Just had another look, you can also get a meter that gives a 4-20ma output so could be used in conjunction with the flow sensor to give a flow rate rather than a pulse. this takes away the need to calculate the flow rate in the PLC. Did not see this on the first glance. so yes, measure both, control one. providing it is set up correctly and nobody messes with your flow settings then it should work reasonably well even on varying flow rates of the water.