I’m sorry, I don’t understand. How do I get a 3m length 5mm diameter tube into the tank that is 4m into the ground and connect it to an esp while the same esp is not under water?
Make your tube 5m long.
But if the sensor is above water level, the water would never reach the sensor through that tube
Aaaahhh, no, wait, now I understand. The sensor is facing downwards, the tube as well, and the water will rise within the tube when the overall water level rises. This will increase the pressure inside the tube and the sensor can measure that. Now I see. That is an interesting approach.
Any idea where I could get a tube that fits so well on the sensor that the amount of air in the tube will stay constant?
The sensor is measuring the air pressure in the tube, like a barometer.
Not the water pressure!
See the Youtube movie in the link I post earlier
On Aliexpress you can find everything, but it looks like a 5mm tube you can find in every hardware store.
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I see, I see, I found this on amazon:
Pneumatik Polyurethan PUN Schlauch 5 meter Außen 4mm x Innen 2,5mm Klar Transparent Flexibel Druckluftschlauch Luft Treibstoff Öl Amazon.de
There was also an item mentioned in the previous link that looks quite similar.
I’ll try that one 
Thanks for taking the time to explain this! I was very slow on the uptake, but I think I figured out the way to go from here. Let’s see if I can make it work!
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I already wrote, try with micro drip irrigation tube. You can also seal it with epoxy for example. Pressure is only 0.3bar.
Sensor doesn’t need to be upside down, you can position it however you want. Just make sure that sensor is connected to the tube before you put tube in water and that it’s stays vertical.
Edit: I see you found tube already.