The most interesting side to the electrical conductivity of lubricating oils is the huge difference between those which are simply residual petroleum stock and those which contains esters which, as noted previously in this chapter, some lubricating oils do. This effect is shown strikingly in work reported by Harvey et al. in which three lubricants from the Shell Vitrea range are compared with Exxon Turbo 23807 oil. The latter contains esters. The Shell oils studied in have kinematic viscosities at 21°C in the range 60 to 600 cSt, and the two examples of the Exxon Turbo 2380 oil examined have values of 35 and 40 cSt at the same temperature.
The electrical conductivity of the Shell oils are in the range 0.4 to 2.1 pS m-1, whereas the Turbo 2380 samples give values of 1370 and 1410 pS m-1. The effect of the esters has therefore been to raise the electrical conductivity by about three orders of magnitude, and this is due to the polar nature of the esters which have been incorporated into it. Returning to the discussion in section 1.5, water at 20°C has a value of 5.4 uS m-1 or 5.4 x 106 pS m-1. Notwithstanding the conductivity enhancement, the ester-containing oil is many orders of magnitude short of being an electrolyte.