I would like to know if anyone has any idear of how i can measure the level of oil in the the underground tanks at petrol stations and remotely keep track of them?
They have the appropiate wireless tank-mounted ultrasonic, pressure, or probe-type tank level devices that are frequently used on underground tanks. Range is much shorter on underground, but several gas station chains use them as there's still enough 'link-margin' to get to a receiver a few dozen feet away. In a gas station use, they meet the EPA requirements for the normal long-stick level measurement required daily.
Mostly the Centeron system is setup so tank level can be monitored over the internet, but they do have some PC software that will talk to the receiver serially or to link the system into a Veeder-Root wired system.
I recently installed a US approved system from Pneumercator. It uses magnetorestrictive level and also has Intrinsic Safe contact inputs for leak detectors.
The interface is Modbus
Roy
Its what is used in a typical gas station.
If you don't have to worry about approvals perhaps use one of the other suggestions, Ultrasonic or Druck DP cell dropped into tank.
Hope this helps.
Roy