The refractive indices of the low-sulphur diesels which feature in the final row of the table at the end of the previous section are 1.4595 and 1.4745. Again a mild trend of increase with density is evident, a typical value of 1.448 having been noted in Chapter 4 for kerosenes and values of around 1.42 having been noted in Chapter 3 for gasolines. The refractive index of n-cetane, the benchmark hydrocarbon against which diesels are assessed, is 1.432.
The dielectric constant for diesel fuels varies across quite a wide range, from about 2 to about 6, making the correlation used for the lighter fractions for calculating the refractive index from the dielectric constant unsuitable for diesels. The meaning of the correlation, which even for a pure hydrocarbon compound is only approximate, is not altogether lost when diesels are considered. Taking the mean of the two values of the refractive index earlier in this paragraph and squaring it gives a value for the dielectric constant of 2.15, which is within the range given though certainly at the low end of it. The dielectric constant of n-cetane at 20°C is 2.08, giving a value of the refractive index of: √2.08 = 1.442
A reader utilizing the correlation in the box above might well make the judgement that the term v0.003 is so close to unity across the range of values of the kinematic viscosity of diesels that it can be omitted without any loss of precision.