Conductivity and Permittivity of Midel 7131: Effect of Temperature, Moisture Content, Hydrostatic Pressure and Electric Field

Knowledge of the conductivity and permittivity of materials used in subsea connectors is needed to predict the electric field distribution in the connectors. The synthetic ester Midel 7131, being one of several dielectric liquids used in subsea connectors, has been chosen as a relevant candidate material. The conductivity and permittivity of Midel was measured by an IRLAB conductivity meter and an IDA insulation diagnostic analyzer. The temperatures were 30, 60 and 90 degrees C, the hydrostatic pressures were 0.1, 1 and 10 MPa (1, 10 and 100 bar), the water content varied between 20 and 5600 ppm(w), and the applied electric fields varied between 2.5 V/mm and 9.4 kV/mm. The results show that the conductivity increases linearly with increasing levels of water content, whereas the temperature dependency follows an Arrhenius relation. The conductivity was found to increase slightly with increased pressure, the differences between samples were higher, rendering this increase insignificant. Measurements of the permittivity showed a linear increase in permittivity with increasing water content. The measured permittivity was found to follow the Clausius-Mossotti relation when varying the temperature and hydrostatic pressure. Based on the dependence of the conductivity and permittivity on water content, it seems that water molecules in Midel have a reduced degree of mutual interaction compared to liquid water.