I am sorry I can not give much detail, but my customer did not wish to disclose much information and he also did not give me much to work on.
Since you all offer help, here what I have and can give you as information:
The unit should work on a 13.8Vdc power source. It must be package in a small non corrosive box around 4" x 2" x 1". It should emit an electric field from a 100 to 150 Vp.p. signal. Frequency must range from 150Khz to 200Khz. Antenna or anything similar should be enclose in the box. It mush have a low power drain. It must be low cost. It will be attach to a circular fluid pipe of 1/4" to 1/2" going strait in parallel with the box. The fluid has a low electric conductivity and density lower than water.
Now from what you said, I can add this:
It will probably use a capacitive coupling. It may have to be enclose in a metal box to prevent parasitic emission. It may require a flat fluid pipe for maximum coupling. It will require a sine wave to drive the output transformer or use the antenna as part of the oscillator.
Hope this help.
Bye Jacques