytical performance of the proposed optical-fiber sensor.

Keywords: ethanol; petrol; gasoline; PMMA; plastic optical fiber; evanescent wave; refractive index; optical sensor; fuel analysis; temperature compensation