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The stability of the diesel fuel is related to the fuel composition that evolved to the modern fuels from the historical ones. The instability of modern diesel fuel is mainly due to the presence of unsaturated components and components with oxygen as organic components and cetane index improvers. There are many standards set by national organizations and fuel producers to test and guarantee the stability of diesel fuel under storage conditions. The simple characteristic of serious degradation of diesel fuel is the appearance of resins and sediments. Standard laboratory techniques for measuring fuel stability, like the rancimat methods, are relatively complex and time consuming. The time of examination of initial stages of degradation is in the rank of a dozen of hours. The fast methods of diesel fuel stability pointing with single ultraviolet (UV) wavelength forced degradation and fluorescence reading in the sensor with disposable capillary optrode have been proposed. The present paper concentrates on the possibility of further reduction of time of examination of the diesel fuel that enables initial and advanced stages of fuel instability pointing. The reduction of time of degradation is expected as two light emitting diodes (LED) of UV radiation working at different wavelengths are implemented in sensor head. The long-wave ultraviolet (UVA) - 365 nm high power light emitted diode and the short-wave ultraviolet (UVC) - 265 nm light emitted diode are selected for examination. The procedure of degradation and examination with signal switching of LEDs has been proposed. Experimental results are presented for the dual LED degradation system in comparison of to the single 265 nm LED degradation system. Obtained characteristics show fluorescence emitted signal at selected wavelength bands dependency of initial, medium and advanced effects of fuel degradation. The results show that using dual LED system increases fuel degradation speed as an advanced effects of fuel instability arise in the time less than one hour.
