How can you acces the tank?
You can measure
-the weight of the tank
-measure the distance from the top untill water/fluid comes (watersensor with steppermotor).
-as posted before the pressure at the bottom with good knowledge of the tanks properties
you could add led+lightsensors in certain distances at the inner walls...
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about the waterresistant ultrasonic sensors.
In generall the speed in water is ~4times higher than in air. As far as i know also your range will decrease drastically but i guess with the right sensors/amplifier thats not a problem.
But we wont measure under water? Im not sure if mounted at the bottom these will measure the top of the water as it doesnt reflect. And a swimmer to measure the bottom distance isnt the easiest too.
=> but for cars there are sensors that work in air and are water resistant.
"Waterproof Ultrasonic Sensor II
Waterproof Ultrasonic Sensor for use outdoor and in high-humidity areas. Excellent for alarm systems"