Hello i am new to PAC System and i would like to design amd buiild a control amd automation system for metering fertilizer proportional to flow rate i would like to know what moduels i have to use to control the metering pumps
That depends an awful lot on the pumps. Do you have a datasheet on them? You'll need more information like what voltage they are, etc. before knowing for sure what modules you'll need.
Also you mention flow rate, is that being measured at the moment?If not, we will need to know about that flow transducer and what its electrical output is so we can help select the correct Opto module to measure the flow.