I spoke with Todd at princeton some time back about this. I think part of the confusion is that princeton makes capacitance SENSORS
and they also make a capacitance CONVERTER/INTERFACE
After speaking with him, it was apparent that the sensors (the black puck with the tube that goes in the tank) does NOT need a converter; it outputs a variable voltage based on the capacitance sensed. This output works just fine with the GEA24.
The CONVERTER/INTERFACE is to be used when you use a home rolled setup, usually with plates in the tank. This black box converts a variable capacitance to a variable voltage to be used with the GEA24, and may require more power than the GEA24 can reliably provide.
I have corresponded with several people that are using the princeton sensors with the GEA24 without any issues...
Look at the probe sensors and converters here: