Might be helpful to provide a photo of your setup and a screenshot of the configuration page.

I assume you want to measure the fill level in the reservoir, right? In this case your sensor is configured to the wrong device type. The device type must be level sensor delta 40. The rest of the setup is almost self-explanatory: enter the theoretical maximum fill level which is the height of the tube and also the current fill level. After a click on the button "calibrate" you are done

The tube which is connected to the reservoir should not be connected within an active flow area because in this case the pumps pressure will work against the membrane inside the sensor. Ideally your reservoir has multiple threads at its base where the tube is connected. It confuses me a bit when you say it is connected to the outlet.