April, I don't know where you live, but from your use of the word "petrol", I am assuming "not in the USA".
Depending on what you are trying to do, using air-pressure (petrol fume pressure?) to measure the level of fuel may not work as expected. Furthermore, depending on local environmental laws, modifying an existing fuel tank (or even putting fuel into a non-approved tank) can be against the law.
Also, using a sensor meant for diesel may not work as expected, or may even fail, when using it with petrol fuel; the different composition of the fuels, solvent capabilities, etc - may lead to a failure of the sensor and potentially to a fire or explosion (the same manufacturer, though, may have one specifically designed for petrol).
If you are thinking of doing something with a vehicle's fuel tank, the fuel pump is usually (nowadays) located in the tank, along with a float-valve sender (basically a potentiometer) to measure the fuel level (which is then sent to the fuel level meter on your dash).
If you are working on a custom vehicle (maybe a racing vehicle?), rules may be completely different since the vehicle isn't likely a licensed road vehicle; even so, speak with the manufacturer of your tank and discuss what you want to do with them - they may already have such a system available as an add-on or can custom-incorporate such changes into the tank properly to reduce possible hazards.