From the omega technical page about intrinsic safety:
Intrinsically safe equipment is defined as "equipment and wiring which is incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration." (ISA-RP12.6). More info here, translation:
Hire a competent field electrician to specify the proper zener barrier for a pair or two of RS485 digital Kellers and get one. Also get a length of approved cable.
Buy an approved RS485 intrinsically safe repeater.
Get a couple of approved, intrinsically safe Kellers.
Get a FlyportPro module to "talk" to the Kellers and publish things on the internet, so you can look at production data, tank levels, etc., on your cell phone.
Note: Since the RS485 repeater can drive cable up to 1200m, and since up to 128 devices can be on the same wired network, a single FlyportPRO module can replace several paper chart recorders... and who knows how many tank-level, long, wooden sticks?