siemens systems layout for oilfield location

I want to take a siemens 1200 plc and hook up probes via a 485 connection to the plc. I, however, am having problems with trying to program it. I want to try and get rid of the remote terminal unit and have the plc do the measurements of how much is oil is in a tank with a probe hooked up to it. any help would be appreciated

ndrghrdr

is there some explosive area issues here?
what type of probes?
how are they connected to the plc
RS485 - ok - that is the comm's format