Optical Detection of Inhibitor in Transformer Oil at Silicon-Detectable Wavelength Using Variable Path Length Model

The conventional method of detecting inhibitor content in transformer oil is by using fourier-transform infrared spectroscopy at the mid-infrared region. This wavelength of detection brings several technical challenges in fabricating the photodetector and light source. Therefore, as a fundamental research that can potentially lead to the development of an on-site measuring device, this study reports on the discovery of a new optical detection wavelength (959 nm) that can be used to measure the concentration of inhibitor in transformer oil. A total of 35 samples were manually prepared and measured using the Agilent Cary5000 Spectrophotometer from 950 nm – 970 nm with cuvettes of three different path lengths. The peak at 959 nm shows the second overtone of O-H stretch in inhibitor and has a strong correlation with Morse oscillation model. Two mathematical models were established for fixed and variable optical path lengths based on the correlation between the concentrations of inhibitor and the optical absorbance spectra of the samples. The mathematical models were verified with average errors of 4.00% and 5.49% for single and variable path length models, respectively. Finally, the importance of a variable path length measurement and the application of the newly discovered wavelength are critically analyzed and presented. The combination of the variable path length model and the detection of inhibitor at Silicon-detectable wavelengths (< 1100 nm) enables the utilization of Silicon technology in the development of an inhibitor monitoring device which can be used in various industries.