A monitoring and control system for a seal gas supply system for a non-contacting gas seal. The supply includes several gas conditioning elements or units. The monitoring and control system includes an evanescent wave sensor to sense the presence of liquid in the seal gas. Multiple sensors to sense the temperature and pressure of the treated seal gas are disposed at the outlet of the conditioning elements. A programmable logic device is provided with information regarding the phase of the gas at various pressures and temperatures and compares the sensed data to the baseline data. Recognition of liquid concentrate results in an output signal.