In this application a 1/2" NPT Liquid Level Sensor
is installed in a tee. As the fluid enters the sensing field the sensor switches. If the fitting is metal or the fluid is non-conductive, the sensor must be shielded
. In plastic fittings when the target is conductive (such as water) the sensor should be unshielded
The sensor can be AC or DC and with any output desired.
This is an incredibly economical alternative to other expensive, complicated and unreliable alternatives.