Comparison of the use of internal and external electrodes for the measurement of the capacitance and conductance of fluids in pipes

A simple theory has been developed to explain the behaviour of concave plate electrodes mounted either internally or externally to a pipe wall. The theory has been adequately confirmed by experimental results, and shows that the conductance and capacitance of the external electrode system are each functions of both fluid conductivity and permittivity and as these parameters increase the measured admittance tends to that of the capacity formed by the electrodes, the wall and the wall-fluid interface. Internally mounted electrodes are to be preferred for the measurement of fluid permittivity but externally mounted electrodes may be used where the effective fluid admittance is low.