Moisture sensing in transformer oil using thin film microdielectrometry

Experiments have been conducted using 1-7- mu m-thick layers of plasma-deposited bromobenzene on the interdigitated 50- mu m-wavelength electrodes of a microdielectrometer chip. Gain-phase measurements in the frequency range of 0.005-10000 Hz, normalized to the frequency for peak phase shift, are found to be universal. With account taken of the finite layer thickness, evidence is presented that the coatings act as semipermeable membranes, serving as barriers to the transformer oil but absorbing moisture in proportion to the absolute content in the oil. Sensitivity to moisture from less than 5 to about 50 p.p.m. is found with a transient response of less than a few minutes. Although the sensor is well suited to following the dynamics of water in paper-oil systems over periods of days or perhaps weeks, a long-term loss of sensitivity must be overcome if it is to be used without recalibration for extended periods of time. Its use for discerning abnormal behavior in a pilot transformer monitoring system is described.<>